[Pyrite] alters easily. Because of this, most builders carefully check the [limestone], [granite], [marble], or whatever other building stone they plan to use to be sure that it does not contain large amounts of pyrite. When exposed to the weather, pyrite changes to [limonite] and causes an unsightly rust stain.
[Pyrite] is used as a source of [sulfur], and it is produced for this purpose in several states. In Texas, however, no pyrite deposits have been found that are large enough to be mined.
Pyrolusite. See [Manganese Minerals].
Quartz
[Quartz], silicon dioxide, is one of the most common minerals. It is glassy, waxy, greasy, or dull and is [transparent] or [translucent]. Pure quartz is colorless, but impurities make some varieties white, black, or a shade of red, yellow, blue, violet, or brown. Quartz is a hard mineral. It scratches window glass and cannot be scratched by a pocket knife or even by a steel file. It has a [specific gravity] of 2.65. The curved, [conchoidal] [fracture] shown by many specimens helps identify it.
[Quartz] is plentiful in Texas. It occurs in [igneous rocks], such as [granite], [llanite], and [pegmatite]; in [metamorphic rocks], such as [quartzite], [schist], and [gneiss]; and in [sedimentary rocks], such as some [sandstone], [conglomerate], and [breccia].
[Quartz] crystal, with inclusions, from Burnet County, Texas.
[Quartz] is found as crystals and as masses. Some of the masses are coarsely [crystalline], but some are made up of extremely small crystalline particles called [cryptocrystalline] quartz. Some of the cryptocrystalline varieties of quartz found in Texas are [chalcedony], [chert], and [jasper]. Some of the coarsely crystalline varieties found here are [amethyst], [milky quartz], [rose quartz], [smoky quartz], and [rock crystal].
