Page 77 - Petrology of Sedimentary Rocks

the mineral formed in a very hydrous environment. However, some grains
contain few vacuoles. Vermicular chlorite is very diagnostic of hydrother-
mal quartz, and the presence of zoned inclusions (parallel to old crystal
faces as the crystal grew) is also diagnostic.

Any type of quartz grain that has been slightly deformed may readjust
by developing a mosaic of almost parallel-oriented sub-grains; this process is
analogous to the process of “polygonization” in strained metals, which also
results in semi-composite extinction mimicking the appearance of a comb-
structured vein. Quartz should not be referred to as “polygonized” unless
the formative process is known: “semi-composite” is better as a purely
descriptive, non-genetic term.

II. Metamorphic Quartz

A. Recrystallized Metamorphic Quartz. Formed under intense but non-shearing
stresses when quartz recrystallizes, or else by recrystallization after stress
has passed. In the lab, recrystallization occurs at 600-8OoOC (Tullis, ‘73
GSA). Occurs in many recrystallized metaquartzites, highly metamorphosed
and gneissic rocks. Shape subequant. Extinction is straight (recrystalliza-
tion destroys any previous strain), and grains may be either single individuals
or composites made up of a mosaic of equidimensional grains with rather
straight boundaries, and widely different optic orientation. Contains few
inclusions, mostly vacuoles; appears to have less vacuoles than plutonic
quartz. May have mineral inclusions in small amounts, feldspar, micas,
tourmaline, etc. Usually it is quite difficult to tell from plutonic quartz
(of ten impossible), but despite its lack of recognition probably is an
important contributor to sands. Caution: the chief diagnostic feature is the
presence of composite grains; naturally these will be less evident if the
sandstone is finer, so in comparing samples you should work with ones of
about the same grain size; also on prolonged abrasion or chemical weather-
ing composite grains will break apart into their sub-individuals and look like
single grains of plutonic quartz.

B. Schistose Quartz forms during lit-par-lit injection of schists, also on
recrystallization of schistose rocks. Quartz grows in between parallel mica
flakes, hence is often elongated or platy, with sub-parallel straight edges;
since it forms by recrystallization or growth from solution after stress has
passed, it seldom has strain shadows and usually shows straight extinction.
Composite grains are common. It sometimes has no inclusions and vacuoles
are uncommon, but micaceous inclusions or other metamorphic minerals are
frequent. It is a common type of quartz but difficult to recognize if there
has been prolonged abrasion, because the chief diagnostic feature is the
shape. Its presence can be shown by a statistical count of the length-width
ratios of grains in a slide (Bokman), an idea first proposed by Sorby.

C. Stretched Metamorphic Quartz (or Sheared Quartz) is very easy to recognize
and is quite important in sediments. It forms when a quartz-bearing rock
(be it older sandstone, granite, schist, or quartz vein) is sheared or strained
in the absence of recrystallization. Grains are generally elongated or platy.
Extinction is moderately to strongly undulose, and grains may be single but
are usually made up of sub-individuals of lensoid or “smeared-out” shape.
Often boundaries between sub-individuals are intensely crenulated or su-
tured, sometimes finely granulated. Sometimes metamorphic inclusions--

71