Topic 3: Feldspar Group
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups
Hassan Z. Harraz
OUTLINE OF TOPIC 3:
Possible substitutes of feldspar
Feldspar Products Terminology
21 November Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 2
This group of minerals consists of framework tectosilicates.
Feldspar is the most common rock-forming mineral (~ 60% of
the earth’s crust) (Kauffman and Van Dyk, 1994).
The mineral name feldspar is derived from the German words
feld + spar. The word "feld" is "field" in German and "spar" is a
term for light colored minerals that break with a smooth
The feldspars are by far the most abundant group of minerals
and are found in igneous, metamorphic and many sedimentary
rocks and thus can be found throughout different geological
environment. It is more commonly found in igneous and metamorphic
Feldspar minerals are essential components in igneous, metamorphic
and sedimentary rocks, to such an extent that the classification of a
number of rocks is based upon feldspar content.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 3
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 4
• Felsdpars are framework silicates where
each silica tetrahedra share all corners
with its four neighbouring tetrahedra.
• A portion of the tetrahedra contain Al3+
instead if Si4+.
• The charge is balanced by incorporating
Na+, Ca2+ or K+.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 5
Framework silicate (feldspars): all corners shared; no octahedral sites.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 6
Feldspars are tectosilicates
with every oxygen atom
shared by adjacent silicon or
The tetrahedra are arranged
in four-member rings that
are stacked to form
“crankshafts” parallel to
the a-axis of the
monoclinic or triclinic
The crankshafts are joined
together in an open
structure with large voids to
hold the alkali metals K+ or
Na+, or the alkaline earth ion
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 7
Low Sanidine and Orthoclase
are more ordered. For these
minerals to be monoclinic, the
center of symmetry in each ring
must be preserved.
At still lower temperatures, the Al+3
will be completely ordered:
always on the two t1 tetrahedra.
This ordering will destroy the
center of symmetry and the
mineral will become triclinic
High Sanidine is fully disordered
with a statistically random Al-Si
distribution: each tetrahedron has,
on averaging over a reasonable
volume, 0.25 Al atoms and 0.75 Si
atoms. The Al+3 can be anywhere.
Looking down the a-axis
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 8
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 9
Looking down c-axis
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 10
A second group of alumino-silicates,
tetrahedra form three-dimensional frameworks with Ca, Na and K as the balancing
Feldspar is a common name that applies to a group of minerals with a general chemical
where: x can be sodium (Na) and/or calcium (Ca) and/or potassium (K).
Chemically, the feldspars are silicates of aluminium, containing sodium, potassium, iron,
calcium, or barium or combinations of these elements.
Most deposits offer sodium feldspar as well as potassium feldspar and mixed feldspars. The
term feldspar encompasses a whole range of materials.
The mineralogical composition of most feldspars can be expressed in terms of the ternary
system Orthoclase (KAlSi3O8), Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8).
Compositions of major elements in common feldspars can be expressed in terms of three
Potassium-Feldspar (K-spar) endmember : KAlSi3O8
Albite endmember NaAlSi3O8
Anorthite endmember CaAl2Si2O8
Feldspar are subdivided into:
K-Na bearing alkali feldspars and
Ca-Na solid-solution series called the plagioclase feldspars.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 11
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 12
The K-feldspars or alkali felspars:
Microcline: Potassium aluminum silicate
Sanidine: Potassium sodium aluminum silicate
Orthoclase: Potassium aluminum silicate
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 13
1) Sanadine (K,Na)AlSi3O8
Al randomly distributed
often see carlsbad twinning
2) Microcline (K,Na)AlSi3O8
much more ordering of Al in the structure
any heating will randomly redistribute the Al
often see “tartan” twinning
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 14
Perthite (albite exsolution in microcline)
Triclinic K-spar “Microcline”
At low temperatures solid solution
(ss) is unstable, ss exsolves to
Albite + Microcline. We say the two
phases are immiscible
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 15
Limited Solid Solution and
e.g. Alkali Feldspar
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 16
TWO-COMPONENT SYSTEM WITH SOLID SOLUTION,A EUTECTICAND A SOLVUS
PLAGIOCLASE AND ALKALI FELDSPAR
WITH A EUTECTIC
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 17
Two-Component System with a Solvus
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 18
Plagioclase Feldspars Series
Essential minerals in most
igneous, sedimentary, and
dependant) solid solution
Minor solid solution of K+
increasing Ab content
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 19
Plagioclase series: complete solid solution
Because of the similarity in
ionic radius between Na+1
(0.95 Å) and Ca+2 (0.99 Å)
Isomorphous solid solution
between end members
Plagioclase Feldspars Series (Cont.)
