Petrographic textures and quartz types of silicified evaporite nodules.

1. JOURNALOFSEDIMENTARYPETROLOGY,VOL. 49, NO. 1, p. 0245--0256
FIGS. 1-14, MARCH, 1979
Copyright © 1979,The Society of Economic Paleontologistsand Mineralogists
0022-.4472/ 79/0049-0245/ $03.00
THE SILICIFIED EVAPORITE SYNDROME---TWO ASPECTS OF
SILICIFICATION HISTORY OF FORMER EVAPORITE NODULES
FROM SOUTHERN KENTUCKY AND NORTHERN TENNESSEE t
KITTY LOU MILLIKEN
Dept. of Geol.Sciences
University of Texas
Austin, Texas 78712
ABSTRACt: Silicified evaporite nodules from Mississippian rocks of south-central Kentucky and adjohaing
Tennessee typify in many respects silicified evaporite nodules which have been reported in rocks
of widely differing ages and diagenetic histories. The silieified evaporite syndrome is an assemblage
of characteristics shared by evaporite-replacement quartz nodules and includes both mineralogical
and textural aspects. The sequence of textures in most nodules begins with interlocking quartzine
spherules near the outer nodule edge and ends with a mosaic of equant megaquartz in the nodule
center. Proportions of the different textures vary considerably between nodules. Anhydrite inclusions
are abundant in the centers of megaquartz crystals. Megaquartz in the nodules is characterized by
strongly undulose, radial extinction. Euhedral terminations often have a cubic appearance. Many nodules
contain crusts of zebraic chalcedony on the euhedral megaquartz bordering internal cavities.
Oxygen isotope analyses are useful m understanding the silicilication history. Microcrystalline qaartz
and fibrous quartz types have ~OIs values which are heavier than those of megaquartz. The
micrccrystaUine quartz nodules, associated with the silicified evaporites, formed earliest in fluids related
to sea water. Length-slow fibrous quartz types formed later in water of intermediate composition.
Megaquartz developed at somewhat higher temperatures and in fluids derived from meteoric water.
Zebraic chalcedony in the nodules is also of meteoric origin. All silicification of these samples has
probably occurred at temperatures of less than 40° C.
INTRODUCTION
Folk and Pittman (1971) recognized the
importance of quartzine and lutecite as
evaporite mineral replacements, and nu-
merous studies have confirmed this idea
(Siedlecka, 1972, 1976; Chowns and Elkins,
1974; Tucker, 1976a, 1976b). Examples of
silicified evaporite nodules are known from
rocks ranging in age from Precambrian to
Cretaceous, in rocks of widely differing lith-
ologic character, and in rocks with greatly
different diagenetic histories.
The two-fold aim of this paper is to de-
scribe: l) the silicified evaporite syndrome--
an assemblage of features, in addition to
quartzine and lutecite, which characterize
silicified evaporite nodules (formerly anhy-
dfite and/or gypsum), and 2) results of some
oxygen isotope analyses which have clarified
the silicifieation history of the nodules in
the study area.
LManuscript received March 15, 1978; revised Sep-
tember 27, 1978.
In this study, nodules from four Missis-
sippian formations were examined (Fig. 1).
The formations sampled include the Fort
Payne Formation, Warsaw Limestone, St.
Louis Limestone, and the Girkin Formation.
Most of the nodules examined are fist-sized,
but range in size from less than 2 cm to
25 cm in diameter. Silicffied evaporite nod-
ules from Kentucky and Tennessee have
been reported previously by Chowns and
Elkins (1974) and Fisher (1977).
The large number of textures and quartz
types which can be recognized in thin section
follow a consistent sequence from the nodule
edge to the nodule center (Milliken, 1977).
Comparison with nodules from other locali-
ties reveals that these quartz textures and
their sequence are shared by many silicified
evapodtes.
PETROGRAPHY
Quartz Types
In addition to quartzine and a small amount
of lutecite, the nodules contain several other

2. 246 KITTY LOU MILL1KEN
• - . ~ ~-? 26 O 26 km
r~ /
~%~ ~ Z5 mm quod
....... i.... :~L.~.__L_ .[~ .~ 7: KY
Flo. l.--Map of the study area.
quartz types which are common to quartz-
replaced evaporite nodules. Figure 2 summa-
rizes the properties of major quartz types.
Megaquartz.--Undulose extinction and
crystal development are two important as-
pects of the megaquartz in the nodules.
