Page 87 - Petrology of Sedimentary Rocks

It is possible to set up a classification of chert types for detailed work, based on
internal structure (i.e., pellets, oolites, fossils, etc.), grain size of the microcrystalline
quartz, color or inclusions. Chert types have been split in great detail by workers in
insoluble residues, who find it an invaluable aid in correlation.

Chert, when metamorphosed, grades into fine-grained metaquartzite. Unfortu-
nately, many fine-grained and/or silicified volcanic rocks resemble chert when broken
to sand-sized grains, and these must be distinguished carefully. Often the volcanic
grains may have a few surviving feldspars with lower index, or may show some trace of
phenocrysts.

Feldspar

Feldspar is an important mineral of sediments, and a good clue to the interpreta-
tion of paleoclimate. The following feldspars are the ones found in sedimentary rocks
(see also J.V. Smith, 3 vol. set, and Van der Plas (1966) on identification).

Orthoclase, KA I Si308, monoclinic. The name adularia is often applied to low-
temperature, vein-filling orthoclase but both are identical in all properties.
Both may contain some NaA I Si308.

Sanidine, KA I Si308, monocl init. Differs only from orthoclase in that it has a
small 2V and different optical orientation, grains are clear and are usually
free of bubble inclusions, in contrast with orthaclase. Sanidine is the high-
temperature form, found in lavas. It may also contain considerable
NaA I Si308.

Microcline, KAISi308, triclinic, grid twinning usually with tapering twin lamellae.
Often contains considerable NaA I Si308, thus grades into anorthoclase.

Anorthoclase, NaA I Si308 and thus grading into microcline. Triclinic with very
close grid twinning, distinguished from microcline by optic orientation;
found in soda-rich volcanic rocks.

Plagioclase, a continuous series ranging from Albite (NaAISi308) through Oligo-
clase, Andesine, Labradorite, and Bytownite, to Anorthite (CaA I 2Si208).
May contain a little KAISi308. Triclinic with albite twinning, and twin

lamellae are straight and parallel. Metamorphic plagioclase is untwinned.

Perthite is a wormy or patchy intergrowth of albite in orthoclase or microcline.
It is common in granites and pegmatites.

The feldspars have a hardness of 6, and three cleavages. All the K-feldspars have
slight relief with all indices below balsam. Extremely sodic albite (limit An21 has all
indices below balsam; between An2 and sodic oligoclase (AnI 7) the three indices
straddle balsam; and plagioclases more calcic than An I 7 have all indices above balsam.

Plagioclases more calcic than An3o even have indices higher than quartz. Birefringence
of the common feldspars is notably lower than quartz, so that interference colors are
gray-white to gray.

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