Page 96 - Petrology of Sedimentary Rocks

Gibbsi te Kaolin lllite Muscovite* Montmor- Chlorite
illonite

SiO2 -- 44-47 47-52 45 50-56 20-30

30-35 37 22-30 38 16-20 15-25
*‘2’3
Fe203Fe0 -- 0.5-I 2-7 -- l-4 I-40

MgO -- 0.3 l-4 -- 3-6 3-35

CaO -- 0.3 o- I -- l-3 o-3

-- O-I .5 6-8 I2 o-o.5 --
K2°
No20 -- o-o.5 O-I -- o-2 --

*Sericite is probably just fine-grained slightly impure or K-deficient muscovite.

Although clays are difficult to identify with the microscope, an educated guess can be
made with careful work. Chlorite may be recognized by its color; montmorillonite is the
only common clay with index below balsam; kaolin, illite and sericite are all above
balsam, but kaolin has very low gray birefringence, and the other two appear as bright
white to yellowish flakes under crossed nicols; illite and sericite are impossible to
differentiate microscopically except that sericite is a little coarser (hard to distin-
guish by X-ray too, except sericite has sharper peaks). Gibbsite is found only in highly
aluminous rocks.

NY Bn Color and Remarks
Gibbsite 1.55 - 1.58 .020 Colorless, 20-45Oextinction

Kaolin 1.55 - 1.57 .006 Colorless
lllite 1.55 - 1.63 .02 - .05 Colorless to very pale green
Serici te 1.55 - 1.62 .03 Colorless; probably just
coarse illite
Montmorillonite 1.49 - 1.52 .02 Colorless (appears spurious
brown because of index)
Chlorite 1.57 - 1.60 .oo - .Ol Pale to deep green; often
anomalous birefringence

It must be realized that the names above refer to groups of minerals, some groups
(chlorite, for example) containing a dozen more species differing slightly in composition
and optical properties. Properties listed above refer to the most prevalent ranges for
the common members of each group. In addition, a great percentage of natural clays
are “mixed layer” minerals, composed of intimately interstacked sheets of two or three
different species often in no definite proportions-- e.g., illite with montmorillonite, or
chlorite with montmorillonite. These are actual intergrowths, not simply physical
mixtures. An illite-montmorillonite mixed layer clay is easily produced by stripping
some of the K+ from a series of illite sheets; this happens readily in weathering. If all
the K+ is removed from illite, a material results that gives a montmorillonite X-ray

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