1. STRUCTURAL MINERALOGY OF CLAYS
By GEORGE W . BRINDL.ET *
INTRODUCTION Structural Grou-ps and Suh-Groups. The minerals
The investigation of clay minerals utilizing X-ray with which we are concerned lie in the broad chemical
crystallograpliie techniques has been very actively pur- category of silicates, which, following W. L. Bragg
sued for the last 20 years and a considerable wealth of (1937) and others, may be classified into structural
information has been collected and organized into a co- groups and sub-groups. The question of the naming of
herent body of knowledge. This interest is reflected in these structural groups has been discussed by Fleischer
the number of books dealing with the mineralogy of (1947), Strunz (1941), and others, and need not be con-
clays that has appeared in recent j-^ears. The following sidered further here. Clay minerals come mainly in the
may be mentioned t : laj'er-silicate grOTip and these can be subdivided accord-
Sedletskij, 1. D., KoUoidno-Dispersnaja Mineralogia, ing to the type of laj-er striicture. The naming of these
layer tj^pes will doubtless also be a matter for discvis-
IMoseow, 1945. sion. In. addition to the single-layer types, a structural
Marshall, C. E., The Colloid Chemistry of the Silicate sub-group is also required for the mixed-layer struc-
Minerals, New York, 1949. tures, and eventually more than one mixed-layer sub-
Hoyos de Castro, Angel, and Gonzalez Garcia, Jose, group may be necessary.
Genesis de la Arcilla, Madrid, 1949. In addition to the layer-silicate clays, there are those
Kerr, P. F., et al., Clay Mineral Standards, Am. Petro- which appear to be more closely allied to chain-silicate
leum Inst., Proj, No. 49, Now York, 1950. structures (pyroxenes and amphiboles), namely paly-
Jasmund, K., Die Silikatischen Tonminerale, Leipzig. gorskite or attapiilgite and possibly sepiolite. These are
1951. less fully understood than the layer silicates and exist-
Garcia Vicente, Jose, Bstructura Cristalina de los Mine- ing knowledge of these minerals up to 1950 has been
rales de la Arcilla, Madrid, 1951. summarized by Caillere and Henin (1951).
Brindley, G. W., X-ray Identification and Crystal Struc- Any scheme of classification is in danger of imposing
tures of Clay Minerals, London, 1951. a rigidity which the subject will eventually outgrow
In the present contribution, it is assumed that the and it is therefore desirable to retain flexibility in the
reader is familiar with the main features of the struc- scheme. This is especially necessary in connection wath
tures of clay minerals, namety the arrangement of atoms the classification of hydrated minerals and of mixed-
in well-defined sheets and layers with the anions, mainly layer minerals.
0, Oil, and occasionally F, grouped tetrahedrally and Hvdrated minerals can often be associated most con-
oetahedrally about the cations, mainly Si and Al in veniently with the corresponding dehydrated forms, but
tetrahedral positions and Al, Mg, and Fe in octahedral on other occasions they may be treated as mixed-layer
positions, together in some minerals with interlayer minerals. At present it seems undesirable to allocate
cations such as K, Na, Ca, Mg. Familiarity with the them rigidly in either way.
main features of the kaolin- and mica-type layers is In the mixed-layer group, it may eventually be useful
assumed and also the existence of water layers and to distinguish between randomly mixed and regularly
hydration complexes between the silicate layers in such mixed structures, and particular examples of the latter
minerals as montmorillonite and vermiculite. may be classified at some future date as distinct struc-
Even with these assumptions, some limitation of the tural sub-groups. In fact, this is already true of the
subject matter is necessary and the discussion is there- chlorites which consist of a regular alternation of mica-
fore confined to particular aspects which are chosen type and hydroxide-type layers, and, in view of their
partly by their relevance to the publication as a whole general importance it is most useful to regard them as
and partly on the grounds of personal interest. constituting a particular structural sub-group. Other
examples of regularly mixed layer structures, which are
A CLASSIFICATION OF CLAY MINERALS less well understood or which are of rare occurrence, can,
for the present, be left in the mixed-layer group. As illus-
Clay minerals, for the most part, belong to the group trations, clay minerals showing long spacing regularities
of silicates having layer structures, and their structural may be mentioned, such as a montmorillonite with a 32A
investigation cannot be divorced from that of layer sili- spacing (Alexanian and Wey, 1951), a mica-type min-
cates generally. A classification of clay minerals must eral with a 22A spacing (Caillere, Mathieu-Sicaud, and
therefore be developed within the wider context. The Henin, 1950), rectorite with a 2 5 A spacing (Bradley,
scheme set out in table I is largely self-explanatory. 1950) and various weathered clays discussed by Jack-
Successive subdivisions are based alternately on struc- son et al. (1952).
tural features and on chemical compositions. This is
particularly important in relation to problems of clay- Chemical Species. Within each structural group or
raineral identification, for it is clearly necessary for the sub-group the chemical species are divisible according to
methods employed to be sensitive both to structure and composition. In some cases this gives clearly defined
to composition. species (e.g. the kaolin minerals, the serpentine minerals)
but in others where continuous or largely continuous
* Reader in X-ray Physics. Physics Laboratories, The University, composition ranges exist, (e.g. the montmorillonite-bei-
Leeds, England ; now Researcli Professor of Mineral Sciences,
Pennsylvania State University, Pennsylvania. dellite series, the chlorites) boundary lines must be
t To this list can now be added Grim, R. E., Clay Mineralog:y, New
York, 1953. drawn in a largely arbitrary manner.
(33)
2—91001

2. 34 CLAYS AND CLAY TECHNOLOGY [Bull. 169
Ttihle 1. A .•srhenw of cltiifsifivution of cliiys (iiui relnted silicnte Diineruh.
CHEMICAL
SILICATES
CATEGORIES
1
1
STRUCTURAL Layer Chain
GROUPS silicates silicates
•* 1
•
1
1
1 1 *" ^ ^
Kaolin Mica Chlorite Mixed-layer Chloritoid
SUB-GROUPS
type type type type
CHEMICAL Kaolin Talc Penninite Anauxite 'alygorskite
SPECIES minerals Pyrophyllite Clinochlore Bravaisite (attapulgite)
Serpentine Muscovite Prochlorite etc.
minerals Ptilogopite Daphnite Sepiolite
Chamosite Biotite etc.
Amesite Glouconite
Greenalite lllite(s)
Cronstedtite Montmorilionoids
etc. Vermiculite
etc.
STRUCTURAL Halloysite Polymorphic Polymorphic
VARIETIES Kaolinite varieties varieties
Dickite
Nacrite
Chrysotile(s)
Antigorite
and hydrated and hydrated
forms forms
There may be some disagreement about grouping to- diff'erent chemical species within the mica-type sub-
gether the kaolin minerals proper, the serpentine min- group, having various hydration properties.
