Page 73 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS
CORDIERITE
Grain size of all clay minerals is extremely small and optical determi- Cordie rite Cyclosilicate
nations are generally useless. Serious studies and precise identification
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of clay minerals are carried out either by X-ray diffraction techniques c urdlerite Al,(Mg,Fe ),Si,AIO 1s orthorhombic
pseudo-hexagonal
(XRD) or by using a scanning electron microscope (SEM) or electron
microprobe. 0.568: 1 : 0.549
The occurrence of all clay minerals will be discussed together after
montmorillonite.
c=e<
I
I
I
monoclinic I
I
na = 1.54-1.57
np = 1.57-1.61
n y = 1.57-1.61
a = o.o3
2V = small ( < 10°) -ve
OAP approx. parallel to (010)
D = 2.6-2.9 H = 1-2
Properties similar to kaolin with the exception of birefringence, which is
much stronger with second order colours.

Montmorillonite group (%Ca,Na) 0 AAI,Mg,Fe).(Si,Al)s0 20 (0H) •. nH,O monoclini
(smectites)
1.48-1.61
1.50-1.64
1.50-1.64
0.01-0.04
2V. = small -ve
OAP is parallel to (010)
Properties similar to those of kaolin and illite.
occuRRENCE Kaolin is the most common of the clay minerals and forms by hydro
II , 1.522- 1.558
thermal alteration or weathering of feldspars, feldspathoids and other ,,/1
silicates. Kaolin, therefore, usually forms from the alteration of a id 1.524- 1.574
II , I. 27- l.578
igneous rocks (granites etc.), with non-alkaline conditions being
., 0.00 - 0.020
required.
Illite is the common clay mineral in clays and mudstones, and it Is '
1', 1)() 7 ° - ve
0
formed by weathering of feldspars or by alteration of other clay mineruls
() Pis porallcl to (LOO)
during sediment formation. Illite formation is favoured by alkalin •
conditions and high AI and K activities. " 2 ... 2.78 1-1 = 7
Montmorillonite and smectites are principal constituent of benton lit• llllllH ( 'olon il ss, but occasionally blue. The precious variety of cordierite
clays, formed from alteration of pyroclastic ash deposits (tuff 1 ·. ), ( 11 1 Sll pphirc) is pale violet and pleochroic.
Montmorillonite (particularly Fuller's earth) is formed by alteration ol 111111 II •h vn ri 1 i s show a c lourles , {3 and y violet, whereas
basic igneous rocks in areas of poor drainage when Mg is n 1 r movt•d, 11111 111 11111 ri ·h vuri ·tics only h w pie chroi min thick sections with a
An alkaline environment is preferred, with low K and hi !11 I' ' 1 prlt t llow 1' • n, {3 ond y pal blue.
Vermiculite is another clay mineral derived from biotit olt ru t ion, 11 1111 I 1111 111 1 i111lll ll 111 ·LUmorphos d ro ks, ordi rit occurs as large 'spongy'
with prop rti s los I r lat d to til srn · ·tit roup, I IY•Iul •onl linin in ~·lus lon s of n1us tWit , hiotit nnd qunrtz. AIIIIOSI
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