Undulose Megaquartz
The megaquartz in the nodules is distinc-
tive because of its undulosity. Extinction is
fan-like m many elongate crystals or crudely
radial in more equant crystals. Some crystals
have extinction which radiates from a central
zone of anhydrite inclusions. Patches of
elongate vacuoles arranged in paraUel stria-
tions are common in undulose megaquartz.
Extinction near the striations is aligned into
elongate composite areas paraUel with the
striations. The overaU effect of the striations
is to give some areas of the crystal an almost
fibrous appearance. Zones of this extinction
type are often developed in strips perpen-
dicular to the prism faces of the quartz (Fig.
3). Parallel striations and the associated ex-
tinction are equivalent to the flamboyant
spectral extinction described by Chowns and
Elkins (1974) and Rubin and Friedman (1977)
in megaquartz associated with evaporites.
Wilson (1966) observed strongly undulose
extinctionin megaquartz replacing Limestone.
He observed that undulosity of megaquartz
increased as it graded into fibrous quartz.
Radially undulose megaquartz and quartzine
spherules can be viewed as end points in
a continuum of quartz types with differing
amounts of lattice distortion and vacuole
inclusions.
Euhedral Megaquartz
Quartz with euhedral terminations occurs
adjacent to internal cavities in the nodules.
The crystal forms developed on the crystals
are always simple. The crystals consist of
either a prism terminated by two sets of
rhomb faces (r and z)* that form a quasi-
hexagonal pyramid, or a prism terminated
by the three faces of a single rhomh. The
near fight angle between the faces of the
single rhomb gives those crystals a cubic
appearance (Fig. 4).
Quartz pseudomorphs after cubic minerals
are known (Staples, 1950). However, grada-
tion of "cubic" quartz into quartzine, and
the correct angle of nearly 86° between the
MAJOR OUARTZ TYPt~$
Microcryltollino Quarlz (20,1~ equont
Magoquorlz )20~ equant
Microflemboyant Quartz ( @redational )
Quartzine fibrous fiber~I t~©-oxi=
Lutecite fibrout I /
Chalcedony fibrou| I --
Zebraic Chalcedony fibrous I t~;z -©
Fro. 2.--Majorquartztypcs. Compiled from Sosman,
1927; Frondel, 1962; Folk and Pittman, 1971; and this
paper.
Fro. 3.--Strips of nearly fibrousextinctionin mesa-
quartz. Zoned holes in center of crystal were produced
by removal of anhydrite.
* [10|1] and [01il i respectively.

3. SILICIFIED EVAPORITES
FIG.4.-"Cubic" quartz crystals, coated with zc-
braic chalcedony, projecting into central cavity of nod-
ule. Scale is in millmeters. FIG.5.-Microflamboyant quartz.
rhomb faces indicate that the quartz is not
a pseudomorph.
The occurrence of simplequartz formsand
"cubic" quartz in evaporite deposits has
been noted before. Tarr(1929)reported doub-
lyterminated quartz crystalsingypsum. Most
of these crystals were terminated by a single
set of rhomb faces (r), though some of the
larger crystals had faces of a second, smaller
rhomb (z). Tan and Lonsdale (1929) found
crystals in the same deposit which were
laclung prism faces and consisted only of
singlerhombohedrons. Euhedral quartz with
anhydrite inclusions was found by Brownell
(1942), Stewart (1951), and West (1964) in
gypsum and anhydrite beds. Silicified
evaporites in the Triassic Dockum Group of
northern~exasare characterized by "cubic"
terminations on much of the quartz bordering
internalcavities in the nodules (S. Seni, pers.
cornm., 1977). "Cubic" zoned quartz in the
Devonian Caballos Novaculite may be an-
other example of evaporite-related "cubic"
quartz (McBride and Folk, 1977).
Microflamboyant Quartz.-If megaquartz
is composite and undulose to the point that
it is no longer clear where one crystal ends
and another begins, the term microflam-
boyant quartz is applied. Microflamboyant
quartz is considered a variety intermediate
between quartzine and megaquartz (Fig. 5).
Randomly Fibrous Microcrystalline
Quartz.-A minor but interesting variety of
microcrystalline quartz occurs near the outer
edgeof a fewnodules. This variety is unusual
because of its low birefringence and because
of the presence of randomly distributed laths
with undulose, inclined extinction (Fig. 6).
Dimensions of the laths average 20 pm by
5 pm. The inclined extinction of these small
fibers suggests that they may be related to
lutecite.
Zebraic Chalcedony and Other Fibrous
Quartz Types.-Ordinary chalcedony
(length-fast) was not observed in these nod-
ules, although it does occur in the silicified
carbonate rocks associated with the nodules.