erals, and others placed in the kaolin-type group and also
about this choice of a name for the group. The struc- Structural Varieties. The structural varieties of each
tural scheme is essentially the same for all these minerals chemical species differ in the manner in which the struc-
and the onlj' point of difference is that some are dioctahe- tural layers are arranged with respect to each other. To
dral and others trioctahedral. * This distinction, however, recognize them in clays by the X-ray powder method, it
occurs in all the sub-groups and appears to be of rela- is necessary to record carefully the details of their dif-
tively small structural importance. To achieve consist- fraction diagrams. With well-crystallized and relatively
ency, the same policy must be adopted with all the groups pure materials, many of the varieties may be recognized
and the simplest solution is to place di- and tri-octahe- by their powder diagrams (see Brindley, 1951a) but with
dral members always together; this also eliminates any poorly crystallized and/or impure materials considerable
difficulties with minerals having an intermediate num- difficulties can arise. To the kaolin varieties and serpen-
ber of octahedral ions. As regards the naming of the tine varieties, which can be recognized optically, separate
group, the term "kaolin-type" suggests the type of names have been given. Polymorphic varieties of the
structure without actually taking the name of one of the micas, studied by Hendricks and Jefferson (1939), and
members. The use of a jiumerical ratio, such as 1:1 and of the chlorites, studied by Brindley, Oughton, and
2:1 for designating the kaolin- and mica-type layers, has Robinson (1950), have not been given separate names
some advantages but it is not easily extended to all the and reqtiire rather detailed X-ray examination to be
layer types which are involved; for example, is a chlorite recognized. They are most usefully described in terms
layer to be designated 2 : 2 or 1 - | - 2 : 1 ? of the unit cell symmetry and the number of structural
layers per imit cell, a system of nomenclature which has
It will be observed that in the mica-type group there already been usefully applied to the polymorphs of
have been included talc and pyrophyllite, the micas silicon carbide (liamsdell 1947; Ramsdell and Kohn,
proper (nmscovite, biotite, . . . ), the clay micas (illites, 1951, 1952) and of wurtzite (Frondel and Palache,
etc.,) and the swelling minerals (montmorilionoids, ver- 1950). Grim and Bradley (1951) have shown that some
miculite). The justification for placing all these minerals of the mica polymorphs can be recognized from X-ray
together in a single structural group instead of separat- powder diagrams, but no such evidence is yet available
ing them into " l O A " and " 1 4 A " groups or into non- for the chlorite polymorphs which have hitherto been
recognized only from single crystal diagrams.
ivvelling and swelling groups (as is usually done), is
that in view of the very clo.se inter-relations of these LATTICE PARAMETERS AND CHEMICAL COMPOSITIONS
minerals, the simplest |)rocedure is to regard them as OF LAYER SILICATES
* Tbp^e terms siynifv the number of cations in octahedral coordina- A consideration of the relation between the lattice
tion per half unit cell. They correspond to the terms heptaphyl-
lite and octaphyllite. parameters and the chemical compositions of layer sili-

3. Part I] GEOLOGY AND MINERALOGY
cates is of interest in connection with (a) the identifica- more loosely coordinated, such as the K* ions in musco-
tion of mineral species by means of X-rays, and (b) vite which are in positions of twelve-fold coordination,
certain questions related to the morphology of clay min- are iinlikely to have more than a small effect on 6o-
erals. The question of identification is considered in a A second parameter which may be considered is the
second contribution to this symposium; here considera- layer thickness. If c,, is the third unit cell dimension and
tion is given only to the morphological aspects. if there are n layers per unit cell, then the layer thick-
The variation of the lattice parameters with chemical ness is given by
composition has been discussed by several authors in C()/n for orthogonal axes,
X-ray Identification and Crystal Structures of Clay C (sin P)/)i for monoclinic axes,
o
Minerals for particular mineral groups, namely the
montmorillonoids by IMacBwan (Ch. IV), the micas by and
Brown (Ch. V), and the chloritcs bj- Brindley and Co(l — cos-a — cos-(31 ''''/n for triclinic axes with
Kobinson (Ch. VI). The development of a more general a ^ 90", P ^ 900, and y = 90".
treatment has been attempted by Brindley and MacBwan The layer thickness depends on composition in a more
(1953) in a paper in the process of publication, which complex way than the parameter b„. The thickness of the
is briefly outlined here.* silicate network itself depends on the sizes of the tetra-
Two parameters call for consideration. In the first hedrally and octahedrally coordinated ions. The separa-
place, in so far as the individual layers have hexagonal tion of the networks from each other depends partly on
symmetrj^, a single parameter sufficies to express the the size of the interlayer cations, but also on the magni-
tude of the cohesive forces between these cations and
the negativelj' charged silicate layers. It is to be antici-
pated, therefore, that the layer thickness will be less
easilj' interpreted in terms of composition than the pa-
rameter &o-
The ho parameter was first discussed by Pauling
(1930) who compared the dimensions of the hexagonal
Si-O networks in ^-tridymite and ;8-cristobalite with
those in gibbsite, A1(0H)3, and brucite, Mg(0H)2. On
the basis of this comparison he considered that a hy-
drated alumino-silicate layer of the kind found in the
kaolin minerals was dimensionally feasible, and that a
corresponding magnesian silicate was unlikely to exist
because of the misfit of the component layers, (see data
in table 2). In the light of present knowledge, it seems
better to start with the Si-O distance, 1.62A, found in
many silicates. This leads to a value bo = 9.16A for a
hexagonal layer of regular Si-O tetrahedra, and when
this is compared with the data for A1(0H)3 and
Mg(0H)2 (see table 2, section 3) there is no difSculty
in visualizing the formation of both aluminum and mag-
nesium layer silicates.
A difficulty in deriving any relations between bo
FIGURE 1. Projections of Si-O layer (left), and X ( O H ) 2 layer values and chemical composition is to decide just how to
(right) on the al) plane of layer lattice silicates. Small black circles effect a compromise between the dimensions of the com-
represent Si, small open circles X, and large open circles, O or
( O H ) . Each Si is tetrahedrally coordinated to O ; the fourth O of ponent tetrahedral and octahedral sheets. Section 3 of
each tetrahedral group (not shown in the diagram) projects on to table 2 suggests that the arithmetic mean gives values
the Si atom at its center. Each X atom is octahedrally coordinated close to the observed values. An important point for con-
to 3 (OH) above the plane of projection, shown by heavy circles, sideration is the precise value for the Si-O distance or
and 3 ( O H ) below the plane, shown by light circles. The right-hand
diagram represents a trioctahedral layer; one-third of the X atoms for bo for the Si-O tetrahedral net work, since Si-O =
are missing in dioctahedral layers. 1.62 or ?Jn = 9-16 was never intended to be taken as a
precise value.
dimensions in the plane of the layer; 5o, (see fig. 1), Taylor and his co-workers (see, for example, Cole,
is a convenient value since it can be obtained in most Sorum and Kennard, 1949) have considered the effect
cases from the easily observed (060) reflection. For all of random substitution of Al for Si in feldspars and
layer lattice silicates the ratio of the orthogonal param- have concluded that the mean (Si,Al)-0 distance in-
eters, bo/an, is equal to or very nearly equal to the creases bj' about 0 . 0 2 A for each Si replaced by Al in a
value V'^ which corresponds to a perfect hexagonal ar- group of four. This would give a corresponding increase
rangement. in bo of 0 . 1 2 A . Brindley and MacEwan (1953) have
The variation of h„ with composition will depend therefore taken the 6o parameter of a tetrahedral net-
mainly on the effective sizes of the cations in tetrahedral work of composition (Si4_xAlxOio) to be
and octahedral positions in the oxygen and hydroxyl bo = T + 0.12x
networks (see fig. 1). The interlayer cations, which are
* This has now appeared in Ceramics—A Syinpositivi, published by
where T is likely to be about 9.1-9.2A, the actual value
the British Ceramic Society, 1953. being found from experimental data.

4. 36 CLAYS AND CLAY TECHNOLOGY [Bull. 169
Values of 60 for octahedral layers have been obtained A detailed comparison of bo values calculated from
from the hydroxide structures and are given in table 2, these formulae and obtained experimentally is given by
section 1. Brindley and MacEwan (1953), and on the whole a
The calculation of ho for combined tetrahedral-octa- close agreement is obtained. Some of the discrepancies
hedral layers involves taking the appropriate mean may arise from experimental errors, but others may
values. Empirically the arithmetic mean has been used have real significance; for example, discrepancies found
and the best value of T found by comparison with ex- with the micas may arise from the effects of the inter-
perimental data. For dioctahedral structures, T is found layer cations. Generally the best agreement is found with
to be about 9.16A which agrees exactly with Si-0 = minerals having no interlayer cations, such as the kaolin-
1.62A. For trioctahedral structures, T = 9 . 0 0 A . type minerals and the chlorites, and with minerals like
the montmorillonoids where the interlayer cations are
Table 2. Lattice parameters in some layer minerals. relatively few in number and are generally hydrated.
