Page 72 - Petrology of Sedimentary Rocks

Quartz is the most durable of the abundant minerals because of its
hardness, ness, and lack of cleavage. To round a quartz grain takes a tremendous
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amount of time; thus quartz is usually subangular in sediments (see section on shape).
Grains are usually subequant, but those derived from schists are usually slightly more
elongated or platy. In silts it is fairly common to find the grains elongated parallel to
the c axis, probably the result of obscure prismatic and rhombohedral fracture.
Terrigenous quartz grains as fine as 3 microns or even less occur, but .OlO-.020 mm is
about the practical lower limit of quartz in most mudrocks. Fractured quartz breaks
easily on transport (Moss, ‘72 JSP), as does poly-crystalline (Harrell and Blatt ‘78 JSP).

Stability. Quartz is ultra-stable under nearly all surface conditions, and it is
probable that very little quartz is ever dissolved by weathering; but some solution of
quartz in tropical areas has been reported with corrosion of grains (Crook, 1968) and
corrosion (by organics) can even occur in soils of S.E. USA (Cleary and Conolly ‘72 JSP),
producing skeletal grains by solution along fractures (microgriking). Folk (‘78 JSP)
reviews quartz solution. However, on burial in the sub-surface, even under only
moderate pressures and temperatures, quartz may occasionally dissolve; thus, some
sandstones show sutured quartz grains along styolites, and even styolites in limestone
often cut through and dissolve quartz grains. Dolomite or calcite rarely replace the
edges of quartz grains, to form etched or pitted contacts (quartz in limestones or
dolomi tes is often thought to be replaced when it is really just an optical illusion caused
by “overlap” of carbonate around the curving edge of the quartz grain). It can also be
replaced by pyrite. On the other hand, quartz grains commonly have authigenic quartz
overgrowths, showing that the mineral may either grow or be destroyed depending on
condi tons.

Depositional Characteristics. Quartz in sandstones is uniformly distributed, in
mudrocks usually occurs as laminae or clots, and in limestones may occur as laminae or
as scattered grains or nests. Grains often show physical orientation parallel with the
bedding, in addition, they also more rarely show an optical orientation with the slow ray
(c axis) parallel with the bedding. Check this by inserting the gypsum plate; a
dominance of blue grains in one direction and yellow grains in a direction of 90”
indicates optical orientation. This is partially due to differential abrasion, with the “c”
axis direction being harder so the grains tend to become elongated parallel to “c”, and
partially due to a weak tendency to fracture into chips parallel to “c” rather than
perpendicular to it.

Quartz Varieties. The study of quartz types is one of the most fascinating and
valuable aspects of sedimentary petrography. It is of great value in paleogeographic
interpretation, and is increasingly used in the correlation of formations. H.C. Sorby
(1858, 1877, 1880) first studied inclusions and extinction in quartz grains as a clue to
their source. Later work along this line was done by Mackie in the 1890’s, and the
subject achieved great height under Krynine in the 1940’s. Blatt (1960’s) has made an
intensive quantitative study of quartz in source rocks and sediments, and Basu, Suttner
and others have made further advances in the ‘70’s. Quartz is an ideal mineral to use
because there are so many possible varieties, and because many of them can be assigned
to a definite type of source area. Each environment of quartz formation--batholith,
volcano, schist, pegmatite, fault zone --lends its characteristic impress to the proper-
ties of the quartz grains, and these properties remain when the grain is deposited in
sediments derived from these areas, as first shown by Sorby.

There are two systems of classification of quartz types. In the genetic
classification, developed by P.D. Krynine, an attempt is made to allot each quartz grain
to a specific environment of formation: plutonic, volcanic, schistose, stretched
metaquartzi te, recrystallized metaquartzite, hydrothermal, etc., based on type of

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