Allen, Wright and Clement- Composition Mg 'Si 3 . 385 



Art. XXXYII. — Minerals of the Composition MgSi0 3 ; A 

 Case of Tetramorphism ; by E. T. Allen, Feed Eugene 

 Weight and J. K. Clement. 



Part I. — Formation and Properties of the Four Crystal 



Forms. 



The study of magnesium silicate, to which the following 

 pages are devoted, is part of a general investigation of the 

 pyroxene and amphibole groups, and had for its immediate 

 object the preparation of its several crystal forms, the deter- 

 mination of the conditions under which these may be produced, 

 their relative stability, and the measurement of their acces- 

 sible constants. A combination of accurate data of this kind 

 with the geologic study of the occurrence of minerals in nature 

 constitutes the only reliable basis for the science of mineral 

 genesis. 



The materials for this synthetic work consisted of the pur- 

 est quartz and magnesia which could be obtained. The quartz 

 contained about 0*1 per cent of non-volatile impurities and the 

 magnesia only a few hundredths of one per cent of ferric and 

 calcium oxides. Some slight additional contamination, how- 

 ever, usually resulted from repeated fusion of the same material. 

 In one of the preparations, which had been remelted many 

 times, we found by direct analysis 0*3 per cent of ferric and 

 aluminic oxides and practically no other impurities. 



Calculated 

 Found. for MgSi0 3 . 



Si0 9 59-85$ 60-00$ 



MgO 39-77 40-00 



Al O and Fe„0„ -30 



99-92 100-00 



Four distinct crystal forms of magnesium metasilicate were 

 found to exist and to be reproducible at will in the laboratory. 

 These forms agree in their optical and other physical proper- 

 ties closely with the following minerals : (1) the monoclinic 

 magnesian pyroxene* ; (2) enstatite ; (3) kupfferitef ; (4) a mono- 

 clinic amphibole corresponding to kupfferite; our products 

 form in fact the end members of certain solution series toward 

 which the natural minerals approach, and sometimes almost 



* Discovered by Fouque" and Levy in certain meteorites (Bull. Soc. Min., 

 p. 279. 1881) and by the authors in the Bishopville meteorite. We shall 

 show later that this form differs considerably in its axial ratio c : a from 

 other pyroxenes. 



fSee Hintze, Mineralogie, Bd. II, p. 1196. 



Am. Jour. Scl— Fourth Series, Vol. XXII, No. 131.— November, 1906. 

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