Zebraic chalcedony, another type of length-
fast fibrous quartz, occurs as crusts on euhe-
dral quartz in the centers of a few nodules.
(Fig. 7). This form of quartz has been known
for many years (Sosman, 1927; Frondel,
1962), but little is known of its significance.
The esthetically pleasing properties of ze-
braic chalcedony result from the twisting of
the fibers about the axis of fiber elongation.
Frondel (1978) has described the crystal-
lographic aspects of twisting in length-fast
quartz fibers and he states states that the
twist periods vary over five orders of magni-
FIG. 6.-Randomly fibrous microcrystallinequartz.

4. 248 KITTY LOU MILLIKEN
Fro. 7.--Crust of zebraic chalcedonyon euhedral
megaquartz.Crossedpolarizers.Scalebar is 0.01nun.
rude. Zebraic chalcedony as used here in-
eludes length-fast fibrous forms of quartz
with relatively short twist periods--on the
order of 0.1 mm--such that the fibers have
alternating black and white bands under cross
polars.
Zebraic chalcedony has been reported in
other evaporite-related deposits (Siedlecka,
1972; McBride and Folk, 1977)It also occurs
in the evaporite-replacement nodules from
the Triassic Doekum Group in northern
Texas (S. Seni, collector). However, the
exact relationship of evaporites to formation
of zebraic chalcedony is not clear. Zebraic
chalcedony does not contain evaporite inclu-
sions or relict textures and it appears to be
a cavity filling. It has also been observed
m agates from volcanic rocks (Jones, 1952),
in geodes from volcanic rocks (Keller, 1977),
and in hydrothermal deposits (Wahlstrom,
1941).
Other crusts on megaquartz are inter-
growths of length-slow and length-fast vari-
eties, and others are in optical continuity
with the megaquartz substrate--thus being
length-fast in one part of the crust, length-
slow in another part, and of intermediate
orientation in other parts.
Sequence of Textures
The basic structure of the nodules, a rind
of fibrous quartz (usually quartzine) which
grades inward to megaquartz, has been rec-
ognized in sedimentary quartz nodules for
many years (Worthen, 1866). The proportions
of fibrous quartz and megaquartz in the
nodules vary considerably. The rim of quart-
zinc may form 10% or less of the volume,
the rest of the nodule being composed of
megaquartz. At the other extreme, fibrous
quartz may comprise almost the entire nod-
ule.
Within the framework of the basic struc-
ture, there is a recurring sequence of textures
which characterize the nodules. (Figs. 8, 9).
The textures in the following fist are given
in order of their occurrence from the nodule
edge to the nodule center. None of the
nodules observed contain this entire se-
quence; one or more of the textures are
usually missing, especially from the begin-
ning or the end of the succession.
1. Isolated spherules of quartzine in ran-
domly fibrous microcrystalline quartz.
2. Interlocking spherules of quartzine,
lutecite, or microflamboyant quartz.
3. Spherules of quartzine, lutecite, or
microflamboyant quartz grading into mega-
quartz on spherule edges; much megaquartz
has "cubic" zonations or terminations.
4. Mosaic of megaquartz spherules and
undulose megaquartz.
5. Simple quartz crystals (normal or
"'cubic") and spherules of quartzine, micro-
flamboyant quartz, or megaquartz isolated
in secondary cavity filling.
6. Crust of fibrous quartz (zebraic or with
one of several possible fiber orientations)
on euhedral quartz terminations.
7. Breccia of evaporite-replacement
quartz types in secondary cavity filling.
Most nodule rinds begin with textures 2
or 3. If the internal cavity is large, texture
4 may be missing and 5, 6, or 7 may follow
3. None of the nodules have both texture
6 and a filling of secondary carbonate miner-
als. Texture 7 may follow texture 3 or texture
7 alone may be present.
Refict anhydfite textures are restricted to
quartzine and microflamboyant quartz.
These two quartz types appear brownish in
plane polarized fight and have abrupt bound-
aries with megaquartz which is clear and
contains anhydrite inclusions. Anhydrite
inclusions in spherules are generally concen-
trated in a zone of the megaquartz near the
gradational boundary with microflamboyant
quartz. In equant megaquartz anhydrite is
concentrated in the center of the crystal,
usually forming a zone with boundaries par-

5. S1LICIF1ED EVA PORITES 249
Flo. 8.--(A) Texture I. Quartzine spherules isolated in grouadmass of low birefringent randomly fibrous
microcrystalline quartz. Scale bar is 0.05 mm. (B) Texture 2. Interlocking quartzine spherules. Scale bar is
0.01 ram. (C) Texture 3. Quartzine spherules grading outward into megaquartz. Scale bar is 0.01 mm. (D)
Texture 3 seen in plane polarized light. Quartzine spherules are brownish and contain no anhydrite inclusions.