1. Octahedral layers.
T H E MORPHOLOGY OF CLAY MINERALS
Cation radius 6o parameter, A. Lamellar Forms. Lamellar crystals of hexagonal out-
line would be expected from layer structures consisting
A 1 ( 0 H ) J gibbsite 8.621 of hexagonal networks of atoms, but when the morphol-
bayerite Al =0.57 8.68/ "
ogy of clay minerals is considered in more detail, many
Fe(0H)3 Fe'+=0.67 9.001" qiiestions arise which still require elucidation.
Mg(0H)2 Mg = 0 . 7 8 9.36 In the case of kaolinite, it is probably generally true
that well-formed hexagonal flakes give clear X-ray dia-
Fe(0H)2 re'+==0.83 9.72
grams indicating a well-ordered succession of layers. The
type of kaolin mineral found in many fire clays seems
ti> Mean value.
<2* Interpolated value. usually to be very poorly crystallized and the X-ray
powder diagram indicates considerable disorder in the
2. Si-O Tetrahedral layers. stacking of the layers, with many displacements of
layers by nho/S, n being integral (Brindley and Rob-
6o parameter, A. inson, 1947). Both in the clay domain and on a mega-
scopic scale, disordered sequences of this type can be
From hexagonal networks in g-tridymite and 3-cristobalite obtained with material showing a well-developed morphol-
(after Pauling, 1930) 8.71
ogy. One cannot therefore necessarily associate quality
On basis of Si-0—1.62 .. ._ . 9.16 of morphological development with degree of crystalline
regularity. Kaolinite crystals from different localities
show considerable variations in their (thickness/size)
3. Comparison with Kaolinite and Chrysotile. ratio and in the elongation of the hexagonal forms. No
explanation of these variationshas yet been offered.
bo, in A 60, in A The poor development of montmorillonite crystals,
few of which show more than an occasional 120° angle
Si-0 layer 9.16 Si-O layer 9.16 between edges, may be associated with the irregularly
Al-OH layer 8.65 Mg-OH layer 9.36 superposed and easily separated layers of this mineral.
On the other hand, vermiculite which is similar in many
Mean value 8.90s Mean value 9.26
respects to montmorillonite, exists as mega-crystalline
Observed value f Observed values for material.
kaolinite 8,93 chrysotile
Tubular and Belated Forms. The non-orientation of
With these values for T, and on the assumption that halloj'site by simple sedimentation pointed to a different
a mean bo parameter for the combined tetrahedral and morphology from that of kaolinite and this has been
octahedral networks is to be taken, the following formulae strikingly confirmed and amplified by electron micro-
are obtained: graphs, especially by Bates and his co-workers (1950)
who have indicated the existence of tubular crystals as
Dioctahedral minerals well as split and partially unrolled tubes. The X-ray
bo = 8.90 + 0.06a; -f OWq + 0.18r -f 0.27s diagrams are consistent with a randomly displaced
Trioctahedral minerals sequence of layers (Brindley and Robinson, 1948). The
bo = 9.18 + 0.06a; - - 0.12p — OMq -f 0.06s occurrence of tubular forms has been attributed by Bates
where et al. to curvature of the layers resulting from the strain
X = no. of Al for Si substitutions when the for- imposed by binding the Si-0 and Al-OH laj^ers together.
mula is expressed in the form (Si4^xAlxOio) A minimum strain will exist in a layer which is curved
p = no. of Al atoms so that the Si-0 and Al-OII sheets have, as nearly as
q = no. of Fe^* atoms possible, their own characteristic dimensions, namely bo
in octahedral positions = 9.16 for the Si-0 sheet and bo = 8.75 for the Al-OH
r = no. of Mg atoms
s = no. of Fe^+ atoms layer. This corresponds to a diameter of about 150A.
(This is similar to, but not identical with, Bates' calcu-
p--q--r--s = 2 for dioctahedral minerals lation). Following Bates, we may suppose that in hy-
3 for trioctahedral minerals. drated hallovsite a silicate sheet of this diameter forms

5. Part I] GEOLOGY AND MINERALOGY 37
mmmnmuL WATER LAYER
SILICATE LAYER
FIGURE 2. Diagram to illustrate (a) Bates' suggestion for the structure of a tube of hydrated halloysite, (6) Brindley's sugges-
tion for the structure of a curved fragment of imperfectly dehydrated halloysite; water inclusions lie in the imperfectly fitting,
curved silicate sheets. The mean layer thickness is about 7.5A as compared with 7.15 when flat layers are packed together. The
diagrams are drawn correctly to scale as regards the water and silicate layer thicknesses and the inner tube diameter is equivalent
to about 200A. Silicate layer is represented by white spaces between hachured areas.
tlie innermost layer. Alternate layers of water and sili- are curved alternately in opposite directions so that a
cate may then be added externally with increasing radius sine-curve distortion is impressed on the structure along
and therefore increasing strain, until the strain imposes the a-axis. This requires a reversal of the structure
a limitation preventing further growth (see fig. 2a). after every half wave.
Yfhen placed in the electron microscope, the system
will largely dehydrate, probably to a metahalloysite Other Morphological Forms. The existence of lath-
with a residual water content given by the composition like and fibrous forms has been previously attributed to
Al2(Si20.5) (OH)4 - H2O, and a lattice spacing in the a lattice strain or curvature limiting crystal growth in
range 7.2-7.5A. The suggestion is made that on dehy- one direction and permitting growth in a perpendicular
dration, the silicate layers (with the possible exception direction. While this is feasible in the case of a kaolin-
of the innermost layer) collapse by removal of the inter- type structure, it is difficult to see how it can apply to
layer water, producing broken fragments having radii mica-like structures, such as nontronite and hectorite,
of about 150A, but retaining a certain amount of residual which develop as fibrous forms. Beautiful ribbon-like
water in the process. This is depicted in figure 2b. This crystals have been observed in electron micrographs of
view replaces that described by Brindley and Robinson allevardite, a mica-type mineral studied by Caillere,
(1946b) based oh flat layers which was suggested prior Mathieu-Sicaud, and Henin (1950), but no explanation
to the discovery of the tubular habit. has yet been given for their occurrence.
A similar explanation may be given of the tubular SOME STRUCTURAL TOPICS
habit of chrysotile, discovered by Bates, Sand, and
Mink (1950), but in this case the Si-0 sheet will be on Relations Between the Kaolin Minerals. The prin-
the concave side of each curved layer whereas in halloy- cipal kaolin minerals, kaolinite, dickite, nacrite, and
site the reverse is true. No evidence has yet been brought halloysite, are now well understood and require little
forward for the existence of a hydrated chrysotile analo- discussion. Their structural characteristics are sum-
gous to hydrated halloysite. marized in table 3.
Brief reference may also be made to a description by The best crystalline kaolinite gives a clear powder
Onsager (1952) of a possible explanation of the peculi- diagram in which some 60 lines have been indexed with
arities exhibited by the single crystal photographs of the triclinic parameters given in table 3 and their in-
antigorite, which was found by Aruja (1945) to have an tensities interpreted in terms of a structure with one
ao parameter eight times greater than that of chrysotile. layer per unit cell (Brindley and Robinson. 1946a). This
The suggested explanation is that the structural layers is in contradiction to an earlier monoclinie structure

6. 38 CLAYS AND CLAY TECHNOLOGY [Bull. 169
'I'ahle ?i. StiKctiiinl data for the prinviixil kaolin minerals.