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 20
Plagioclase Feldspars Series (Cont.)
In the plagioclase series, there is usually high and low temperature structural
At high temperatures the structure remains the same over the
complete solid solution.
e.g., anorthite has the same structure as high albite. This would
be the case for rapidly quenched volcanic rocks.
At lower temperatures the series is often not continuous but contains
structural divisions (due to position of Al in the crystal structure).
An0-3, Ab97-100 low albite
An3-22, Ab78-97 Peristerites
An22-73, Ab27-78 Intermediate Structure
An73-100 Ab0-27 Anorthite Structure
Peristerites: sub-microscopic intergrowth of Na rich and Ca rich phases with
alternate low albite structure and intermediate structure. Optically peristerites
produce a “schiller” (iridescent) effect on cleavage surfaces.
If you reheated these low temperature inhomogeneous mixtures you would get
a member of the series with high albite structure.
All the Plagioclase is triclinic but in High Albite, the position of the Al is random,
while in Low Albite the Al position is more ordered and fixed.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 21
Feldspar minerals are usually:
i) They usually crystallise in the monoclinic or triclinic system. The crystalline
structure of feldspars consists of an infinite network of SiO2 and AlO4
ii) white or very light in color and
iii) have a hardness of 6 on the Mohs’ Scale of Hardness and
iv) perfect to good cleavage (plane of breakage) in two directions.
v) Another physical property of the feldspar group is that the mineral has good
cleavage in two directions. The cleavage breaks are about 86o for plagioclase
feldspars (albite, oligoclase, andesine, labradorite and anorthite), at 90o for
orthoclase and 89.5o for microcline.
vi) Feldspar weathers to kaolin which is the main clay mineral used in ceramics
and fine pottery.
Feldspars are primarily used in industrial applications for their alumina and alkali
Most of the products we use on a daily basis are made with feldspar: glass for
drinking, glass for protection, fibreglass for insulation, the floor tiles and shower
basins in our bathrooms, and the tableware from which we eat. Feldspar is part
of our daily life.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 22
Optical and Physical Properties
Form: Laths or euhedral xtls
RI, Bire, 2V and Optic
Figure: All vary
Cleavage: 3 directions (two
Extinction: Varies with
Twinning: Albite twinning
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 23
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 24
Plagioclase Composition from Albite Twins
Albite twins in
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 25
Form: Phenocrysts or euhedral to
Cleavage: 3 directions (2 good)
Interference figure: Biaxial
2VAngle: Varies with composition
Extinction: Varies with composition
and axial plane
Twinning: Carlsbad, Albite and
Pericline can be present
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 26
The feldspar group approximates a ternary
system consisting of three components: Albite,
Anorthite and Orthoclase.
There are continuous and limited solid solution
between the three groups.
Plagioclase -solid solution (continuous) between
Anorthite (CaAl2Si2O8) and Albite (NaAlSi3O8).
Alkali feldspars -solid solution between Albite
and Orthoclase (KAlSi3O8) is limited (dependent
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 27
Plagioclase Feldspar Solid Solution Series
• There is a complete solid solution
between Albite and Anorthite.
• Intermediate phases are
Oligoclase, Andesine, Labradorite
• The amount of water in the melt
PH2O will change the crystallization
range of plagioclase e.g., PH2O of
5000 bars will drop the
crystallization temperature of
Anorthite from 1553°C to 1234°C.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 28
Alkali Feldspar Solid Solution Series
1) Volcanics -rapidly quenched, at very high temperatures get
High Albite→Sanadine. It would appear to be uniform
(homogeneous) in thin section.
e.g: Or25 →Or60 on Fig 46. However,
submicroscopically, every crystal has two
components: Laths of plagioclase in a sanadine
host. X-ray diffraction would show two peaks.
