Page 79 - Petrology of Sedimentary Rocks

There is of course some degree of tie-up between the genetic and empirical
classifications. As shown on page 71, types I and 2 come mostly from granites and
gneisses, to a smaller extent from recrystallized metamorphic rocks, and to some
extent from veins. Types 3 and 6 come chiefly from gneisses and stretched
metamorphic sources, but type 3 is in some strained granites. Type 4 is typical of
hydrothermal veins (but also occurs in lesser amounts in other rocks), and type 5 occurs
chiefly in schists or recrystallized metamorphic loci, but to a small extent in granites.

The second basis for the empirical classification is the type of inclusion present.
Here some leeway is present and for each problem a different classification might be
set up; for example, in some areas vermicular chlorite inclusions might be diagnostic
and would be set up as a separate class for counting; or, the presence of negative
crystals or of tourmaline inclusions might be of value and would be counted separately.
For general purposes, the following classification is suggested, operated under the
principle that if a grain does not fit in the first category, one goes on down the line
until he finds the first category that applies (for example, a grain with abundant bubbles
and rutile needles would be placed in class “a”). The types are as follows: (a) Abundant
vacuoles (grain may even appear milky in reflected light; (b) Rutile needles or other
hairlike inclusions; (c) Microlites (inclusions of minerals other than rutile); (d) Sparse
vacuoles, no microlites. For detailed work, microlites should be identified and counted
separately. Of these classes, type “a” is diagnostic of hydrothermal veins; ‘lb” is an
excellent stratigraphic and regional marker, but its genetic significance is characteris-
tic of metamorphic terranes, but also occurs in plutonic environments; and “d” is the
commonest type, derived from all sources.

Empirical Classification of Quartz Types
(After R. L. Folk)
A. BY EXTINCTION (ALL SHOWN AS UNDER CROSSED NICOLS)

1. Single grain, stra ight 2. Single grain, slightly 3. Single grain, strong lY
extinction. undulose extinction. undulose extinction.

- -
_’
-
c3

4. Semicomposite grain, 5. Composite grain, 6. Composite grain,
straight to slightly straight to slightly strongly undulose
undulose extinction. undulose extinction. extinction.
. :,:.
. *
.:.. .-,;a.
.
.* .<-
B. BY INCLUSIONS . . . ..*
.A:-‘.‘:’
. ,*
0 0 .
a. Abundant b. Rutile c. Microlites d. Few vacuoles
vacuoles needles no micro,1 i tes

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