Co „
Mineral Layers per unit cell, symmetry (in I) (in A) References
(in A)
Hallovsito (dehydrated) 1 la.yer^ _ _ _ S.14 8.90 d(OOl), 7.2-7.5 Brindley and Robinson (1948)
1 layer, triclinic . - . 5.14 8.93 7.37 a91.8°, gl04..5° Brindley and Robinson (1946a)
5.14 8.94 14.42 &96° .50' Gruner (1932b)
6 layers, rhombohedral -- - ^ 8.94 5.14 43.0 ggO" 20' Hendricks (1938)
containing two layers per unit cell (Gruner, 1932a). The question as to whether a name should be given to
Although the structure is essentially well-ordered, it the disordered tj'pe of kaolin mineral should be viewed
may (and probably always does) contain some random in relation to the situation existing in other layer-min-
layer displacements, as Hendricks (1942) has stated, eral groups. The occurrence of ordered and disordered
Avhich are more numerous than those occurring in dickite. (or, more ordered and less ordered) forms is very com-
There is a marked difference in the powder diagrams of mon and therefore it seems undesirable to attach a par-
well-crystallized kaolinite and of poorly crystallized ma- ticular name to a form w4iich appears to be characterized
terial such as occurs in manj^ fireclays. The essential mainly by its disorder. The writer advocates '' disordered
crystallographic feature of the poorly crystallized ma- kaolin mineral," or "6-axis disordered kaolin mineral"
terial is the virtual disappearance of all reflections with when a more specific description is required.
k ^ 3w. This indicates the existence of a great number The writer, in collaboration with R. H. S. Robertson
of random layer displacements of the type nhn/^. No and R. C. Mackenzie, has recently examined an excellent
evidence of triclinic character remains and such lines example of a &-axis disordered kaolin mineral which
as can be observed may be indexed with a single laj^er, under the electron microscope shows very regular hexag-
monoclinic cell (Brindley and Robinson, 1947).* onal crystals about 0.2 micron in size. An account of
Halloysite represents a more extreme form of disorder, this work will be published soon t (see also Meldaii and
from which the only reflections observable are the OOJ Robertson, 1952).
which correspond to the stacking of layers at intervals
of about 7.2 — 7 . 5 A in dehydrated halloysite and lOA in The Kaolin-Type Minerals. The principal addition to
hydrated halloysite, together with (hk) bands which the list of kaolin-type minerals in recent years has been
arise from the regularitv within the lavers. A treatment the mineral chamosite (Brindley 1951b). This is a fine-
of X -ray diffraction from this mineral taking account grained hydrated ferrous silicate frequent^ associated
of its curved or tubular form has not so far been at- t See preceding footnote.
tempted, but a good general interpretation of the diffrac-
tion data has been obtained on the basis of a randomly 3 Al
displaced laver structure (Brindlev and Robinson,
1948). Kaolinite
It is of interest to consider whether a series of min-
erals exist extending from an ideally well-ordered kao-
linite to a fully disordered halloysite, with the fireclay
type fully disordered in one direction (6-axis), as an
intermediate stage. This question calls for a more ex-
tended survey than it has yet received. The present
writer is inclined to the view that all stages probably
exist between well-ordered kaolinite and a kaolin mineral
highly disordered along the 6-axis, but is less certain of
the existence of intermediate stages between the ft-axis
disordered type and halloysite.
At this point reference must be made to a very careful
study by Bramao, Cady, Hendricks, and Swerdlow
(1952) of "kaolinite, halloysite, and a related mineral
in clays and soils"; the latter is fine-grained, poorly
organized material, "which may be the kaolin mineral
of fireclay." In the electron microscope, it exhibits fea-
tures intermediate between those of kaolinite and halloy-
site, and consists of "irregular layered particles having 3 Fe 2 +
curved surfaces." In view of these statements, it is not 3 Mg
impossible that a continuous series may exist from kao- FiGXJKE 3. Composition of octahedral layers in kaolin-type
linite to halloysite and, indeed, Bramao et al. consider minerals. Strictly the diagrnm rotors to trioetnhedral members only
a possible genesis of elaj^ minerals from hydrated halloy- having the seneral formula (MK,Fe.Al)3(Si,Al)203(OH)4. The
site to the largely dehydrated form, to the disordered only dioctahedral member, kaolinite (and its polymorphic forms),
can be included if the octahedral ions, 2A1, are multiplied by 3/2.
variety of kaolin mineral. The trioctahedral members lie below the dashed line yhich corre-
sponds to the composition (:MR,Fe):Al(SiA10o) (OH) i. Any tri-
* See also Robertson, R. H. S., Brindley, G. W., and Mackenzie, octahedral mineral lying al)0ve the dashed line would have more Al
R. C, 1954, Mineralogy of kaolin clays from Pugu, Tanganyika : than Si atoms in tetrahedral positions.
.A.m. Mineral., vol. 39, j)p. 118-13S.

7. P a r t 1] GEOLOGY AND MIXERALOGY 39
with siderite and kaoliiiite; it is common in sedimentary minerals into the t r u e micas, which are normally non-
ironstone deposits. The name, however, was originally swelling types, at one extreme and the montmorillonoids
applied to a material from Chamoson whieh has been a n d vermiculites, which normally swell readily, at the
shown to have a chlorite-type s t r u c t u r e (Oreel, Caillere, opposite extreme, and to place illites and k i n d r e d min-
a n d Henin, 1950). Ilallimond (1951a) has expressed the erals in an intermediate position. I t is becoming increas-
opinion that the orifi'inal material maj' have been reerys- ingly recognized, however, t h a t this classification m a y be
tallized and that " i t would not be surprising- if . . . the less fundamental t h a n it was once considered to be.
original chamosite t u r n e d out to be b a v a l i t e " , which is "Weaver and Bates (1951), who have discussed the ques-
a well authenticated chlorite. Taylor (1951) also has tion generally, underline the fact that the swelling prop-
stressed the necessity for caution regarding the t y p e erties depend largely on the interlayer cations, which
material, some of which may be t h u r i n g i t e , which also m a y vary with the environment; they cite m a n y results
is a chlorite. The use of the term " c h a m o s i t e " for two in support of this view. Thus AYhite (1950) has treated
minerals chemically similar but s t r u c t u r a l l y very dissim- illite with MgClo, precipitated the exchanged potassium,
ilar creates a nomenclature problem which can scarcely and obtained a montmorillonite X - r a v p a t t e r n . Caillere
be solved by taking the view t h a t the material from a n d Ilenin (1949) treated montmorillonite with K O H
f'hamoson has historical p r i o r i t y ; a p a r t from the doubt- a n d obtained an illite-type diaaram. Barshad (1948)
ful character of the type material (cf. Hallimond, showed t h a t biotite leached with MgCU over long pe-
1951b), tlie term chamosite is now too widely used for riods is converted to an expansible vermiculite, and
the silicate mineral in sedimentary ironstones for it to Walker (1949) has studied similar transformations pro-
be easily changed. duced by weathering processes. Thus passage from a
The discovery that chamosite is a kaolin-type mineral swelling to a non-swelling mineral and vice versa seems
has provided a ferrous analogue of kaolinite and chryso- to be r a t h e r easy, a n d the swelling p r o p e r t y to be con-
tile. I t s composition is somewhat variable but a tj'pical nected p r i m a r i l y with chemical composition. Therefore,
example has the formula : in accordance with the principles of the classification set
out in table 1, these minerals must be regarded as es-
(Fe^^.7sMg„,4iAl„.4.sPe+ j)(Si:.4oAlo.fioO.,)(OH)4
sentially different chemical species r a t h e r t h a n different
The mineral is therefore trioctahedral and mainly fer- s t r u c t u r a l arrangements. The range of basal spacings,
rous. The mineral greenalite, the formula of which may extending from lOA to 14A or more, is connected p r i -
be written approximately as marily with h y d r a t i o n which is extremely variable in
m a n y of these minerals. The most logical procedure is
( F e - o . 2 F e - V , 0 (Si.O.,) (011)4, therefore to place all these minerals in the mica-type
comes still nearer to being a purely ferrous iron analogue s t r u c t u r a l group, to differentiate chemical species ac-
of chrysotile (Gruner, 1936). cording to composition, and to regard h y d r a t i o n as a
A t r i a n g u l a r diagram representing the compositions of p r o p e r t y not affecting the broad lines of the classifica-
the octahedral layers of kaolin-type minerals is shown in tion. This perspective does not detract from the intrinsic
figure 3. This represents principally the trioctahedral interest of the swelling properties, which will now be
minerals, the corner compositions being 3Mg, 3Fe++ a n d considered.
