Identifying detrital feldspar.
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Quartz may be the most common mineral in sandstone, but feldspar is the most abundant mineral in pretty well every other rock type; in fact, it is the most abundant mineral in the Earth’s crust. Unlike quartz, feldspar is an essential ingredient in nearly all igneous rocks, felsic through ultrabasic. It begins to crystallize in magmas at temperatures about 1000oC – 200o warmer than quartz crystallization. Feldspars are also common in metamorphic rocks. As such, feldspar is an important (usually subordinate) component of most terrigenous clastics, reflected in its inclusion in QFL classification schemes.
The two major groups of feldspar are potassium feldspars and plagioclases. All have low relief in plain polarized light (similar to quartz). Both groups have two good cleavage planes at 90o to each other such that broken crystal fragments tend to be blocky. Twinning is common. Continue reading