Page 80 - Petrology of Sedimentary Rocks

In practice, one examines a series of slides to set up the diagnostic quartz types
present, patterning it after the above classes. If necessary he erects additional classes
to take care of types he thinks might be significant. Then he proceeds to count 100 or
more grains (always under high power), and classifies each grain as “I b”, “3a”, etc.,
recording them in a binary table. If special types, such as volcanic quartz or reworked
abraded grains are present, they may be counted in a separate class. After this has
been done, he re-examines the slide with the genetic classification in mind, refers to
the graph, and decides the probable lithology of the source area. To do this he also
takes into account the associated minerals, and the effects of grain size and abrasion on
his count (less composite grains will be found if the material is fine-grained, etc.).
Criteria for an older sedimentary source have already been indicated. In general grains
from a sedimentary source area are of diverse types; if quartztypes are few the source
is probably a primary igneous rock.

An Opposing View on Quartz Extinction. H. Blatt (I 963-l 967) has recently made
study of quartz in igneous and metamorphic rocks, in the immediate disintegration
products of such rocks (grus), and in consolidated sedimentary rocks from many areas.
His results do not support the “classical” interpretation given above. (I) Blatt measured
the true degree of undulatory extinction in 586 medium sand-sized quartz grains from
freshly disintegrated granites, schists, and gneisses. He found (a) that 91% of these
grains have less than seven degrees of undulose extinction; (b) there is no correlation
between rock type and amount of strain of the grains; (c) no grain had undulose
extinction greater than eighteen degrees. From these data, Blatt concluded the
strength of the quartz structure does not generally allow the development of more than
ten degrees of undulose extinction. This result is consistent with the results of
experimental deformation of quartz. The important point is that Blatt claims that no
real difference exists in degree of undulose extinction in quartz grains among igneous
and metamorphic rocks, and therefore that any “averaging” method of determining
amount of undulose extinction on a flat stage is at least as useless as attempts at
differentiation with universal-stage measurements. (2) Blatt further says that if one
defines “strongly” undulatory as greater than five degrees of undulose extinction, as
Folk does, there are very few truly strongly undulatory grains, and most grains that
appear to be “strongly” undulatory appear that way only as a result of the relationship
between the plane of the thin-section and the plane containing the optic axes in a
strained grain. (3) Blatt’s dat a indicates that, based on universal-stage measurements,
the percentage of grains with greater than one degree of undulose extinction (Folk’s
slightly and strongly undulatory grains) is statistically identical among granites, schists,
and gneisses. Therefore, there appears to be no way of distinguishing between quartz
grains from each of these three rock groups on the basis of either the unstrained/-
strained ratio or on the basis of the slightly strained/strongly strained ratio. (4) Blatt
feels that medium sized polycrystalline quartz grains composed of ten or more sub-
grains are an excellent indicator of metamorphic source (composite quartz of Folk).
However, he considers that medium sized grains composed of less than five sub-grains
are as commonly derived from granites as from schists and gneisses. (5) Blatt’s data
shows that well-rounded, pure quartz sandstones (i.e. supermature orthoquartzites) have
a very high percentage of “straight” quartz, much higher than any conceivable source
rock. He interprets this as being due to selective destruction of undulatory quartz,
while the unstrained quartz survives. This had been suspected for some time by many
workers. Basu et al. (I 975 JSP) and Young (I 976 JSP) have come to a practical middle
ground in a detailed re-evaluation of undulosity and polycrystallinity.

A key point to the Cambro-Ordovician sandstones of the central U.S., which
consist almost entirely of well-rounded common (straight to slightly undulose) quartz;

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