Page 155 - Petrology of Sedimentary Rocks

I. Terrigenous Minerals
a. Quartz (stretched metaquartzite should be counted sepa-
rately and its percentage specified because it is sometimes
assigned to the “Ml’ or “RF” poles, not the ‘W pole).
b. Chert
C. Feldspar
d. Mica
e. Rock fragments (metamorphic, plutonic or volcanic, re-
worked sedimentary). Their internal composition texture
and structure should be described, and they should be
specifically identified if possible (e.g. muscovite schist,
andesite, biomicrite, illite-slate).
f. Other terrigenous minerals--such as tourmaline, zircon,
etc.; each of these should be described separately.
90 Clay minerals.
At the end state what percentage you allot to the Q, F, and
R poles of the composition triangle, and give the composi-
tional rock name (e.g. arkose, or feldspathic phyllarenite,
etc.). This should be based on a point-count of at least 100
essential (QFR) grains.
2. Allochemical Grains (here describe separately the fossils,
mlites, intraclastic particles, pellets, cellophane, glauconite and
other allochems, using the complete outline I - 25 below).
3. Orthochemical minerals. Also describe each variety of these
individually, using the following outline, I - 25.
For each of the properties, for each of the minerals, ask yourself
why this is so; for example, if zircon is smaller than quartz and
more angular, and tends to be placered into streaks, you reason
that it is smaller and placered because it has a higher density;
also it is more angular than quartz because it is harder. A great
amount of valuable information can be unearthed if you will
continually ask yourself why; it will stimulate you to look for
clues that otherwise might be completely missed. This is the
outline to be followed for each mineral (Terrigenous, Allochemi-
cal, and Orthochemical) in the order set forth above.
I. Name. If the mineral varies in composition (such as
plagioclase or hornblende) identify the composition opti-
cally.
2. Method of identification if the identity is not obvious.
3. Percentage present in section. State method, whether
sheer estimation, point-count, etc. A t error (01 .sb~ltl
always be attached.
4. Occurrence in slide and distribution pattern (uniformly
distributed, randomly scattered, or patchily distributed.
For example, heavy minerals often show a patchy distribu-
tion as they are concentrated along certain bedding planes
as placers; or certain cements may occur in patches and be
absent in the rest of the rock; or certain minerals may
occur only in the vicinity of joints or near weathered
surfaces; or certain minerals like pyrite may be selectively
associated with the more clayey layers, etc.).
5. Physical orientation
6. Grain size of this mineral: median, extreme range, and
sorting; if desirable the l6-84% range should be specified.

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