Feldspar Feldspar is a white to pink or red mineral having a glassy luster on its flat broken surfaces (cleavage faces). It will scratch window glass. It is the most abundant mineral in granite and usually controls the color of that rock; for example, the red granite at Graniteville contains red feldspar, and the pink-gray granite in the Knoblick region has feldspar of those colors. Small bodies or bands of very coarse feldspar, quartz, and mica (pegmatite dikes) which cut the granite may contain crystals of feldspar large enough to be recovered as small, hand specimens, but otherwise it does not occur in coarse fragments. The recognizable crystals, or phenocrysts, in the porphyry are mainly feldspar. [Illustration: A large piece of feldspar showing cleavage surfaces.] Feldspar is really a family name for a group of several minerals, all of which are crystallized in the igneous rocks. The potassium (potash)-containing varieties, named orthoclase and microcline, occur in the granite and porphyry, whereas plagioclase, a calcium-sodium (lime-soda) feldspar is in gabbro, diabase, and basalt. Plagioclase commonly has a thin, lath shape, is a shade of gray, and makes up the lighter colored part of the greenish to dark gray igneous rocks. Further differences between it and orthoclase may interest the mineralogist but are of little concern to the non-technical person. Pulverized feldspars are used extensively in the ceramic industries, but Missouri does not have any productive deposits. Under natural, long-time weathering processes feldspar usually decomposes to clay which may be used technically, but the usual fate of it is soil formation. Mica Mica, incorrectly called isinglass, is an elastic, fairly soft, platy mineral, which may be split into flakes of paper thinness. The relatively clear variety is called muscovite, and the brownish black to black variety is biotite, both being members of the mica family. They may occur in Missouri in small grains in the igneous rocks, except that muscovite may be present in sandstone, where it was deposited along with the quartz sand. Mica is used chiefly as insulating material in the electrical industry where large sheets are required. Another use is as window or chimney material in stoves or lanterns. Missouri has no mica which is satisfactory for these purposes. Porphyry, Rhyolite, Rhyolite Porphyry