Megaquartz is clear and contains abundant inclusions. Scale bar is 0.01 ram.
allel to the crystal faces of the quartz. Past
the zone of anhydrite concentration much
of the megaquartz has a virtually inclusion-
free rim.
Chowns and Elkins (1974) suggested that
anhydrite-free zones in the megaquartz they
studied formed after all of the anhydrite was
removed from the nodules. However, nod-
ules from the Permian San Andres Formation
of west Texas are only slightly silicified and
yet still contain quartz crystals with inclu-
sion-free rims. Some other factor, possibly
the rate of silica precipitation or concomitant
anhydrite solution, is responsible for the
distribution of anhydrite inclusions in mega-
quartz.
Spherules and crystals of quartz isolated
in anhydfite have also been observed in
incompletely silicified nodules. Apparently,
the quartz euhedra and spherules isolated
in secondary calcite or dolomite were left
suspended when silicification did not go to
completion and the anhydrite was replaced
instead by a carbonate minerai. Many of the
nodules are klappersteins--or rattling rocks.
In these, secondary carbonate minerals did
not form and crystals of quartz and quartzine
spherules, formerly suspended in the anhy-
drite, are left as loose particles in the nodule
cavity.
Breccias made of broken spherules and
fragments of quartz mosaics formed after
nodules coUapsed after silicification. Broken
quartz euhedra are evidence that precipi-
tation of calcite aided in fracturing and
movement of the fragments. Even in nodules
containing breccias, the nodule rinds may
remain intact. This suggests either that the
calcite was solely responsible for the break-
age or that only a portion of the nodule
collapsed, leaving most of the rind unbroken.
The SilicifiedEvaporite Syndrome
Most of the minerals and textures present
in these nodules are also found in other

6. 250 KITTY LO U MILLIKEN
FiG. 9.--(A) Texture 4. Mosaic of equant megaquartz. Note large holes leftin the centers of some crystals
by removal of anhydrite inclusions. Scale bar is 0.5 ram, (B) Megaquartz spherules typical of texture 4. Scale
bar is 0.5 ram. (C) Texture 5. Euhedral megaquartz isolated in secondary calcite.Scale bar is 0.5 ram. (D)
Texture 6. Beautiful zebraic chalcedony crust on euhedral quartz bordering internal cavity of nodule. Scale
bar is0.5 ram,
randomly f ibfous
microctysla#line quartz
quartzine spherules
arlhydrite inclusions ~_~
calcite []

7. SILICIFIED EVA PORITES 251
silicified evaporites of different ages and
diagenetic histories. Similar sequences of
quartz types have been observed in nodules
from the Permian San Andres Formation of
west Texas (Mejia, 1977), the Triassic Dock-
um Group of north Texas, the Cretaceous
rocks of central Texas, and the Ordovician
Knox Dolomite of Tennessee. The sequence
also characterizes many of the numerous
examples of silicified evaporites reported in
the literature (Chowns and Elkins, 1974;
Tucker, 1976a).
The assemblage of quartz types and tex-
tures shared by all these nodules can be called
the silicified evaporite syndrome. The
important features of the syndrome are sum-
marized in the following list and in Figure
10,
1. Length-slow fibrous quartz--in spher-
ules with relict evaporite textures, commonly
as the outer nodule rind and gradational to
megaquartz on spherule edges.
2. Strongly undulose megaquartz--anhy-
drite inclusions concentrated in the central
part of the quartz crystal or in a zone around
a central spherule of quartzine or microflam-
boyant quartz. The crystals often have
crudely radial or fan-like undulosity.
3. Simple quartz crystals--many "cubic"
terminations, many doubly terminated crys-
tals.
4. Fibrous quartz crusts on eulaedral
megaquartz--either zebraic chalcedony, fi-
brous quartz in optical continuity with a
megaquartz crystal, or intergrown fast and
slow fiber orientations.
5. Textures in characteristic sequence.
SILICIFICATIONHISTORY
The timing of replacement of anhydrite
by silica is one of the most difficult aspects
of the nodules to determine. Little is known
about climatic variations and pore water
changes which may have occurred in the
immense span of time since the formation
of these nodules in the Carboniferous.