3A1 respectively. The only known dioctahedral s t r u c t u r e The relation of swelling to the t y p e of interlayer ca-
is the kaolin layer itself a n d this may be included in the tion has been discussed especially by Barshad (1950)
diagram by multiplying the dioctahedral ions by the fac- who considers that, " T h e ionic radii, the valency, and
tor 'Vo. (The same procedure may be followed with any t h e total charge of the interlayered cations, as well as
other dioctahedral s t r u c t u r e which m a y be discovered). the n a t u r e of the interlayered substance seem to de-
I n this representation, kaolinite, serpentine, and green- termine the extent of the interlayer expansion of the
alite come at or near the corner positions, chamosites mica type of crystal l a t t i c e . " A useful synthesis of data
occupy a small field near the Fe*+ corner, and amesite is obtained by considering the interlayer cations in order
lies roughly between serpentine and kaolinite. The out- of the potential V := ne/r, where ne is the ionic charge
standing feature is the emptiness of the diagram. I t may of an interlayer cation of valency n, and r is the ionic
well be the case, however, t h a t trioctahedral minerals radius. This potential, conveniently measured by n/r,
cannot be expected to lie much n e a r e r the Al corner than determines approximately the tendency of icms to form
the dashed line which corresponds to a t e t r a h e d r a l com- h y d r a t i o n shells with water molecules and therefore,
position of (SiAlO.-,) ; minerals nearer the Al corner vuider hmnid conditions, to expand the lattice structure.
would have more Al t h a n Si in the tetrahedral layer. I t Table 4 summarizes d a t a b y B a r s h a d (1950) and W a l k e r
is difficult, however, to m i d e r s t a n d why ferric iron sub- (1951) for the spacings of, and water layers in, air-dry
stitutiou in dioctahedral minerals docs not occur, or vermiculite and montmorillonite. It is clear t h a t as 7i/r
occurs to only a small extent. The writer is u n a w a r e of decreases, the h y d r a t i o n tendency diminishes a n d the
the existence of a kaolin mineral with extensive substitu- nmnber of water layers diminishes from two to one to
tion of Al by other trivalent ions. zero.
The tendency of a lattice to swell owing to h y d r a t i o n
Relations Between the Mica-Type Minerals. In the
s t r u c t u r a l group of the mica-type minerals, the non- of the ions is opposed by the electrostatic force b i n d i n g
swelling and swelling minerals wdth roughly ] 0 A a n d the negatively charged silicate layers to the positively
14A basal spacings have been collected together, a n d charged interlayer ions. This force depends on the magni-
some justification for this procedure is required. It has t u d e of the layer charge, ne, (Avhich also determines the
been customary (cf. Brown, 1951) to separate these nund)er of interlayer ions) ? and also on the seat of the

8. 40 CLAYS AND CLAY TECHNOLOGY [Bull. 169
Table J,. Lattice spaeings of, and numbers of water layers in m ontmorillonite and
vermiculite with different interlayer cations.
Ions H+ Mg++ Ca++ Li+ Ba++ Na+ K+ NH,+ Rb+ Cs+
0.30 0.65 0.99 0.60 1.35 0.95 1.33 1.48 1.48 1.69
n/r 3.3 3.08 2.02 1.67 1.48 1.05 0.75 0.68 0.68 0.59
(i) Data for vermiculites
Lattice /Barshad. _ _ __ 14.33 14.33 15.07 12.56 12.56 12.56 10.42 11.24 11.24 11.97
spaeings Walker . 14.36 15.0 12.2 12.3 14.8 10.6 10.8
2 2 2 1 1 1, 2 0 0 0 0
(ii) Data for montmorillonites
L a t t i c e spaeings B a r s h a d 14.5 14.8 15.1 13.4 12.9 11.9 12.0 1 >A 12.3 12.9
2 2 2 1 1 1 1 1 1 1
Sources of data: Barshad (1950); Walker (1951).
charge, e.g., whether it arises predominantly in the oc- particularly useful for studying fine-grained products
tahedral or tetrahedral part of the lattice. and for intimate mixtures which cannot readily be sep-
I t is therefore useful to consider the magnitude of the arated. The second and possibly the more important use
layer charge as determined by cation-exchange measure- of X-rays lies in the possibility it provides of studying
ments. Some typical values are set out in table 5, from the actual transformation process itself. So far such
which it is evident that there is no great difference be- studies have been confined to observing the orientation
tween illites, vermiculite, and montmorillonoids as re- of a new phase in relation to the old phase. It may then
gards the charge of the layers. be possible to arrive at a plausible picture of the mech-
anism of the transformation.
The question now arises as to whether the non-swelling
of such micas as muscovite and biotite is to be attributed Thermal transformations of layer silicates are of two
to the greater layer charge and therefore greater elec- or possibly three kinds. Dehydration and recrystalliza-
trostatic attraction or to the kind of interlayer ions. tion processes are well known, but a third may be added,
Barshad (1950) has shown that the sodium mica, para- namely oxidation processes.
gonite, will expand in water when finely ground (to less Dehydration Processes. Dehydration processes may
than 0.5 micron), but muscovite when similarly treated be subdivided into those concerned with release of water
does not expand. Crystal size therefore appears to be an without change of silicate structure and those which
additional factor. involve a structural change. The former processes in-
clude release of adsorbed water from external surfaces,
Table Layer charge on mica-type minerals. from internal surfaces (cf. montmorillonoids and ver-
miculites) and from channels in a structure (cf. atta-
Mineral L a y e r charge, per u n i t cell pulgite, zeolites). The latter processes involve the release
of so-called "structural water," generally hydroxyl
Margarite _ , _„ _ 4 groups, with concomitant modification or recrystalliza-
2
tion of the structure.
Illites__. _._ 1 . 0 - 1 . 5 ( M a c k e n z i e , W a l k e r a n d H a r t , 1949)
A partial release of "structural water" may result in
a partial change of structure, but the final release of
0 . 9 - 1 . 4 ( B a r s h a d , 1948) water is generally followed by recrystallization. As ex-
0 . 7 - 1 . 0 (Foster, 1951) amples of partial release of water, the chlorites may be
0 . 4 - 1 . 1 (Ross a n d H e n d r i c k s , 1943-4) specially mentioned. Brindley and Ali (1950) first
0 snowed in detail that the hydroxide or brucite-type layer
of the structure can be largely dehydrated without modi-
fying the mica-type layer and without any major col-
A comparison of the data for vermiculite and mont- lapse of the usual 14A basal spacing of the structure.
morillonite in table 4 indicates somewhat greater swell- The X-ray reflections are characteristically modified,
ing by the latter mineral. A full interpretation of this especially as regards the basal intensities. The mica-type
cannot be offered but it may well arise from the smaller layer dehydrates at a temperature usually about 100°-
crystal size of the montmorillonites. 150° C. higher than the first process. Chamosite provides
another example of a structure which can be partially
SOME T H E R M A L AND CHEMICAL TRANSFORMATIONS dehydrated. Two distinct stages of dehydration indicate
OF LAYER SILICATES CONSIDERED FROM the presence of hydroxyl ions with different degrees
A STRUCTURAL ASPECT of binding in the lattice, and this is especially clear in
the ease of chlorites.
Structural studies of mineral transformations are con-
cerned with the nature of the new phases formed and Similar deductions can be drawn with regard to ad-
with the processees by which the changes occur. Chem- sorbed water and water of hydration which is shown
ical and optical methods have been employed for many by the doubled, low temperature endothermic peaks
years in the recognition of phases and the X-ray method recorded in differential thermal analyses of some mont-
of identification has provided yet another method. It is morillonoids.