2) Plutonic -called PERTHITES if they can be seen
(cryptoperthite appears homogeneous). They consist of Na rich
plagioclase intergrowths in K rich feldspar host (Fig 34 handout).
low albite -Microcline
most common Perthite is orthoclase or microcline host
with albite lamellae. Called Antiperthite If you have
Alkali feldspar in a plagioclase host.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 29
The feldspar chemically weather to kaolinite.
Sericite (a mica) is also an alteration product
The feldspars often have a “dirty’ or “dusty”
appearance when altered.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 30
First examine the Alkali Feldspar Series (Or-Ab)
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 31
Alkali Feldspar: Miscibility gap
Wrong size “bricks”
results in alkali feldspar
Single alkali feldspars
formed at high temp
exsolve (unmix) at
lower temp (if slowly
• Alkali Feldspar: Miscibility gap
– Because of dissimilarity in size between the Na+1 (0.95 Å) and
K+1 (1.33 Å) complete solid solution occurs only at high temp
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 32
This diagram (Fig 6-4a) is for 1 atms and a dry melt (no H2O).
a) There is a minimum temperature E (eutectic) that all melts move towards.
Example: Melt of composition O cools and hits the liquidus at point P. Start to crystallize crystals of
composition C. As cooling continues the melt becomes more Ab rich. The final product is
homogeneous crystals of phase D. Only a melt of eutectic composition would ever crystallize E.
b) What happens when the temp drops below the solidus? There is no longer complete miscibility in the
alkali feldspar series.
c) At lower cooling temperatures we get immiscing in the middle range of feldspar compositions into 2
separate solid phases. This occurs below the area called the solvus and where immiscing occurs,
defines the solvus line.
d) Any quick cooling would freeze the reactions and immiscing would not occur.
e) The exsolution of K rich phase results lamellae. The slower the cooling, the more time for diffusion
and exsolution and the larger the lamellae.
Figure 6-4b and Figure 20. When a melt contains H2O the fields of the phase diagrams change drastically.
i) The leucite field disappears with increasing H2O. Only occurs in volcanics (quenched).
ii) Crystallization temperatures are reduced.
iii) Composition changes continuously along liquidus-solidus during cooling.
iv) On Figure 6-4b a liquid L cools to N. At point N crystals of M form. The liquid cools and changes
to E while the crystals change to composition D. This is the limit of the solid solution.
At E, the eutectic you get crystallization of two solids D and F until all the melt is used up. Cooling
continues. The composition of 2 stable, co-existing phases would occur along the solvus from D→G→J
and F→H→K. Na and K end members. Get lamellae. Perthite intergrowths should occur, but are
dependent on the kinetics of diffusion of K and Na ions in the solid media. Kinetics are slow with lower
temperatures. Slow cooling in nature usually does not produce perthitic intergrowths. Usually two
separate feldspars will occur.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 33
Remember the Plagioclase
solid solution series (An-Ab),
• If water is added, the whole
curve is depressed by
300°C. If pressure fluctuates,
you get zoned crystals
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 34
• Took Ca plagioclase, added crushed SiO2, H2O and soda (NaHCO3) and heated to 300°C
for three days.
• The result was that much of the plagioclase had converted to albite.
• Therefore, in many low temperature and metamorphic rocks, albite is a stable end
member. [Implications to hydrothermal, seawater regional metamorphism].
• Ca plagioclase is the high temperature end member.
Now lets look at the system NaAlSiO4-KAlSiO4-SiO2
There is no solid solution between these end members. They are simple binary
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 35
This phase diagram is similar to the Forsterite-
Enstatite-Silica system we looked at before
(previous handout Figure 13-7). Depending on
the original composition we can end up with K-
feldspar and Quartz or K-feldspar and Leucite.
BUT, we can not end up with Leucite and Quartz
because they are not stable together (the same
way olivine and quartz can not occur together).
Note, however, that there is the complication of incongruent melting in
the Leucite-K-feldspar portion of the right diagram. Point R is a
Peritectic. Leucite is not stable in the presence of melt below 1150°C
and will undergo the reaction:
KAlSi2O6 + SiO2 → KAlSi3O8
(leucite) (quartz) (k-feldspar)
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 36
Now lets look at the 3-D version of this system :
Examine Figure 8-13 and note some of its
1) Structure is temperature
dependent (i.e., from orthoclase to
2) As discussed before, there is
limited solid solution at low
temperatures but almost complete
solid solution at high
Under low pressure, dry conditions,
can get quite a large portion of
leucite (Figure 8-14).