Silicification has not been reported in
modem evaporite deposits and it cannot be
determined from petrographic evidence how
early in the sediment's history replacement
of anhydrite may have begun. Silicification
has been largely selective--affecting mainly
certain alloehems and the evaporite minerals.
Evidence bearing on the timing of silicifica-
tion relative to other diagenetic events such
as cementation and compaction has not been
observed.
Petrographic observations do provide a
few clues useful in understanding the timing
of silicification. A small cavity near the
surface of one nodule contains a geopetal
fdi of silt, clay, and iron oxides apparently
derived from the soil (Fig. 11). A mosaic
of equant megaquartz l'dls the upper part
of the cavity. This indicates that precipitation
Flo. 11.--Geopetalstructure in cavity near nodule
surface. (A) Cavitysurrounded by megaquartz(a) is
f'dled with a crust of chalcedony(b) followed by a
soil-derivedf'dlingof silt, clay,and ironoxides(c) and
amosaicofequantmegaquartz(d).Planepolarizedlight.
Scale bar is 0.1 ram. (B) Schematicdrawing of thin
sectionseenin A.
FIo. 10.--Thesiticifiedevaporitesyndrome--examplesofsomecommonvariants. 1)Typicalnodule.2)Nodule
with a large proportionof fibrousquartz. 3) Nodulewith a large proportionof megaquartz. 4) Nodule with
tittle megaquartzand a largeinternalcavityIdledwithsecondarycalcite.

8. 252 KITTY LO U MILLIKEN
of the quartz has occurred sometime after
the nodule was removed from the host rock
and may even be going on today. The ob-
served sequence of quartz types in the nod-
ules, together with oxygen isotope measure-
ments on the different quartz types, gives
further information on the timing of silici-
fication.
Twenty samples were analysed for oxygen
isotopes. Different quartz types were
separated by chipping sawed slabs with a
hammer. The samples were crushed to
TAnL£ l.--Oxygen isotope samples--descriptions and 18~Oquartz values
Sample number ~ 018(~.3) Description
ACU 28.4
BN-9 27.1
F-5 28.8
F-ii-A 31.1
F-ii-B 31.2
F- 7- 3A 29.7
F-7-3B 30.2
G-2-1A 29.2
G-2-1B 31.4
GP-6A 28.4
GP-6B 28.7
H-4-3 22.5
H-4-8 30.7
H-4-10A 28.2
H-4-10B 24.0
L-I-iA 23.4
VU 6691A 28.0
VU 6691B 26.3
VU 6691C 26.7
Exploded 29.0
blastold
Microflamboyant quartz from outer edge of pit and ridge
nodule; probably St. Louis Ls., Simpson Co., Ky.
Chert nodule, very fine grained compared to other cherts,
few recognizable allochems, still contains large amounts
of calcite; Girkin Fm., Bowling Green North Quad., Ky.
Spherules of quartzine grading into megaquartz; St.
Louis Ls., Franklin Quad., Ky.
Outer rim of chert nodule; St. Louis Ls., Franklin
Quad., Ky.
Inner zone of chert nodule, same as previous sample.
Crust of evaporite replacement quartzine and microflam-
boyant quartz lining cavity in chert nodule; St. Louis
Ls., Franklin Quad., Ky.
Chert portion of same nodule as previous sample.
Quartzine spherules scattered through megaquartz in
nodule center; Ft. Payne Fm., Gamaliel Quad., Ky.
Quartzine spherules scattered in a matrix of randomly
fibrous microcrystalline quartz; same as previous nodule.
Insoluble residue of silicified pelmatozoan debris;
Girkin Fm., Hadley Quad., Ky.
Quartzine from rim of nodule removed from same sample
as silicified pelmatozoan debris.
Megaquartz from nodule center; Ft. Payne Fm., Holland
Quad., Ky.
Bed of replacement chert from a locality of abundant
silicified evaporites; Ft. Payne Fm., Holland Quad., Ky.
Quartzine from outer edge of nodule; Ft. Payne Fm.,
Holland Quad., Ky.
Cubic quartz with thin coating of 2ebraic chalcedony,
from inner portion of same nodule as previous sample.
Megaquartz from nodule center; Ft. Payne Fm., Lafayette
Quad., Tenn.
Quartzine and microflamboyant quartz from outer edge of
nodule; Ft. Payne Fm., near Woodbury, Tenn., Vanderbilt
University Collection.
Euhedral megaquartz from central part of nodule, same
nodule as previous sample.
Zebraic chalcedony forming botryoidal crust in internal
cavity, same nodule as previous sample.