9. Part T] GEOLOGY AND MINERALOGY 41
Becrystallization Processes. The final dehydration of than y-alumina is the essential recrystallization product.
a structure often leads directly to recrystallization, with They worked with kaolinite crystals which were ob-
a structural reorganization. Investigations tend to show served with an electron microscope and obtained evidence
that when blocks or units of structure of an old phase of the development of mullite needles having particular
can be directly built into a new phase, the transforma- orientations with respect to kaolinite crystals.
tion is rapid. Such transformations tend to develop by
an orderly process. X-ray photographs of single crystals Oxidation Processes—Chamosite. Oxidation processes
of chlorites, for example, have shown the co-existence have not been studied to any large extent from a struc-
of the old and the new phases, and in this way informa- tural standpoint, but Brindley and Youell (1952) have
tion is obtained of the orientation relations of the completed an investigation of the transformation of the
phases (c.f. Brindley and Ali, 1950). Chrysotile has normal ferrous form of chamosite to a ferric form with
been similarly studied by Hey and Bannister (1948) accompanying dehydration. Detailed X-ray and chemical
and by Bradley and Grim (1951). The latter have dis- data indicate two processes of dehydration, the first
cussed generally a number of transformations of this being essentially part of an oxidation process, as follows:
type. Pe ++ -> Fe+++ + e-
e-+ (OH)--»0~ + H
Dehydration and Recrystallization of Kaolin Minerals. 2H + O (atmospheric) -> H 2 0
Space does not permit a detailed discussion of recrystal-
lization phenomena generally in layer silicates, but The oxidation of Pe ++ to Fe+++ liberates an electron, e~,
reference may be made to the dehydration and recrystal- which attaches itself to (OH)" forming an O"" ion and
lization of the kaolin structure. The practical importance setting free H which is oxidized to water by atmospheric
of the changes in kaolinite has stimulated many investi- oxygen. The combined process can be written:
gations (c.f. Bichardson, 1951) but in some respects, the 4Pe ++ + 4 ( O H ) - + Oo (atmospheric)
results have been surprisingly discordant. Dehydration = 4Fe+++ + 4 0 - + 2H 2 0
at about 550° C. leads to a disordered phase generally
said to be amorphous and recrystallization commences Thus the lattice loses only hydrogen, but in a certain
around 900-950° C. with the formation of Y - a ' u m i n a sense the process is partly one of dehydration since
or mullite together with cristobalite. So much is broadly (OH)-^> 0—. The oxidation of Fe ++ to Fe +++ is shown
ascertained, but the details seem to lack precision and quantitatively by chemical analysis, while X-ray powder
different workers have emphatically maintained different diagrams show a shrinkage of the silicate structure
points of view as regards the details. Brindley and which is consistent with the radii of the two ions con-
Hunter (1952) have attempted to obtain further infor- cerned, namely Fe ++ , 0.83A; Fe+++, 0.67A. That atmos-
mation by studying the polymorphic varietv, nacrite, pheric oxygen plays an important part in the process
which has the same layer structure as kaolinite. The is proved by the fact that in vacuo or in an inert atmos-
onlv differences which can arise, therefore, between phere the reaction does not proceed in this way.
kaoHnite and nacrite are those connected with crystal Associated with the oxidation-dehydration process, is
size. a normal dehydration of the outermost hydroxyl sheet *
of the structure, proceeding as follows:
Von Knorring. Brindley, and Hunter (1952), work-
ing on nacrite from a newly discovered source, have 2(OH)- = H 2 0 + 0 "
found that the mineral is well-crystallized and (inter An important structural aspect of the entire oxidation-
alia) that on dehydration, although most of the water dehydration reaction which takes place at about 400°C,
is lost at about 550°C, some water is retained to about is that only the outermost hydroxyls take part in it. Out
750°C. This is consistent with data for kaolinite. The of the initial (OH) 4 of the structural formula, 3 (OH)
retention of some water to quite high temperatures radicals are dehydrated, but the fourth persists in the
seems well established. The single crystal photographs of structure to a higher temperature, about 450-500°C.
heated nacrite crystals (this work is not yet published Here we appear to have a clear indication of difference
in detail) show persistence of some degree of structural in chemical reactivity of the hydroxyl ions within the
order almost to 900°C. The question, therefore," must be silicate layer and those in the external sheet of each
asked whether persistence of order and retention of silicate layer.
water can be correlated. A broad diffraction ring indi- It is perhaps wishful thinking to correlate this be-
cating considerable disorder increases in intensity up to haviour of chamosite with that of kaolinite and to sup-
900°C. At 950°C, the diffraction diagram clarifies and pose that the retention of hydroxyl units by kaolinite
a powder pattern of mullite and some cristobalite is ob- is related to the inner positions of one-quarter of the
tained.* It was hoped that by working with a single hydroxyl radicals. Although this may well be partly
crystal of nacrite, a strongly orientated diagram of mull- related to the behaviour of the kaolin minerals, so far
ite would be obtained. This, however, has not emerged, we lack any clear quantitative explanation because the
although there are some small indications of a prefer- amount of water retained by kaolinite and nacrite to
ential orientation. It still remains to work out the de- high temperatures is a small percentage only, of the
tails of these photographs, but broadly the findings are order of 5 percent, and is not a fraction which might be
in agreement with those - of Comeforo, Fischer, and given a simple structural interpretation. At present,
Bradley (1948) who emphasized that mullite rather therefore, we can go no further than to note that the
* Further work by Brindly and Hunter has shown that v-alumina release of (OH) ions as water may be related to their
occurs as a transient phase, and it appears to be oriented with
respect to the original nacrite. It still remains to elucidate the * I.e., those which lie in the external sheet of each silicate layer;
structural significance of the results. the expression does not refer to the external surface of a crystal.

10. 42 CLAYS AND CLAY TECHNOLOGY [Bull. 169
structural environment and that in one case, that of Stephen and I have obtained a material which we think is a
type of hydrated chlorite and therefore half-way between mont-
chamosite, we have clear evidence for such a conclusion. morillonite and chlorite. I t is a mixture of both swelling and non-
swelling ehloritic material. This material gives 14 A and 28 A
Dissolution of Chlorites in Acids. Another example spacings when it contains no water, but hydrated it gives higher
of chemical reactivity being related to structural en- spacings. We have also obtained a mixed-layer material which
vironment has been obtained by Brindley and Youell contains both montmorillonitic material and the ehloritic material.
(1951) from a study of the acid attack on chlorites. It This gives spacings of about 30 A and 24 A. The spacings do not
necessarily follow in a rational series.
