Page 89 - Petrology of Sedimentary Rocks

much more microcline. Volcanic rocks contribute small quantities of plagioclase and a
Ii ttle sanidine. In general, orthoclase is the dominant feldspar in sediments because of
its great availability; microcline is a close second despite the fact that it is more stable
than orthoclase; sodic plagioclase is a poor third; and calcic plagioclase is almost
lacking. Formations in which plagioclase is abundant are consequently unusual. Thus, if
a formation contains more plagioclase than K-feldspar, suspect a volcanic source--
especially if the plagioclase crystals are zoned. If a formation contains very little or no
feldspar, people often assume that it was because of prolonged abrasion or a high rate
of weathering; however, it may just as easily mean that little or no feldspar was
available in the source area. Schists, phyllites, slates, or older sediments contribute
little or none. Occasionally feldspar can be reworked from an older feldspathic
sediment into a younger one; but nearly all feldspar comes from a primary igneous or
metamorphic source.

Durability. Feldspar is softer than quartz and has a much better cleavage, hence
abrasion reduces size and increases roundness much more rapidly. Thus the relation
between the size of quartz and feldspar and the roundness of quartz and feldspar is one
of the most valuable keys to the history of the sediment and in determining direction of
the source area. In youthful sediments quartz and feldspar will be the same size (or
feldspar may even be larger since it starts out in the parent rock as larger crystals), and
of essentially similar shape. As the sediments suffer more abrasion, feldspar becomes
smaller and rounder than quartz; by the time quartz becomes well-rounded, the feldspar
is usually much finer. Of course, during the process of abrasion the feldspar continually
diminishes in quantity relative to quartz; thus one may start with a good arkose
composed of angular grains of quartz and feldspar of equal size, and simply by abrasion
alone end up with a supermature orthoquartzite in which all the grains are well-
rounded, and the feldspar is reduced to a very small percentage concentrated in the silt
size while the sand fraction is nearly all quartz (Folk).

Stability. Feldspars are unstable under deuteric, hydrothermal, and weathering
conditions. Four types of alteration are important: kaolinization, sericitization (or
illitization), solution, and vacuolization. Kaolin forms as tiny flakes throughout the
feldspar grain; the flakes have an index slightly higher than the feldspar, and give a
gray birefringence. They do not impart a cloudy appearance to the grain, as the
brownish turbidity that textbooksassign to kaolin is actually caused by bubbles. Most
kaolin forms on intense weathering by the complete removal of K, but sometimes forms
hydrothermally. Sericite and illite form tiny flakes with indices higher than feldspar,
giving a white to yellow birefringence and often having crystallo-graphically-controlled
distribution. They are most commonly formed hydrothermally but also result on
weathering if it is not intense (as in semi-arid climates). Water-filled vacuoles are
responsible for the cloudy, brownish appearance of altered feldspars, and vacuolization
is the most common type of alteration (Folk). Under high power the vacuoles are
brownish (an effect of the index) with index much lower than feldspar. They form on
weathering, on hydrothermal alteration, and also are a primary inclusion trapped within
the crystallizing feldspar (thus feldspars forming in hydrous environments like granites
or pegmatites contain abundant water-filled vacuoles, while the feldspar of volcanic
rocks is clear because of the relatively anhydrous environment). T.W. Todd and D.N.
Miller, Jr. (U. T. 19.55) have discovered post-depositional cavernous alterations of K-
feldspars with development of huge holes inside the feldspar grains. Todd (I968 J.S.P.)
has studied the complexities of feldspar weathering in soils. Elsie Begle (U.T. 1978) has
found much evidence of solution of feldspars in soils with production of deep
channelways and skeletal grains.

83