Quartzine from nodule edge; Girkin Fm., Rockfield
Quad., Ky.

9. SILICIHED EVA PORITES 253
around 1 mm with a steel mortar and pestle
and different quartz types were further
separated by hand picking under a binocular
microscope. Samples were then powdered
in a tungsten carbide ball mill and treated
with concentrated HC1 to remove carbonates
and other trace minerals. After extensive
washing with distilled water the quartz was
reacted with BrF5. The GO'a~,,~ values are
reported relative to SMOW and calculated
according to the usual formula:
GOj8 (O~/O L6sample- O~/O'6 standard )
= - - - - 1000.
qu=rtz 018/016 standard
Results of these analyses, together with sam-
ple descriptions, are listed in Table 1.
In order to interpret these data it is neces-
sary to know the possible range of tempera-
ture and fluid isotopic composition to which
the nodules may have been exposed. Maxi-
mum possible depth of burial is estimated
as 600 m based on the total known thickness
of Mississippian and Pennsylvanian rocks in
the study area and adjoining parts of Ken-
tucky. According to paleogeographic maps,
the area has been emergent since the Late
Pennsylvanian (Dunbar, 1960). It is thus
unlikely that post-Pennsylvanian rocks have
added anything significant to the overburden.
Temperatures in well waters in Warren
County, Kentucky, reported by Lambert
(1976) range from 13° to 21° C and average
15° C. Using a geothermal gradient of 1.20 F
per 100 feet of burial (2.2°C per 100 m)
(Amer. Assoc. of Petroleum Geologists and
U.S. Geol. Survey, 1976), the temperature
is probably no more than about 30° C at 600
m. At the end of the Paleozoie when this
area was nearer the equator (Dott and Batten,
1971) the temperature would have been
somewhat higher, perhaps 40° C at 600 m.
Assuming that the isotopic composition of
the ocean has not changed, the isotopic
composition of the water in the depositional
envkonment would have been equal to that
of sea water (GO's = 0) or slightly higher
if the environment was restricted.
A value of -6.3, determined from a map
,8
by Taylor (1974), can be used for the GOH:o
in modern meteoric water in the study area.
Fluids causing silicification may have varied
between this value and a value slightly greater
than that of sea water.
An equation derived by Knauth and Ep-
stein (1976) for oxygen isotope fractionation
in quartz was used to determine the most
likely combinations of temperatures and
80~20 values in effect at the time of silicifi-
cation. This equation was chosen over sever-
al others in the literature because it is specifi-
cally intended for microcrystalline quartz
formed at low temperatures.
The relationship between oxygen isotope
ratios and quartz types for the 20 samples
measured is shown in Figure 12. Figure 13
shows the spectrum of temperature and
18
808 o values possible for these samples2 .
according to the equation of Knauth and
Epstein (1976).
Over the temperature range permitted
there is no singJe 80 TM value which canH20
METEORIC WATER MIXED WATERS SEA WATER
QUARTZ TYPES
• rnlcrocrystolhne quGrtz
o fibrous quortz
c megaquortz
[] 0 0
.I = , , , I , I
22 25 30 32
18
d" Uquortz • zebroic chalcedony
X silicified pelmotozoon
debris
Fic. 12.--Correlation of quartz types with G O '~ values and a schematic description of silicificationhistory.

10. 254 KITTY LO U MILLIKEN
oC
40-
18 .
/arOquartz- 25.0%,
20- ,x~
I0,
* 0 -I -2 -3 -4 -5 -6.25
sea meteoric
water water
18
c('O HZ0
I
• 15 mean
groundwater
temp.
103In =3.09 (106 T'~-3.29
Knauth and Epstein {1976)
Microcrystalline quartz
Quartzine
Megaquartz
Zebraic chalcedony •
progression of silicification
F[o. 13.--Spectrum of temperature and 80 ~s values possible for the quartz samples according to the equationH20
of Knauth and Epstein. Diagonal lines are 80~.~ z values. Sofid diagonals are for reference. Dashed diagonals
represent the high and low BO~..= values for the quartz types indicated, Patterned areas represented interpreted
conditions of formation of the different quartz types.
f O tsaccount for the entire range o B a~=**~values
observed. Thus, the isotopic composition of
the fluid from which the quartz precipitated
must have changed between the onset of
silieification and the end of the process.
