fae 
’ 
> G. H. Williams—Pyroxene and Hornblende. 259 
| q Art. XXXTIL—On the Paramorphosis of Pyroxene to Hornblende 
‘ in Rocks; by Gro. H. WILLIAMS. 
Ir has long been recognized that pyroxene and hornblende. 
_ are two different crystallographic forms of essentially the same 
_ molecule, of which the former is most stable at high, the latter 
at ordinary temperatures. As early as 1824 Mitscherlich and 
_ Berthier melted tremolite at the potteries of Sévres and found 
_ that on slowly cooling the mass crystallized in the form of 
_ augite.’ In 1831, G. Rose repeated the same operation with 
 actinolite from the Zillerthal with the same result. The con- 
_ Stancy of this change at temperatures of fusion has also been 
_ abundantly verified by the more recent experiments of Profes- 
_ Sors Fouqué and Michel-Lévy of Paris, who found it impossible 
_ to artificially produce hornblende. In every case where this 
_ Mineral was employed, it changed at high temperatures to 
_ augite.’ In natural lavas hornblende crystals are frequently 
_ observed to have undergone alteration around their outer edge 
_ Into an aggregate of minute augite and magnetite crystals.‘ 
_ This is probably due to a caustic action of the magma, which, 
_ ©n account of an increase of temperature, partially dissolves the 
_ completely crystallized hornblende individual. The same 
- Possessed the external form of augite.* Some of these were 
| Pogg. Ann., xxii, p. 338, 1831. 2 Ib, 
‘ Synthése des minéraux et des roches, Paris, 1882, p. 78. 
s K. Oebbeke: Neues Jahrbuch fiir Min., ete. I Beil. Bd., p. 474. 
7 ter and Hussak: Neues Jahrbuch fiir Min., ete., 1884, i, p. 24. i 
Pogg. Ann., xxii, 1831. ‘Tb, xxxii, p. 617, 1834. 
Ax. Jour. Sct—Tuirp Series, Vou. XXVIII, No. 166.—Oor., 1884. 
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