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 37
Let’s look at an example of a quartz rich melt. Initially tridymite (quartz)
crystallizes. The composition moves along the line to a minimum point where
feldspar +tridymite precipitate at T1.
After this temperature, the phases are governed by what we say in Figure 6-4 or
Figure 20 (ie are pressure PH2O dependent). As temperature drops and moves
along the hoop shaped solvus, lamellae form and change composition. This is
dependent on ion mobility and temperature. In plutonic conditions (slow cooling)
formation of perthite is favoured.
If the PH2O is raised, the
solidus and liquidus are
depressed and the leucite
field would shrink and
disappear at 5000 atm
Also, the addition of iron
into the system would
cause pyroxene or iron
oxides to crystallize (Figure
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 38
a) Silica saturated compositions end
crystallization at the minimum
along the SiO2-Feldspar join
(Peralkaline, oversaturated rocks).
b) Silica undersaturated
compositions end crystallization at
the Ternary Eutectic (syenites and
trachyte) (Figure 8-12).
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 39
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 40
Common in Alkaline (Si-undersaturated) igneous rocks
Leucite – KAlSiO4
Nepheline – (Na,K)AlSiO4
Sodalite – Na8(AlSiO4)6Cl2
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 42
Feldspar raw materials are rocks with the prevalent portion of minerals of the feldspar group or their
mixtures in such a form, quantity and quality, which allow their industrial processing.
Feldspars are a group of monoclinic (orthoclase, sanidine) and triclinic (microcline, plagioclases) potassium
and sodium-calcium alumosilicates, and together with quartz they represent the most common rock
For industrial use are suitable are potassium feldspars (orthoclase, microcline) and acid plagioclases
(albite, oligoclase, andesite).
Suitable feldspar resources are dyke rocks (pegmatites, aplites), igneous rocks (granites) and sediments
(feldspar bearing sands and gravel), eventually also residues of incompletely kaolinized rocks. The major
impurities are high content of iron in the feldspar structure (unremoveable) or in the form of admixture
Most often, commercial feldspar are mined from pegmatite or feldspatic (sand) Placer deposits.
Aplite, which is a fine-grained igneous rock with the same mineralogical composition as granite, also
is mined frequently for its feldspar content.
Alaskite is a granitic rock with few or no dark minerals, and a texture ranging from granitic to
pegmatic. The average mineral composition of the alaskites is oligoclase feldspar (40%), quartz
(25%), microcline feldspar (20%), and muscovite mica (15%). Alaskite also contains minor accessory
minerals such as biotite, garnet, epidote, apatite and pyrite.
Another source of feldspar is phonolite, the effusive equivalent of nepheline syenite, which contains
feldspathoids and biotite? Phonolite is produced commercially in West Germany.
In the same country, rhyolite is also exploited for feldspar.
Amongst the numerous rocks in which they are present, feldspars are particularly abundant in
igneous rocks like granite, which contains from 50 to 70% of alkaline feldspar. Granite is
however rarely used for its feldspatic content.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 43
Feldspar Resources (Cont.)
b) Feldspar (sand) placer
d) Nepheline Syenite
Feldspar from Pegmatite:
The initial production of feldspar was from pegmatite bodies. Pegmatites are bodies of rock commonly of granitic
composition that consist mainly of unusually large crystals or masses of quartz, feldspar, and mica. Some pegmatites also
contain potentially economic minerals such as fluorspar, beryl, and spodumene. The constituent minerals may occur
segregated in zones, an advantage in commercial extraction. Pegmatites crystallize during the last stages of injection of
granitic magma. The magmatic fluids are rich in water and cool so slowly that the crystals grow larger than usual. The
grain size can range from less than an inch to more than a foot but rarely greater than three feet. Pegmatites range in
size from small lens, one to a few feet in thickness, to large tabular bodies tens of feet thick and hundreds of feet long.
Past production came from the larger pegmatites. Mining was labour intensive. Ore was hand cobbed (hand-size pieces
are broken by hammer) and sorted by hand. Conventional open-mining methods including removal of overburden,
drilling and blasting, loading, and transport by trucks are used to mine ores containing feldspar. The ore is crushed by
primary and secondary crushers and ground by jaw crushers, cone crushers, and rod mills until it is reduced to less than
841 µm (20 mesh). A froth flotation process is used for most feldspar ore beneficiation.