The samples described as chert in Table
1 consist of limestone-replacement micro-
crystalline quartz. These chert nodules occur
in close association with the silicified
evaporite nodules. They occur adjacent to
one another in outcrops and in one case
(F-7-3) the microcrystalline quartz (contain-
ing clearly identifiable relict aUochems)
forms a uniform rind around a silicified
Fio. 14.--Silicified evaporite nodule (a) with a rind
of limestone-replacement microcrystanine quartz (b).
evaporite nodule (Fig. 14).
The microcrystalline quartz is most logi-
cally interpreted to have formed in fluids
very similar to sea water. The study area
was positioned 10° south of the middle Mis-
sissippian equator (Dott and Batten, 1971)
so temperatures at the time of silicffication
may have been in the range of 25 o to 30° C.
Quartzine probably formed in fluids of
fighter isotopic composition than did the
microcrystalline quartz. Meteoric water in
the present day field area cannot account
for most of the quartzine values because this
quartz type is too enriched in tSO. Therefore
the quartzine must have formedin water with
a composition intermediate between seawa-
ter and modem meteoric water and at the
same or sfightly higher temperature than did
the microcrystalline quartz. Meteoric water
associated with coastal marine environments
18 o
typically has 8Oa~o values of -3 to -4 ]oo.
Megaquartz has most likely formed in
meteoric water. The estimated maximum
burial temperature is not quite high enough
to account for all of the megaquartz values.
Again, this could mean that temperatures in
the area were higher at sometime in the past.
Other possibilities arc: 1) the estimated
thickness of Pennsylvanian rocks is too low,
2) the geothermal gradient for the area has

11. SILICIFIED E VAPORITES 255
been higher at some time in the past, or,
3) disequilibrium precipitation has occurred.
Relationships in thin section indicate that
zebraic chalcedony formed after the mega-
quartz. Therefore, it too must have formed
in meteoric water. The slightly heavier 8018
of the zebralc chalcedony suggests that it
formed at a somewhat lower temperature,
perhaps nearer the surface, than did the
megaquartz.
CONCLUSIONS
The silicified evaporite syndrome is an
assemblage of mineralogical and textural
features which characterize silicified
evapofite nodules.
In addition to length-slow fibrous quartz
types, these nodules are characterized by:
abundant anhydrite inclusions concentrated
in the centers of megaquartz crystals and
in zones in the megaquartz of spherules;
extremely undulose, radial extinction in
megaquartz; euhedral terminations which are
either simple with two rhombohedrons, or
"cubic" with a single set of rhombohedral
faces; crusts of fibrous quartz--zebraic or
with a variety of fiber orientations--adjacent
to internal nodule cavities; and a distinctive
sequence of textures--mainly interlocking
spherules of quartzine on outer edges fol-
lowed by mosaics of megaquartz.
Silicification is a process which apparently
has continued over a long period since the
formation of the original evaporite nodules.
Earfiest replacement by quartzine or micro-
flamboyant quartz occurred in water of
composition intermediate between sea water
and meteoric water. Megaquartz and zebraic
chalcedony formed in meteoric water.
Temperatures of silicification have varied
from near-surface temperatures to burial
temperatures no higher than 40° C.
ACKNOWLEDGMENTS
I appreciate the help given me by Dr.
Robert L. Folk, who supervised the thesis
on which this paper is based, and Dr. Lynton
S. Land, who performed the oxygen isotope
analyses (supported by NSF grant EAR76-
17774). Both of these gentlemen also gave
encouragement and helpful suggestions dur-
ing preparation of the manuscript.
The Geology Foundation of the University
of Texas at Austin provided funds for field
work and a grant from the Owen-Coates Fund
for manuscript preparation.
REFERENCES
AMER. ASSOC, PETROLEUMGEOLOGISTSAND U.S. GEOL,
SURVEY, 1976, Geothermal gradient map of North
America, 1:5,000,000.
BROWNELL,G. M., 1942, Quartz concretions in gypsum
and anhydrite: Univ. of Toronto Studies in Geology,
v. 47, p. 7-18.
CHOWNS, T. M., AND ELKXNS,J. E., 1974, The origin
of quartz geodes and cauliflower cherts through the
silicification of anhydrite nodules: Jour. Sod. Petrol-
ogy, v. 44, p. 885-903.
Dorr, R. H., AND BArrEN, R. L., 1971, Evolution of
the Earth: New York, McGraw-Hill Book Co., 649
p.
DUNBAR,C. O., 1960, Historical Geology: New York,
Wiley, 500 p.
FISHER,I. S., 1977, Distribution of Mississippian geodes
and geodal minerals in Kentucky: Econ. Geol., v.
72, p. 864-869.
