Mineral Facts and Problems: 1960 Edition Page: 202
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MINERAL FACTS AND PROBLEMS, ANNIVERSARY EDITION
perature range, preferably 1,800 to 2,2000 F.
Clays used in manufacturing cement are
selected to supply deficiencies in chemical com-
position, such as silica, iron oxide, and alu-
mina not present in the available limestone
or cement rock.
The most desirable clays for rotary-drilling
muds swell when wetted with water (see Ben-
tonite).
Filtering and decolorizing clays have high
sorption and high ion-exchange activity, such
as characterize the montmorillonite group of
minerals (see Bentonite and Fuller's earth).
For filler clays and paper coating clays,
emphasis is on fineness of grain and whiteness
of dry, unfired clay; for insecticide carriers,
high adsorption, in turn largely dependent on
particle size, is important.
TECHNOLOGY
MINERALOLOGY AND GEOLOGY
Mineralogically the clay minerals have been
classified as hydrous aluminum or magnesium
silicates. The names, chemical composition, and
optical properties of these silicates are given
in table 1. Natural clays consist of many dif-
ferent minerals. Certain minerals or classes of
minerals, however, occur in all clays and are
called clay minerals.
In the ceramic industry the kaolin group is
the most prevalent. Halloysite, kaolinite, dick-
ite, and nacrite are the principal minerals in
this group.
The montmorillonite minerals are less com-
mon than kaolins but comprise an important
group of clay minerals. With the addition ofwater, some minerals of this group swell, re-
sulting in increased volume. The best known
example of this phenomenon is the swelling
bentonites. Montmorillonite, beidellite, and
nontronite form one series where silica, alu-
mina, magnesia, iron oxide, and water are the
principal components. Saponite and hectorite
make up another series containing chiefly mag-
nesia, lithia, silica, and water. Illite (hydro-
mica) represents an important but as yet not
too well understood mineral group.
The magnesium silicate group of palygors-
kite or attapulgite and sepiolite is much less
common than the other groups. The minerals
are essentially hydrated magnesium silicates.
Geologically, clays are of two general types
-residual and transported; but in all in-
stances clay is of secondary origin, having been
formed by the alteration of some other rock.
Clays may be found in a variety of forms--
as a soft paste, soft solid (mudstone), or as
laminated rock (shale). They may occur at or
near the surface or at relatively great depths
(4, 14, 18). The kaolinitic clays of commerce
are derived largely from feldspathic rocks and
the bentonites and fuller's earths from volcanic
ashes. The decomposition is largely a result
of weathering, although hydrothermal reac-
tions are responsible in :many instances (13).
Shales and flint fire clay are clays that have
been indurated by metamorphic processes.
Some china clay and bentonite deposits are
formed by weathering of rocks in place, but
most commercial clay deposits have been trans-
ported, either by water, glacier or wind action.TABLE 1.--Physical properties of the clay minerals (18)
Optical properties
Name Composition
Crystal Specific Hard- Refrac- Birefrin- Interference Crystal
system gravity ness tive gence figure habit Color
index
Kaolin group:
Kaolinite........-....... Al(Si20s) (OH)2-------.... Triclinic.... 2. 61 2.0 1. 56 0. 006 B-neg-...... Plates ..... None.
Halloysite-----...... ..------- Al(Si2Os) (OH)-........... Hexagonal(?). 2.62 .. 1. 52 030 .....do..... Small Do.
needles.
Livesite---..-----.... .. A2(Si2O) (OH)........... Monoclinic.... 2. 63 ..------ 1.54 .020 .....-do-..... Small Yellow(?).
plates.
Dickite....... ....... A12(Si20) (OH)2---- ... -- -....do----.....----. 2.62 2. 5 1. 56 .006 .....do-....... Plates..... None.
Nacrite.... ......-.----- . A12(Si2Os) (OH)_........... Orthorhombic. 2. 54 2. 5 1.56 .006 B-neg(?).... _ __do ..... Do.
Anauxite--.......... AlOs3SiO2H2 ........................ 2. 60 ...---------------------------
Allophane.......-----------... AlOnSiOnHO ......------------ ------ ... .. . 1.49 - -- Isotropic
Montmorillonite group:
Montmorillonite.......... (A1,Mg)sSi4Olo(OH)-...... Monoclinic .. 2. 50 2. 0 1. 51 .021 B-neg-...... Small None.
flakes.
Biedellite...... ------------- Al(Al,Si)40(OH)--............do ....- 2. 50 2. 0 1. 51 .020 .....do ...do...... Cream.
Nontronite----....------ (Al,Fe,Mg)s(A1,Si)4 2. 60 2. 0 1.56 .023 -...do.. Small Brown.
O10(OH)2 laths.
Hectorite...............(Mg,Li,Al)SiOo(OH) .....do.....---- 2. 4 2. 0 1. 49 .022 .....-do....-. Small None.
flakes.
Saponite ----............... Mg(A1,Si)4010o(O H)s-...........do..... 2. 5 2.0 1. 50 . 015 .....do..........do....... Do.
Hydromica (illite) ...-__. Variable ... .... .....do----........ 2. 75 .. 1. 57 021 .....do.......Plates-..... Do.
Magnesium silicate group:
Sepiolite li..c....e . g.ro. . H4Mg2SiOo- nH20........ Orthorhombic 2. O0 2.0 1. 53 . 010 U-neg....... Small Do.
epalygol iite .-------------- Mg fibrous.
Palygorskite (attapulgite). MgsSisO2so(OH)S8H20..........do.......- --------....... -----........ 1.51 .020 .--..-----.do ..-------..
____________________ __________________ ________________I202
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United States. Bureau of Mines. Mineral Facts and Problems: 1960 Edition, report, 1960; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc38790/m1/210/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.