Feldspar from Placer deposits:
Beach sand and alluvial deposits may contain economic quantities of feldspar, and are mine in Spain. Under certain
conditions feldspar decomposes to koalinitic clay; if the alteration is only partial, the deposits may contain feldspar,
kaolin and quartz.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 44
Possible substitutes of feldspar
Feldspar substitutes are materials having alkali metals
confined to other minerals than feldspars, like
nepheline syenites or nepheline phonolites (in the
These replace feldspars as a melting agent.
In other applications (fine abrasives, filler in rubber,
plastics and paints), feldspars can be replaced by
bauxite, corundum, diatomite, garnet, magnetite,
nepheline syenite, olivine, perlite, pumice, silica sand,
staurolite, ilmenite, barite, kaolin, mica, wollastonite,
calcined alumina hydrate, clays, talc, spodumene,
pyrophyllite or their mixtures.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 45
Feldspar Products Terminology: The supplier and user of feldspar worldwide normally have agreed on a standardized definition of the
feldspar products which are placed in the world market.
According to that harmonised definition, feldspar products are aluminosilicates with:
SiO2 content > 58%,
Alkali content (Na2O+K2O) > 2%,
Alumina content (Al2O3) > 3% and
loss on ignition < 3%.
Basically, the two properties which make feldspars useful for downstream industries are their alkali and
On those elements we can distinguishThree
1) Feldspatic sand: with an alkali content
(Na2O+K2O) < 6% or alumina content (Al2O3)
2) Pegmatite: with an alkali content
(Na2O+K2O) >6% and alumina content
(Al2O3) comprised between 8 and 14%
3) Feldspar: with an alkali content (Na2O+K2O)
>6% and alumina (Al2O3) content > 14%.
A further distinction can be made between
Sodium, Potassium and mixed feldspars,
depending on the type of alkali they contain.
For the same purposes there are also used feldspar
substitutes, which are rocks with alkali metals
confined to some other minerals (mostly nepheline -
anhydrous sodium-potassium alumo-silicate).
Nepheline syenites are particularly used abroad to
substitute for feldspar raw materials.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 46
i) Potash Feldspars:
The presence of potash feldspar in a glaze or clay body has a more refractory effect on the ceramic surface
compared to equivalent amounts of soda feldspar.
Although potash feldspar actually begins its melt at a lower temperature than soda feldspar, once the melt
begins, the formation of leucite crystals causes a slower and more viscous flow.
ii) Soda Feldspars:
Kona F-4 and NC-4 feldspars contain a fair amount of potassium oxide, and their total sodium content is not as
high as the total content of potassium in potash feldspars.
These feldspars are hybrids that incorporate some qualities of both potash and soda feldspars.
This is especially evident when they’re compared to stronger sodium materials, such as nepheline syenite.
Hence, it’s often possible to substitute some soda feldspars for potash feldspars without causing a dramatic
Nepheline syenite is a low-silica, high-soda, high-alumina mineral referred to as a feldspathic rock.
The fluxing power and shrinkage rate of nepheline syenite depends on the grade number.
The finest grades (A400 and 700) have the greatest melting power and shrinkage rate and are used in
electrical porcelain and by manufacturers of ceramic wares.
Grade A270 has a medium melting and shrinkage rate and is the most commonly used form of
nepheline syenite in ceramic studios and schools.
Grades A40-A200 (used by glass manufacturers) are the coarsest grades and produce the lowest melting
and shrinkage rates.
Cornwall stone (feldspathic rock):
Cornwall stone, also a feldspathic rock, contains more silica and less melter oxides than do the feldspars. Since
silica has a high melting point, Cornwall stone has a higher melting temperature than the feldspars and appears
stiffer and less melted when fired alone to stoneware temperatures.
This is especially apparent at the lower stoneware temperatures. Even the potash feldspars show more fusion
at the cone 5-6 oxidation firing temperatures than does Cornwall stone, so this would not be a first choice as a
glaze core at these firing temperatures unless a stiffer surface is desired.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 47
Feldspar Beneficiations and Processing Technologies
Feldspars are either selectively mined or processed by flotation and/or
magnetic separation, in order to remove the accessory minerals (e.g.
quartz, mica, rutile, etc.) present in the ore.