FOLK, R. L., AND PITrMAN,J. S., 1971, Length-slow
chalcedony: a new testament for vanished evaporites:
Jour. Seed. Petrology, v. 41, p. 1045-1058.
FRONOEL, C., 1962, The System of Mineralogy, Vol.
3, The Silica Minerals: New York, Wiley, 334 p.
- - , 1978, Characters of quartz fibers: Am. Mineral-
ogist, v. 63, p. 17-27.
JONES, F. T., 1952, Iris agate: Am. Mineralogist, v.
37, p. 578-587.
KELLER, P. C., 1977, Geology of the Sierra Del Gallego
area, Chihuahua, Mexico [unpub. Ph.D. thesis] : Aus-
tin, Univ. of Texas, 124 p.
KNAUTH,L. P., AND EPSTEIN, S., 1976, Hydrogen and
oxygen isotope ratios in nodular and bedded cherts:
Geochim. et Cosmochim. Acta, v. 40, p. 1095-1108.
LAMeERr, T. W., 1976, Water in a limestone terrane
in the Bowling Green area, Warren County, Kentucky:
Ky. Geol. Sur. Rept. of Invest. 17, 43 p.
McBluoE, E. F., ^NO FORK, R. L., 1977, The Caballos
Novaculite revisited: Part II: chert and shale members
and synthesis: Jour. Sed. Petrology, v. 47, p. 1261-
1286.
MmtA, D., 1977, Facies and diagenesis of Permian,
Lower San Andres Formation, Yoakum County, West
Texas [unpub. M.A. thesis] : Austin, Univ. of Texas,
137 p.
MtLLmEr~,K. L., 1977, Silicified evapofite nodules from
the Mississippian rocks of southern Kentucky and
northern Tennessee [nnpub. M.A. thesis]: Austin,
Univ. of Texas, 122 p.
RueIN, D. M., ANDFRIEDMAN,G. M., 1977, Intermittently
emergent shelf carbonates: an example from the
Cambro-ordovician of eastern New York State: Sed.
Geol., v. 19, p. 81-106.
SmDLECro~,A., 1972, Length-slow chalcedony and reficts
of sulphates--evidences of evaporitic environments
in the Upper Carboniferous and Permian beds of Bear
Island, Svalbard: Jour. Sed. Petrology, v. 42, p.
812-816.
- - , 1976, Silicified Precambrian evaporite nodules
from northern Norway: a preliminary report: Sed.

12. 256 KITTY LOU MILLIKEN
Geology, v. 16, p. 161-175.
SOSMAt~,R. B., 1927, The Prolmrties of Silica: Amer.
Chem. Soc. Monograph Series, 37, 856 p.
STAPLES,L. W., 1950, Cubic pseudomorphs in petrified
wood: Am. Jour. Sci., v. 248, p. 124-136.
STEWART,F. H., 1951, The petrology of the evaporites
of the Eskdale no. 2 boring, east Yorkshire, Part
[II, the upper evaporite bed: Min©ralogicat Mag., v.
24, p. 557-572.
T^tm, W. A., 1929, Doubly terminated quartz crystals
occurring in gypsum: Am. Mineralogist, v. 14, p.
19-25.
- - , ANDLONSD^LE,L T., 1929, Pseudocubic quartz
crystals from Artcsia: Am. Mineralogist, v. 14, p.
50--53.
T^YtoR, H. P., 1976, The application of oxygen and
hydrogen isotope studies to problems of hydrothermal
air©ration and ore deposition: Econ. Geol. v. 69, p.
843-888.
TUCKER,M. E., 1976a, Quartz-replaced anhydrite nod-
ul©s(Bristol Diamonds) from the Triassic of the Bristol
District: Geol. Mag., v. 113, p. 569-574.
, 1976b, Replaced evaporites from the Late
Precambrian of Finnmark, Arctic, Norway: Sed.
Geol., v. 16, p. 193-204.
WAHLS~OM, E. E., 1941, Hydrothermal deposits in the
Specimen Mountain volcanics, Rocky Mountain Na-
tional Park, Colorado: Am. Mineralogist, v. 26, p.
551-561.
WEST, I., 1964, Evaporite diagenesis in the Lower
Purbeck beds of Dorset: Yorkshire Geol. Soc. Prec.,
v. 34, p. 315-330.
WILSON, R. C. L., 1966, Silica diagenesis in Upper
Jurassic limestones of southern England: Jour. Sed.
Petrology, v. 36, p. 1036-1049.
WOSTBEN, A. H., 1866, Sub-carboniferous limestone
series: Ill. Geol. Sur., v. 1, p. 77-118.