Page 76 - Petrology of Sedimentary Rocks

Genetic Classification (modified from Krynine)

I. Igneous Quartz

A. Plutonic Quartz, from granite batholiths or granite-gneisses, thus the most
abundant type. Shape xenomorphic, irregular subequant, sometimes with
re-entrant angles. Single crystals average about 0.5 mm. Often has
straight to slightly undulose extinction in sand-size grains, but may be
strongly undulose if the batholith has been stressed after solidification, or if
the rock is gneissic. A few grains may be composites. Usually contains no
inclusions other than a small amount of scattered vacuoles; if these are
randomly scattered they were probably trapped during crystallization, but if
arranged in lines (actually planes) they probably formed upon stressing of
the parent rock and represent incipient, healed fractures. Mineral inclusions
sometimes occur but are not common; rutile, zircon, micas, feldspar,
biotite, hornblende, tourmaline, etc. are most frequent. Because much
quartz from other environments (metamorphic, vein, etc.) has the same
characteristics as “plutonic” quartz, a better name for this type is “com-
mon” quartz.

B. Volcanic Quartz, as phenocrysts derived from erosion of volcanic rocks such
as rhyolites, latites, dacites, etc. or as direct contributions from ash falls, is
not abundant in general but some stratigraphic sequences contain numerous
grains of it, and it is an excellent stratigraphic marker. Recognition is
based mainly on shape --whole or fragmental bipyramidal crystals lacking
prism faces, hexagonal with geometrically straight edges (compare with
hairline microscope), but usually with rounded corners. They often have
large rounded corrosion embayments. Extinction nearly always straight,
usually contain no inclusions and thus are water-clear. There are rarely any
bubble inclusionfiut occasionally bits of volcanic glass or aphanitic volcanic
ground-mass may be trapped, sometimes in the form of negative hexagonal
crystals. On abrasion, the diagnostic features of volcanic quartz are lost.
Be on the conservative side when identifying quartz as volcanic. In recent
sands, volcanic phenocrysts are easily recognizable by their highly polished
surfaces and water-clear appearance, together with the typical hexagonal
bipyramid shape and embayments.

C. Vein Quartz, derived from pegmatites, hydrothermal and much more rarely
sedimentary vein fi Ilings. It is very common but not usually abundant
except in certain stratigraphic horizons; it is possibly the most comon type
of quartz occurring in pebbles. Shape is not diagnostic. Extinction is most
commonly semi-composite with parallel arrangement of individuals almost
but not quite in optical continuity, partly a reflection of the comb structure
produced as the crystals grow out all nearly perpendicular to the walls of
veins. Many grains have straight extinction, however, and sometimes the
extinction may be slightly undulose, because the formation of veins is often
accompanied by stresses; intense stress and shearing causes vein quartz in
some cases to take on a metamorphic appearance with intense strain
shadows and crenulations, but this is then arbitrarily classed as stretched
metamorphic quartz. Grains contain no microlites as a rule (occasionally
mica or tourmaline or feldspar may occur if it is from a pegmatite, or
carbonate if a cooler vein) but in most cases vacuoles are very abundant,
giving the typical milky appearance to pebbles and even sand grains, because

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