Stone or

STONE) or for their play of iridescent colours (see LABRADORITE and AVENTURINE).

With the exception of microcline, anorthoclase and hyalophane,

all the felspars have been synthetically prepared. Orthoclase, celsian and albite require the use of fluxes for their preparation. Anorthite melts sharply at 1,55o° C, but the solid solutions of albite and anorthite exhibit a melting interval. Felspars readily suffer chemical alteration, the commonest products being kaolin, sericite, various zeolites and calcite. In dynamic metamorphism the anorthite-rich plagioclases break up into albite and a group of calcium aluminium silicates including prehnite, zoisite and grossular, this assemblage being referred to as saussurite.

Albite is the most stable of the felspars under the action of

weathering and is commonly found in detrital sediments. Celsian and hyalophane are rare barium—felspars, the latter represented by solid solutions of celsian with orthoclase, the limits being arbitrarily fixed as follows: hyalophane Celsian barium-orthoclase Celsian and hyalophane occur rarely as contact minerals in dolomites, nota bly at Jakobsberg in Sweden and the Binnental in Oberwallis. Tabular crystals of celsian, twinned on the Carlsbad law, have been artificially prepared. Celsian melts at above 1,70o° C.

The felspars are the most important group of rock-forming

minerals. They constitute practically 6o% of igneous rocks and serve as a basis of classification of these rocks. In sediments only the alkali-felspars (orthoclase, microcline, albite and oligoclase) attain any importance as rock constituents, but in metamorphic rocks they are all—with the exception of anorthoclase and the barium felspars—widely distributed. Felspar (orthoclase) is used industrially in the manufacture of porcelain and it also forms an important constituent of many ceramic glazes. Orthoclase has also been used as a source of potash. More detailed information on the individual felspars is given under ALBITE, AMAZON-STONE,