Minerals of the Composition MgSiO z . 437 



gate is found in nature, therefore, where it might be supposed 

 to have had sufficient time for attaining equilibrium, is not a 

 sufficient reason for assuming that it has actually reached the 

 state of greatest stability. 



Summary. 



1. There are four crystal forms of magnesium metasilicate: 

 (I) a monoclinic pyroxene, haying the characteristic prismatic 

 cleayage (92° and 88°), and a similar axial ratio a : h, but a ratio 

 o ; a, which yaries widely from that of the pyroxenes; (II) an 

 orthorhombic pyroxene identical with enstatite and optically 

 very similar to (I); (III) a monoclinic modification correspond- 

 ing to an amphibole in its optical properties; (IY) an ortho- 

 rhombic form, optically also an amphibole and resembling (III) 

 very closely. These forms, with the exception of (III), have 

 been prepared in pure condition or with only traces of other 

 forms; their optical properties have been studied, their specific 

 gravities have been determined, their behavior on heating has 

 been investigated, and, in the case of (I), measurable crystals 

 have been obtained. 



Forms (I) and (II) occur in nature, usually in mixed crystals 

 with ferrous silicate, and it is quite probable that the same is 

 true of (III) and (IY). 



2. (I) is formed in pure condition by crystallizing a melt a 

 little below the melting point (1521°), which may be readily 

 accomplished by cooling slowly. Measurable crystals are 

 obtained by heating any form of the metasilicate to about 1000° 

 in molten magnesium chloride traversed by a stream of dry 

 hydrochloric acid gas. Calcium vanadate, magnesium vanadate, 

 magnesium tellurite, and other fiuxes yield crystals which are 

 not so well developed. Ml the other forms of magnesium 

 silicate pass into (I) at temperatures between 1150° and 1300°, 

 depending on the crystal form taken. (II). This form (ensta- 

 tite) crystallizes at lower temperatures than (I); the purest prep- 

 aration, containing only traces of other forms, is made by heat- 

 ing the glass of the same composition at a temperature between 

 1000° and 1100°; large crystals of enstatite (up to 23 mm in 

 length) were obtained in silicate (magmatic) solutions. (Ill) 

 forms in very small quantities by rapidly cooling the melt: there 

 is also evidence that it forms from (IY) when the latter is heated 

 with water in a bomb to a temperature of 375°-475°. When 

 an aqueous solution of magnesium- ammonium chloride, or 

 magnesium chloride and sodium bicarbonate is heated with 

 amorphous silica or sodium silicate, this substance is probably 

 formed, though the crystal fibers are too small to decide whether 

 the product is identical with (HI) or with (TY). (IY) is obtained 

 by heating the molten silicate high above the melting point, say 

 to 1600°, and then cooling rapidly in air; it cannot be formed 

 by heating the glass. 



