Page 88 - Petrology of Sedimentary Rocks

N N Bn Remarks
X z
Orthoclase 1.5 I7 I .524 .007 2v 70", (-)

Sanidine 1.5 I7 1.524 .OQ7 2V very small, (-1
Microcline I .518 I .525 .007
Anort hoclase .523 1.529 .006
Albi te (An& I .525 1.536 .Ol I Alb. ext. angle I3 to 19”

Oligoclase (An*& .539 1.547 .008 Oligo, ext. angle 0 to 13”
Andesi ne (An4($ I .550 I .557 .007 And. ext. angle I3 to 23”
Labradori te (An60) I.560 I.567 ,007 Labr. ext. angle 28 to 39”

Quartz 1.544 1.553 A09 Uni axi al (+)
Canada Balsam 1.537

All the feldspars are biaxial, and all have large optic angles except sanidine. The
K-feldspars are all optically negative, but plagioclase varies from positive to negative.
For details of optic orientation see standard texts, preferably Winchell. Stains are also
very useful in identifying feldspars.

Inclusions (other than alteration products--see below) are not all common in
feldspar. The scarcity of primary mineral inclusions is really marked. intergrowth of
two feldspars (perthite) and intergrowths with quartz are rather common as detrital
grains, however.

Hints on distinguishing feldspar from quartz. Plagioclase and microcline are
easily distinguished by their twinning. Orthoclase is much more difficult, however, and
is very often missed even by experienced petrographers. In identifying orthoclase, use
high power. The diagnostic features are (I) Becke line-- orthoclase is distinctly lower
than balsam, quartz distinctly higher; (2) because of its lower index, orthoclase looks
very pale pinkish or brownish (with fracture lines being bluish), while quartz looks very
pale bluish. With practice this becomes the easiest method of identification; (3) Most
feldspar grains show cleavage, quartz does not; (4) Feldspar is commonly clouded with
bubbles or alteration products, quartz is usually clearer; furthermore, in feldspar the
bubbles are elongated or arranged in vague grids or lines parallel with crystal
directions, while in quartz they are irregular; (5) Feldspar has distinctly lower
bi ref ringence; with practice, this becomes a valuable method of identification;
(6) Orthoclase is biaxial-negative with large 2V, quartz is uniaxial positive. Untwinned
plagioclase, important in some metamorphic rocks, should be identified by figure. Do
not confuse the quartz flash figure with the feldspar figure.

If feldspar and quartz are coated or embedded in hematite, opal, carbonate or
clay matrix, those methods based on index of refraction fail, and the easiest method is
to dissolve or crush the rock and mount the loose grains in 1.540 oil, then count them.
Or one can also cruise around the edge of the rock chip where the sand grains are in
contact with Balsam.

Availability. Feldspar forms 5-15% of the terrigenous fraction of sediments.
Feldspar is by far the most abundant mineral in igneous rocks, and its much lessened
volume in sediments is because of the ease of weathering and lack of durability, Most
feldspar comes from granites and gneisses, where orthoclase and microcline are
dominant with sodic plagioclase (usually oligoclase) a poor third. Pegmatites contain

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