Note:  Do not rely on this information. It is very old.

Felspar

Felspar, the name of a group of closely related and intimately associated mineral species, the most abundant of rock-forming minerals. They are silicates of aluminium together with silicate of potash, soda, or lime, often coloured by iron-oxides; their hardness is about 6, or that of glass; and. their specific gravity ranges from 2'54 to 276. They are divided into the Orthoclastic, Oblique, or Monoclinic, which yield cleavage forms with some pairs of faces at right angles, and the Plagioelastic, Anorthic, or Triclinic, which do not; or, chemically, into the more acid, containing over 60 per cent. of silica, and having a specific gravity not exceeding 2-7, and the more basic, containing less than 60 per cent, of silica, and having a specific gravity over 2-7. The acid felspars are the more insoluble in acids and the more infusible, and are especially abundant in the plutonic rocks, in which they are associated with free quartz, hornblende and mica. The basic felspars are more characteristic of the volcanic rocks, in which they are associated with augite, but which is afterwards dyed, printed, or otherwise adapted to its purpose. The most extensive felting industry is the manufacture of hats. A coarser. kind of felt is used for various mechanical appliances, such as covering steam boilers, polishing wheels, pianoforte-hammers, etc. In such cases cow-hair is often the chief ingredient. The "asphalted felt" used in roofing is felt of a coarse quality which has been immersed in asphalt, pitch, or coal-tar.