386 K T. Allen, F. K Wright and J. K. Clement— 



reach, when practically free from impurities. The conditions 

 of formation and the properties of these four minerals will first 

 be described ; their relations to one another will then be con- 

 sidered in a subsequent portion of the paper. 



1. Monoclinic Pyroxene. — This form of magnesium silicate 

 is the product usually obtained from fusion, though small but 

 variable quantities of enstatite and kupfferite commonly crys- 

 tallize with it. Ebelmen,* who was the first to synthesize the 

 monoclinic magnesium silicate, accomplished it by melting 

 magnesia and silica with boric anhydride. The latter sub- 

 stance served as a flux and was evaporated later by long-con- 

 tinued heating at a high temperature. Hautefeuillef reached 

 the same result by dissolving amorphous silica in molten mag- 

 nesium chloride with partial exclusion of moisture; Stanislas 

 MeunierJ effected its synthesis through the action of silicon 

 chloride and water vapor on metallic magnesium. Haute- 

 feuille, Daubree and other earlier observers mistook this form 

 for enstatite. Their work was done at a time before modern 

 microscopic methods had been developed, and their conclu- 

 sions were therefore based chiefly on chemical and morpho- 

 logical evidence, which misled them, as was proved later by 

 Fouque and Levy§ and by VogtJ who examined the original 

 preparations of Ebelmen and Hautefeuille preserved in the 

 museum of the College de France. 



In our own experiments we have observed the formation of 

 the monoclinic pyroxene in several different ways : (1) from a 

 melt of the same composition ; (2) by heating the glass to a 

 temperature above 1300° ; (3) by heating any of the other 

 crystalline forms ; (4) from the action of molten magnesirfhi 

 chloride or tellurite on amorphous silica ; (5) by recrystallizing 

 magnesium silicate from a flux of magnesium chloride, mag- 

 nesium vanadate, calcium vanadate, or tellurium dioxide. 



(1.) The first method, except under conditions of slow cool- 

 ing, yields crystalline aggregates, usually in radial spherulites, 

 consisting chiefly of the monoclinic form, generally inter- 

 mixed with more or less enstatite and kupfferite ; but if the 

 molten silicate crystallizes at a temperature only slightly below 

 the melting point, an operation which can readily be carried 

 out in the electric resistance furnace, the product is homo- 

 geneous, and consists entirely of the monoclinic form. This 

 is the best method for preparing this substance in quantity, 

 though the crystals are not individually well developed. 



(2.) The product obtained by heating the glass to 1300° or 



*Ann. Chim. Phys. (3), xxxiii, 58, 1851. flbid. (4), iv, 174, 1865. 



JComptes Rendus, xc, 349, 1880. 



§ Synthese des Mineraux et des Roches, p. 107. 



|| Mineralbildung in Schmelzmassen, p. 71. These observers also studied 

 preparations of their own, and Vogt has described the occurrence of the 

 same mineral in blast furnace slags. 