was found that ions such as M^jPe"^* occupying octahe-
dral positions in the structure, were removed at the same G. W. Brindley:
rate when the amount dissolved was expressed as a frac- I think it is becoming increasingly important to extend measure-
ments into the region of long spacings. In some of the early
tion of the total amount of each ion. Al was found to be chlorite measurements the longest spacing recorded was around
removed differently, however, and this was correlated 7 A and the 14 A "reflection" was missed, and in some of the
with the fact that Al occupies both tetrahedral and octa- earliest kaolin measurements the 7 A reflection was similarly over-
hedral positions in the lattice structure. Octahedral Al looked. Several recent investigations tend to show that spacings
was found to be removed at the same rate as other longer than 14 or 18 A do occur in nature. There is the work of
Bradley (1950) on rectorite, which consists of a fairly regular
octahedral ions and, in fact, on this basis the proportions succession of pyrophyllite and vermiculite layers with a periodicity
of octahedral and tetrahedral Al were separately deter- of about 25 A. There has also been a recent publication (Alexanian
mined by a purely chemical method for the first time. and Wey 1951) in which spacings of the order of 32 A are re-
The results were found to be in agreement with the corded from well-orientated montmorillonites. Long spacings of
the order of 22.5 A have also been recorded from a mica-like min-
allocation of Al to the tivo kinds of lattice positions from eral which has been called allevardite by Caill6re and co-workers
structural considerations and provided an experimental (1950). Allevardite is a rather unusual mineral which has been
proof for the customary procedure. What is more im- shown by electron micrograph to have a ribbon-like form. I t is
portant in the present context is that, here again, there clear, therefore, that periodicities longer than the simple ones of
7, 10, and about 14 A do occur, and it becomes increasingly im-
is evidence for chemical reactivity being related to struc- portant that techniques should be developed for working in this
tural environment. long spacing region using crystal-reflected radiation. I t should be
noted that with filtered radiation a peak may arise from the dis-
CONCLUDING REMARKS tribution of intensity in the white radiation that can be very
confusing. We had such an experience in our laboratory. To de-
In conclusion, it must be confessed that this is a very termine the nature and origin of long spacings, one should look
inadequate account of the structural mineralogy of clays first for a regular succession of orders, the existence of which
and related minerals. The subject is now sufficiently well would indicate a regular succession of layers. In all probability
developed for a classification to be attempted on mod- there will not be a single layer having a thickness of 25 or 30
A but rather a regular succession of two kinds of layers such as
erately detailed lines, but it is still growing and is likely A-B-A-B. If successive reflections do not occur a t equal intervals
to outgrow any classification which is too rigid in frame- of sin 0 , then we probably have a disordered or partially dis-
work. It is hoped that the scheme suggested will be suffi- ordered layer sequence. The general line of investigation is then
ciently elastic to accommodate new developments. to try to modify one of the component layers by dehydration or
by adsorption of organic molecules. In addition one may study in
DISCUSSION detail the actual shapes or profiles of the lines.
D. M. C. MacEwan:
In connection with the charge on the structure of micaceous R. C. Mielenz:
minerals in relation to their behavior, some calculations have been Many X-ray patterns of montmorillonite-type clays show not
done by Brown and Norrish (1952). They have recalculated a only a ring at approximately 13 to 15 A but also a ring at twice
number of analyses of micas which were low in potash, and showed that dimension ranging from 26 to 30 A. Would someone care to
that, in many cases, much better agreement can be obtained with discuss the significance of this determination with regard to the
the expected complement of octahedral ions and the expected present concepts of the size of the unit cell of montmorillonite?
amount of OH" ions if one assumes that the excess water is pres-
ent in the form of H3O* ions. In many analyses there is more E. A. Hauser:
water than expected on the basis of a normal structure. If the At temperatures above 150°C there is a complete change in the
water is assumed to be present in the form of H3O+ ions, the surface composition of clay minerals. This has been demonstrated
analyses can be recalculated to show that there are two Interlamel- by Weyl (1949) of Pennsylvania State College with his new
lar cations per structural unit, in most cases, with H3O* partly technique which I have used quite extensively. A specific indi-
replacing K*. This suggests that more micas than had been sup- cator will show that at a certain temperature there is a pro-
posed are fully charged. Nevertheless, there are materials with nounced release of atomic oxygen. This release causes a complete
low charge, such as Barshad has been finding, and his technique change in the reactivity of the system and explains many of the
for determining exchangeable hydrogen directly may be very im- phen(;mcna.
portant.
REFERENCES
A. Pabst: Alexanian, C , and Wey, R., 1951, Sur I'existence d'une raie
Much longer spacings than those that have been mentioned here intense a 32A enviion dans le diagramme de rayons X de pla-
have been reported for chrysotile (Frankuchen and Schneider, quettes de montmorillonite orientee; Acad. Sei., Paris, Oomtes
1944). The long spacings were interrupted as arising from "parallel Rendus, v. 232, pp. 1855-1856.
fundamental fibrils—hexagonally packed in cross section". The Aruja, E., 1945, An X-ray study of the crystal-structure of
indicated diameters of the fibrils in various samples ranged from antigorite: Mineralog. Mag., v. 27, p. 65-74.
195 to 250 A. I t may be pointed out that this is in excellent accord Barshad, I., 1948, Vermiculite and its relation to biotite as
with the later electron microscope work by Bates and associates revealed by base exchange reactions, X-ray analyses, differential
(1950). The dimensions of the fibrils are similar to the dimensions thermal curve and water content; Am. Mineralogist, v. 33, pp.
found for halloysite tubes by Bates and co-workers to which Dr. 655-678. ~
Brindley has referred. Barshad, I., 1950, The effect of the interlayer cations on the
expansion of the mica tvpe of crvstal lattice: Am. Mineralogist,
D. M. C. MacEwan: y. 35, p. 225-238.
I feel sure that you can obtain spacings greater than 30 A from Bates, T. F., Hildebrand, F . A., and Swineford, A., 1950, Mor-
montmorillonite in certain circumstances. Norrish, working in our phology and structure of endellite and halloysite: Am. Mineralo-
laboratory, has obtained spacings between 35 and 100 A. gist, V. 35, pp. 463-484.

11. Part I] GEOLOGY AND MINERALOGY 43
Bates, T. F., Sand, L. B., and Mink, J . F., 1950, Tubular Grim, R. E., and Bradley, W. F., 1951, The mica clay minerals,
crystals of chrysotile asbestos: Science, v. I l l , pp. 512-513. in Brindley, G. W., Editor, X-ray identification and crystal struc-
Bradley, W. F., 1950, The alternating layer sequence of rec- tures of clay minerals: Chap. 5, part I, pp. 138-154, London, Min-
torite: Am. Mineralogist, v. 35, pp. 590-595. eralogical Society (Clay Minerals Group).
Bradley, W. F., and Grim, K. E., 1951, High temperature Gruner, J . W., 1932a, The crystal structure of kaolinite:
thermal effects of clay and related materials: Am. Mineralogist, Zeitschr. Kristallographie, Abt. A, Band 83, pp. 75-88.
T. 36, pp. 182-201. Gruner, J. W., 1932b, The crystal structure of dickite: Zeitschr.
Kristallographie, Abt. A, Band 83, pp. 394-404.
Bragg, W. Iv., 1937, Atomic structure of minerals: 292 pp., Ox-
ford Univ. P r e s s ; Cornell Univ. Press. Gruner, J. W., 1936, The structure and composition of greena-
l i t e : Am. Mineralogist, v. 21, pp. 449-455.
Bramao, L., Oady, J. G., Hendricks, S. B., and Swerdlow, M.,
Hallimond, A. F . , 1951a, Discussion: Clay Minerals Bull., v. 1,
1952, Criteria for the characterization of kaolinite, halloysite, pp. 136-137.
and a related mineral in clays and soils: Soil Science, v. T3, p. Hallimond, A. F., 1951b, Problems of the sedimentary iron ores:
273-28T. Yorkshire Geol. Soc. P r o c , v. 28, pp. 61-66.
Brindley, G. V., 1951a, Editor, X-ray identification and crystal Hendricks, S. B., 1938, The crystal structure of nacrite AlzOa-
structures of clay minerals; 345 pp., Ijondon, Mineralogical Soc. 2SiOz-H20 and the polymorphism of the kaolin minerals: Zeitschr.
(Clay Minerals Group). Kristallographie, Band 100, pp. 509-518.
Brindley, G. W., 1951b, The crystal structure of some chamo- Hendricks, S. B., 1942, Lattice structure of clay minerals and
site minerals: Mineralog. Mag., v. 29, pp. 502-525. some properties of clay: Jour. Geology, v. 50, pp. 276-290.
Brindley, G. W., and Ali, S, Z., 1950, X-ray study of thermal Hendricks, S. B., and Jefferson, M. E., 1939, Polymorphism of
transformations in some magnesian chlorite minerals: Acta Crystal- the micas, with optical measurements: Am. Mineralogist, v. 24,
lographica, v. 3, pp. 25-30. pp. 729-771.
