[Feldspar] [cleavage fragment] from Llano County, Texas. The two directions of good [cleavage] meet at an angle of about 90°.
The feldspars are important rock-forming minerals. You can find them in [igneous rocks], such as [granite] or [pegmatite], and in [metamorphic rocks], such as [gneiss]. They also occur as fragments in [sedimentary rocks], such as some [sandstone] and [conglomerate].
Although the feldspars can originate in other ways, they form mostly from hot magmas that cool and crystallize into [igneous] rocks. These minerals occur in the rocks as grains, as cleavable masses, and as individual crystals. The crystals may be shaped like prisms, or they may be flat and slabby.
Good places to look for feldspars are in areas where granites, pegmatites, and other [intrusive] [igneous rocks] appear at the surface. The [pegmatite] rocks of Burnet, Gillespie, Llano, and Mason counties in the [Llano uplift] area of central Texas, and those of the Van Horn Mountains in Hudspeth and Culberson counties in west Texas, are especially good sources of [feldspar]. Large cleavable masses and crystals that are more than a foot long are found in some of these rocks.
The feldspars have a number of uses. Some of the [pegmatite] feldspars from Llano County in central Texas have been crushed and used as [granules] for built-up and composition roofs. In addition, some have been shipped to Mexico for glass-making. Some of the other uses of [feldspar] are in making porcelain, ceramic glazes, and scouring compounds. A few of the feldspar minerals, such as the variety of [microcline] known as amazonstone, are used as gemstones.
[Microcline] [feldspar] crystals from near [Granite] Shoals Lake, Llano County, Texas.
Fibrous Gypsum. See [Gypsum].
Flint. See [Quartz].