The feldspar may then undergo a milling step which allows to adapt the
particle-size to the intended use.
The degree of refining and possible milling is very dependent upon the final
use of the product.
For a number of uses, it is perfectly acceptable, and even advantageous,
that the product retains some accessory minerals, e.g. quartz, while at the
other extreme some applications require extremely pure and fine-grounded
Basically, the two properties which make feldspars useful for downstream
industries are their alkali and alumina content.
Normally feldspar concentrates should meet the following requirements:
To maintain a proper ratio of (K2O+Na2O) to SiO2
To be low in iron content
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 48
• Feldspars are either selectively mined or processed by optical, flotation and/or electrostatic separation, in order to remove
the accessory minerals (e.g. quartz, mica, rutile, etc.) present in the ore. The feldspar then undergoes a comminution step.
The degree of refining and possible comminution is very dependent upon the final use of the product. For a number of uses,
it is perfectly acceptable, and even advantageous, that the product retains some accessory minerals, e.g. quartz, while at
the other extreme some applications require extremely pure and fine grounded grades. Basically, the two properties which
make feldspars useful for downstream industries are their alkali and alumina content.
• The flotation process is only used by AKW, INCUSA, and SP Minerals. The feldspar recovered by flotation only represents
about 10 % of the European feldspar production. The flotation process is essential to get a high quality grade (low iron
content and high alumina content) required for some specific and important applications (e.g. TV/computer screens). For
instance, although the Italian producer Maffei is the biggest producer in Europe, the three abovementioned companies
supply the Italian market with these high quality grade products.
• The essential use of the flotation process may be explained by the following figure:
In Sections I and III a primary mechanical separation
(hydrocycloning, centrifugation) can be achieved.
Figure Feldspar particle vs.
recovery graph [39, IMA, 2002]
In Section II, either optical, flotation or electrostatic
separation can be used to separate feldspar from quartz,
depending on both the intrinsic characteristics of the raw
material, and the final product requirements.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 49
In the feldspar process, one may distinguish three
different flotation steps, namely the micas flotation, the
oxides flotation, and the feldspar flotation.
Each of these requires a different reagent regime.
The following flow sheet shows the steps
involved in the recovery of feldspar.
21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 50
Generally, feldspar is used in the manufacture of glass products (70%), in ceramics and other products
Feldspar is also used Filler (in paint, in mild abrasives, urethane, latex foam, and as a welding rod
In the manufacture of ceramics, feldspar is the second most important ingredient after clay.
Feldspars are an important glaze raw material used as the main flux in ceramic industries.
Feldspar is a source for the simultaneous introduction of SiO2, Al2O3, Na2O, K2O and CaO and is the most suitable
material for introducing alkaline oxides into glazes.
Feldspars are used as fluxing agents to form a glassy phase at low temperatures and as a source of alkalis and alumina in
Feldspars play an important role as fluxing agents in ceramics and glass applications, and also are used as functional
fillers in the paint, plastic, rubber and adhesive industries.
Because of their low melting point, feldspars are used as a melting agent in ceramic mixtures, glass batches,
glazes, enamels and also as casting powders in the last years.
Feldspar does not have a strict melting point, since it melts gradually over a range of temperatures. This greatly
facilitates the melting of quartz and clays and, through appropriate mixing, allows modulations of this important step of
Feldspars melt at ~1150oC.
The feldspathic glass they produce surrounds the refractory clay particles and fills up the pores between them.
Due to the free fluxes they contain, feldspathic glasses will also bind to the surfaces of the refractory particles thus
helping to bind the ceramic body together.
The more feldspathic glass a ceramic body contains, the denser the fired body will be.
They improve the strength, toughness, and durability of the ceramic body, and cement the crystalline phase of other
ingredients, softening, melting and wetting other batch constituents.
21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 51
• Glazes are thin glassy coatings usually 0.15 to 0.5
mm thick formed in place on a ceramic body, after
blending the raw materials, spreading the mixture
on the surface and firing at a high temperature.
Important and uses:
• Glazes are usually applied to make the bodies non-
porous, smooth, glossy, mechanically stronger and
chemically more resistance.