Brindley, G. W., and MacEwan, D. M. C , 1953, Structural as- Hey, M. H., and Bannister, F . A., 1948, A note on the thermal
pects of the mineralogy of clays and related silicates, in Green, decomposition of crysotile: Mineralog. Mag., v. 28, pp. ,333-337.
A. T., and Stewart, G. H., Ceramics, a symposium, pp. 15-59, Jackson, M. L., Hseung, Y., Corey, R. B., Evans, E . J., and
Stoke-on-Trent, The British Ceramic Society. Heuvel, R. C. V., 19.52, Weathering sequence of clay-size minerals
Brindley, G. W., Oughton, B. M., and Robinson, K., 1950, in soils and sediments. I I . Chemical weathering of layer silicates:
Polymorphism of the chlorites. I. Ordered structures : Acta Crystal- Soil Sci. Soc. America P r o c , v. 16, pp. 3-6.
lographica, v. 3, pp. 408-416. Knorring, O. von, Brindley, G. W., and Hunter, K., 1952, Nac-
Brindley, G. W., and Robinson, K., 1946a, The structure of rite from Hirvivaara, northern Karelia, Finland: Mineralog. Mag.,
kaolinite: Mineralog. Mag., v. 27, pp. 242-253. V. 29, pp. 963-972.
Brindley, G. W., and Robinson, K., 1946b, Randomness in the Mackenzie, B . C , Walker, Q. F., and Hart, R., 1949, lUite
structures of kaolinitic clay minerals: Faraday Soc. Trans., v. occurring in decomposed granite at Ballater, Aberdeenshire: Min-
42B, pp. 198-205. eralog. Mag., V. 28, pp. 704-713.
Brindley, G. W., and Robinson, K., 1947, An X-ray study of Meldau, B., and Robertson, R. H. S., 19.52, Morphologische
some kaolinitic fire clays: British Ceramic Soc. Trans., v. 46, pp. Einfliisse auf technische Staubeigenschaften: Ber. deut. Keram.
49-62. Ges., Band 29, pp. 27-35.
Brindley, G. W., and Robinson, K., 1948, X-ray studies of halloy- Onsager, L., 1952, Discussion, in Summarized proceedings of a
site and metahalloysite : Mineralog. Mag., v. 28, pp. 393-406. conference on structures of silicate minerals—London, November
Brindley, G. W., and Youell, R. F., 1951, A chemical determina- 1951: British Jour. Applied Physics, v. 3, pp. 281-282.
tion of "tetrahedral" and "octahedral" aluminium ions in a sili- Orcel, J., Caillere, S., and Henin, S., 1950, Nouvel essai de
cate : Acta Crystallographica, v. 4, pp. 495-496. classification des chlorites: Mineralog. Mag., v. 29, pp. 329-340.
Brindley, G. W., and Youell, R. F., 1953, Ferrous chamosite and Pauling, Linus, 1930. The structure of micas and related min-
ferric chamosite: Mineralog. Mag., v. 30, pp. 57-70. erals : Nat. Acad. Sci. Proc., v. 16, pp. 123-129.
Brown, G., 1951, Nomenclature of the mica clay minerals, in Pauling, Linus, 1930, The structure of the chlorites: Nat. Acad.
Brindley, G. W., Editor, X-ray identification and crystal structures Sci,, P r o c , V. 16, pp. 578-582.
of clay minerals; Chap. 5, part I I , pp. 155-172, London Mineral- Bamsdell, L. S., 1947, Studies of silicon carbide: Am. Min-
ogical Society (Clay Minerals Group). eralogist, V. 32, pp. 64-82.
Brown, G., and Korrish, K., 1952, Hydrous micas: Mineralog. Bamsdell, L. S., and Kohn, .T. A., 1951, Disagreement between
Mag., V. 29, pp. 929-932. crystal symmetry and X-ray diffraction data as shown by a new
Caillerc, S., 1951, Sepiolite, in Brindley, G. W., Editor, X-ray type of silicon carbide, l O H : Acta Crystallographica, v. 4, pp.
identification and crystal structures of clay minerals: Chap. 8, 111-113.
pp. 224-233, London Mineralogical Society (Clay Minerals Group). Ramsdell, L, S., and Kohn, J. A., 1952, Developments in silicon
Caillere, S., and H^nin, S., 1951, Palygorskite—attapulgite, in carbide research: Acta Crystallographica, v. 5, pp. 215-224.
Brindley, G. W., Editor, X-ray identification and crystal structures Richardson, H. M., 1951, Phase changes which occur on heating
of clay minerals: Chap. 9, pp. 234-243, London Mineralogical So- kaolin clays, in Brindley, G. W., Editor, X-ray identification and
ciety (Clay Minerals Group). crystal structures of clay minerals: Chap. 3, pp. 76-85, London,
Caillere, S., Mathieii-Sicaud, A., and Henin, S., 1950, Nouvel Mineralogical Society (Clay Minerals Group).
essai d'identifieation du mineral de Ija Table pres AUevard, I'alle- Boss, G. S., and Hendricks, S. B., 1931, Minerals of the mont-
vardite: Soc. Francais MlnSralogie Crystallographie, Bull., v. 73, morillonite group, their origin and relation to soils and clays: U. S.
pp. 193-201. Geol. Survey Prof. Paper 205-B, pp. 151-180.
Caillere, S., and Henin, S., 1949, Transformation of minerals of Strunz, Hugo, 1941, Mineralogische Tabellen ; eine Klassifizierung
the montmorillonite family into lOA micas: Mineralog. Mag., v. 28, der Mineralien auf kristallchemisehes Grundlage, mit einer Ein-
pp. 606-611. fuhrung in die Kristallchemie: 308 pp., Leipzig, Akademische
Caillere, S., and Henin, S., 1949, Experimental formation of Verlagsgesellschaft.
chlorites from montmorillonite: Blineralog. Mag., v. 28, p. 612-620. Taylor, J . H., 1951, In discussion on Observations of the chlor-
Cole, W. F., Sorum, H., and Kennard, O., 1949, The crystal ites of iron ores: Clay Minerals Bull., v. 1, p. 137.
structures of orthoclase and sanadinized orthoclase: Acta Crystal- Walker, G. P., 1949, The decomposition of biotite in the soil:
lographica, V. 2, pp. 280-287. Mineralog. Mag., v. 28, pp. 693-703.
Comeforo, J. E., Fischer, R. B., and Bradley, W. F., 1948, Mulli- Walker, G. F., 1951, Vermiculites and some related mixed-layer
tization of kaolinite: Am. Ceramic Soc. Jour., v. 31, pp. 254-259. minerals, in Brindley, G. W., Editor, X-ray identification and crys-
Fleischer, Blichael, 1947, Some problems in nomenclature in min- tal structures of clay minerals: Chap. 7, pp. 199-223, London,
eralogy and inorganic chemistry: Am. Soc. Testing Materials P r o c , Mineralogical Society (Clay Minerals Group).
v. 47, pp. 1090-1108.
"Weaver, C. E., and Bates, T. F., 1951, Privately circulated
Foster, M. D., 1951, The importance of exchangeable magnesium report. See Weaver, C. E., 1953, A classification of the 2 : 1 clay
and cation exchange capacity in the study of montmorillonite clays: minerals: Am. Mineralogist, v. 38, pp. 698-706.
Am. Mineralogist, v. 36, pp. 717-730.
Frankuehen, I., and Schneider, M., 1944, Low angle X-ray scat- White, .1. L., 1951. Transformation of illite into montmorillonite:
tering from chrysotiles: Am. Chem. Soc. .Tour., v. 66, p. 500. Soil Sci. Soc, America Proc. 19.50, v. 15, pp. 129-133.
Frondel, 0., and Palache, C , 1950, Three new polymorphs of Wey], W. A., 1949, Surfage structure and surface properties of
zinc sulfide: Am. Mineralogist, v. 35, pp. 29-42. crystals and glasses: Am. Ceramic Soc. Jour., v. 32, p. 367.