Page 78 - Petrology of Sedimentary Rocks

micas, sillimanite, garnet, etc,--are present. This type of quartz is common
in graywackes. Cautions: (I) do not confuse fine-grained stretched
metamorphic quartz with chert. Microcrystals of chert seldom are over IO
microns, and are randomly oriented as a rule; the component individuals of
stretched metamorphic quartz are usually elongated, coarser than 20
microns, and are in sub-parallel orientation (test with gypsum plate). (2) On
prolonged abrasion metamorphic quartz may possibly abrade or fracture
faster than plutonic quartz, so in very well-rounded sands the percentage of
metaquartz may be abnormally low. (3) Be careful not to be confused by
post-depositional strain-shadows when studying quartz types; sandstones
made entirely of straight-extinction quartz develop strain shadows where
strongly folded or faulted, with most strain occurring where a grain is
“pinched” by corners of a neighboring detrital grain.

III. Reworked Sedimentary Quartz.

Sedimentary authigenic quartz is sometimes found as reworked detrital grains.
The most common kind is as quartz grains with reworked overgrowths, formed in an
older sandstone or limestone. Overgrowths survive river transport, and probably most
beach transport, with very little sign of abrasion; so do not try to identify a reworked
overgrowth by its so-called “abrasion”, because this cannot be seen in thin section
(unless the grain is broken right slap in two). If, in a sandstone, scattered grains have
good overgrowths and others have none, and particularly if the nuclei of the overgrowth
grains have a uniform shape (e.g. all are well rounded), then they are probably
reworked. Indigenous overgrowths usually interlock with long or sinuous contacts, and
occur on most of the grains in the slide regardless of the shape of the nucleus.
Occasionally quartz nodules are reworked from older carbonate rocks, where they occur
associated with chert nodules. These consist of flamboyant quartz and are quite scarce;
they often originate through replacement of evaporites.

Empirical Classification (Folk). Six extinction types are recognized for counting
purposes; (I) Straight, the grain extinguishes uniformly all over at once. (2) Slightly
undulose, the grain is a single individual and the extinction shadow sweeps smoothly and
without breaks across the grain; however the sweep is accomplished on very slight
rotation of the stage. (3) Strongly undulose, the grain is a single individual and the
extinction shadow sweeps smoothly across, but a large rotation of the stage is required
betwen the time one part of the grain is extinguished to the time another part is
extinguished. (It is difficult to establish objective boundaries between the three
classes, but experience has shown that in grains of .I!%.35mm, “straight” grains
extinguish with a stage rotation of less than lo, “slightly undulose” grains require a
rotation of lo to 5” before the deepest part of the extinction shadow sweeps from one
side of the grain to the other, and “strongly undulose” grains require a rotation of over
5”, often as much as 20” or more. Blatt says that this measurement is useless if made
on a flat stage because we do not see the true extinction angle). (4) Semicomposite, the
grain is made up of two or more sub-individuals with very close optical orientation, but
there is a distinct break between individuals and the extincxshadow does not sweep
smoothly across; the individuals may however have straight to slightly undulose
extinction. The presence of crenulated borders or strong undulose extinction throws
this into class “6 I’ . (5) Composite, th e grain in made up of two or more sub-individuals,
with wide1 differing orientation, but these same individuals have normal (not crenu-
lated + boundaries and straight to slightly undulose extinction. (6) Composite metamor-
phic, the grain is made up of two or more sub-individuals with strongly undulose
extinction, and may or may not have crenulated boundaries.

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