Page 71 - Petrology of Sedimentary Rocks

Quartz

Composition, SiO2, essentially constant with no isomorphous substitution. Hexag-

onal (rhombohedral). Hardness 7, cleavage for practical purposes lacking No 1.544,
Nel.553 (both almost constant), both indices slightly higher than Balsam.

Bn 0.009 (quite constant), giving gray to white or cream colors. Extinction in crystals
parallel (or symmetrical to rhomb faces), lengthslow. Often shows undulose extinction
or “strain shadows”, usually as the result of stress after crystallization, occasionally the
result of the method of growth (even unstrained quartz crystals in geodes and veins
sometimes show undulose extinction --partially this may represent a transition to
chalcedony).

Strain shadows usually ocur in bands subparallel to the c axis, and in strongly
strained grains the c axes may deviate as much as 30” (true angle). Intense strain may
result in the formation of warped, subparallel lines made of minute bubbles (B&m
lamellae). Quartz is uniaxial positive, but becomes slightly biaxial when strained, with
2V of 5” or more. Please get a few interference figures on quartz grains showing higher
interference colors (pale yellow), so that you will become familiar with the quartz flash
figure and not confuse it with the truly biaxial feldspar figure. Luminescence
Petrography (Sippel) reveals much occult strain, fracturing and growth zones.

The vast majority of quartz grains contain inclusions (see publications by
Roedder). Most commonly a few isolated vacuoles or trains of vacuoles are found;
these are usually filled with liquid alone, but often include a tiny spherical, “dancing”
gas bubble, and rarely are filled with gas alone. Liquid-filled vacuoles appear brownish
(a complex dispersion effect), and have a high relief with index below quartz; in
reflected light they appear silvery. Gas-filled bubbles have such a tremendous negative
relief (I .OO vs. 1.544) that they appear black and opaque; but they are silvery in
reflected light. Sometimes the vacuoles have the external shape of a quartz crystal
(“negative crystals”), but usually they are highly irregular with rounded protuberances.
Most quartz grains have only a few vacuoles or bubble trains; some, however, are so
crowded with bubbles that they appear milky in reflected light. These excessively
bubbly types come from hydrothermal veins. Mineral inclusions are fairly common;
tourmaline, mica, magnetite, hair-like rutile needles (sometimes in sets intersecting at
60” angles), chlorite, feldspar or zircon. A zonal arrangement of fluid or mineral inclu-
sions, parallel with quartz crystal faces, indicates that the quartz grew in an open
cavity as a vein or geode filling.

Availability. Quartz forms 35-50% of the terrigenous fraction of sedimentary
rocks. Most quartz, especially that in Mesozoic and later rocks comes from reworking
of older sandstones or sandy limestones. This can be recognized by anomalous
size-roundness relations, high roundness standard deviation within the same grain size
(i.e., association of well-rounded with angular grains of the same size), or more rarely
presence of abraded quartz overgrowths. See Syllabus on Grain Shape for details. Also,
“side evidence” may be used such as the presence within the same specimen of
considerable chert, shale or limestone fragments, or the character of the heavy
minerals. A considerable amount of quartz comes from metaquartzites or schists, and a
small amount from hydrothermal veins or volcanic material; however, the ultimate
source of most quartz is to be found in the granites and granite-gneisses. Criteria are
given below for distinguishing these types.

65