• They improve the aesthetic appearance of ceramic
ware glazes are required to fit different ceramic
bodies, to mature at different temperatures and to
exhibit various specific properties, which explains
the great variety of grazes.
21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 52
Feldspar (Naturally occurring forms of devitrified glass)
Potassium Feldspar (Orthoclase or
K2O Al2O3 6SiO2
Sodium Feldspar (Albite) Na2O Al2O3 6SiO2
Lime Feldspar (Anorthite) CaO Al2O3 2SiO2
Each of the three components of feldspar is discussed below
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 53
The Na2O, K2O and CaO in the above formulas are called alkaline
metal oxides because they are strong bases when added to water.
These oxides are used as fluxes.
Fluxes have very active molecular structures at high temperature,
and they attach to and combine with the surface molecular
structure of otherwise hard crystalline materials, causing the
surface molecules in the crystals to "dissolve".
This exposes deeper layers of the crystal to the dissolving action of
other flux molecules and so on until the entire crystal melts away.
In other words, fluxes cause crystalline structures to melt at lower
temperatures than would otherwise be possible, a bit like water
melts a cube of sugar at room temperature.
Without fluxes present, none of the other constituents in the
ceramic body would be able to melt at normally attainable
temperatures, and the fabrication of pottery would have been
beyond the reach of prehistoric peoples.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 54
Aluminum oxide (Al2O3) (Stabilizer).
Aluminum oxide exists in two separate forms within clay and porcelain
When chemically combined in molecular form with the other constituents
of feldspars, aluminum oxide acts as a stabilizer, and is a part of the glass
Aluminum atoms can bond with silicon via a shared oxygen atom and can
thus be an integral part of the amorphous silicon matrix. In this form, it
does NOT affect the transparency of the glass.
However, aluminum oxide is also added to clays as a separate constituent
in the form of kaolinite. Because of the large amount of flux contained in
the feldspar, some of the kaolinite also melts into a glass, like the feldspar
itself. But the by-product left over when the kaolinite melts is a precipitate
of pure crystalline aluminum oxide called alumina.
The alumina crystals remain unmelted (i.e., they are refractory particles)
and scattered throughout the glass melt, and in this form, aluminum oxide
causes the glass to become cloudy or opaque.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 55
iii) Glass Former
Silica silicon dioxide (SiO2) (Glass Former)
Silica is silicon dioxide, The SiO2 portion of the feldspar
formulas shown above.
Like alumina, silica also exists in two entirely separate
forms within clay and porcelain bodies.
When chemically combined with flux and aluminum
oxide, as it is in feldspar, silica exists as a molecular
component in the amorphous melted glass gel.
Silica also exists as unmelted crystalline particles of
quartz scattered throughout the glass melt. This form
is part of the refractory substructure which supports
clay and porcelain bodies.
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 56
Bourne, H.L., 1994, Glass raw materials: in Carr, D.D. and others, eds., Industrial
minerals and rocks (6th edition): Littleton, CO, Soc. for Mining, Metallurgy
and Exploration, Inc., p. 543-550.
Kauffman, R.A. and Van Dyk, D., 1994, Feldspars: in Carr, D.D. and others, eds.,
Industrial minerals and rocks (6th edition): Littleton, CO., Soc. for Mining,
Metallurgy, and Exploration, Inc., p. 473-481.
Lesure, F.G., 1968, Mica deposits of the Blue Ridge in North Carolina: U.S. Geol.
Sur. Prof. Paper 577, 129 p.
Potter, M.J., 1991, Feldspar, and Nepheline Syenite, and Aplite: Annual Report, US
Dept. of Interior, US Bureau of Mines.
Potter, M.J., 1996, Feldspar and Nepheline Syenite: Minerals Yearbook, US Dept. of
Interior, US Geological Survey; (//minerals.er.usgs.gov/minerals).
Potter, M.J., 1997, Feldspar: Mineral Commodity Summaries, US Dept. of Interior,
US Geological Survey; (//minerals.er.usgs.gov/minerals).
USM 1993, Short Course on : Industrial Processing of Kaolin, quartz/silica sand and
Feldspar, 20th-22nd. April 1993, School of Materials and Mineral Resources
Engineering in Collaboration with The Department of Mining and
Metallurgical Engineering, University of Queensland, Australia.