Page 82 - Petrology of Sedimentary Rocks

Note: Two geologic realms where straight extinction occurs, at opposite ends of the
scale: (rystallization from magma or solution, (2) recrystallization of a solid. In
between is the realm of undulose quartz. Undulosity is a function of size as well as
strain. Thus schists have so much straight quartz because they are the fist-grained,
having been formed from fi ne-grained source rocks, e.g. shales and siltstones. This
diagram is a fusion of ideas of H. E. Blatt and the writer--December 1963.

Explanation of page 76. Quartz is more diagnostic of formative conditions rather
than specific lithologic types of source rock, but the two are somewhat associated,
which led to the original confusion. The following are recognizable quartz types in
terrigenous sediments:

A. Special Varieties

I. Volcanic quartz, mainly recognized by shape.

2. Vein quartz, mainly recognized by abundance of vacuoles.

3. Reworked sedimentary quartz, reworked overgrowths.

B. Def i ni te Metamorphic Types

I. Fine polycrystalline quartz, crystals finer than .I-.2 mm, usually with
straight extinction because of their small size; straight borders.
Origin: recrystallization probably in the solid state. Source: schists,
recrystallized siltstones and sandstone, recrystallized chert or sheared
vein quartz, etc.

2. Sheared quartz, undulose with crenulate boundaries and two or more
elongate individuals. Origin: intense deformation without recrystalli-
zation. Source: gneisses, metaquartzi tes, shear zones in granites,
crushed veins, sheared sandstone, etc.

C. Equivocal Types, indicative only of the latest structural event, not indica-
tive of any particular source rock.

I. Single crystal units with straight to slightly undulose extinction (under
2” on U-stage?). Any relatively unstrained or recrystallized source
rock:. granite, vein, coarse metaquartzite or schist, etc. This is the
dominant quartz type in most sandstones, and becomes much more
abundant in the finer grain sizes. On prolonged abrasion all other
quartz types except this are probably selectively eliminated.

2. Single crystal units with stronger undulose extinction. Any relatively
strained rock: granite, vein, coarse metaquartzite, coarse restrained
schist, etc.

3. Semi-composite quartz. Any source rock that has been weakly
strained and subsequently polygonized; or a comb-structured vein.

4. Coarsely polycrystalline quartz with non-sutured boundaries. Most
likely from metaquartzites, but can also come from finer-grained
granites, ccxrrser schists, etc.

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