1870.] Mineralogy. 417 



9. MINERALOGY. 



To determine with accuracy the true nature of those colouring 

 matters which, although present in extremely minute proportions, 

 nevertheless impart to many of our gems much of their value and 

 their beauty, is a task which has often sorely taxed the resources of 

 the mineralogical chemist. It is still an open question, for example, 

 whether tho green tint of the emerald is due to the chromium which 

 the mineral contains, as originally suggested by Vauquelin, or whe- 

 ther it is referable to the presence of certain organic constituents de- 

 tected in the emerald by M. Lewy. Towards a solution of this 

 question a contribution has recently been published by M. J. Bous- 

 singault,* who has had at his disposal more than a couple of pounds 

 of the amorphous emerald from the celebrated mines of Muzo, near 

 Santa-Fe-de-Bogota, in New Granada. In the carbonaceous schist 

 two kinds of emerald are found — known respectively as canutillos 

 and morallones — the former being crystallized, transparent, and of 

 great value, while the latter are uncrystallized, less translucent, much 

 fissured, and comparatively valueless : it was, of course, a kilo of the 

 latter that found its way to Boussingault's laboratory. Whilst Lewy 

 has asserted that the emerald becomes opaque and colourless by cal- 

 cination, Boussingault finds that nothing of the kind takes place 

 with the morallon: the green colour is preserved at a bright red 

 heat, but the mineral suffers a loss of nearly 2 per cent., consisting 

 partly of water and partly of carbon. The water is regarded as ex- 

 isting in a state of chemical combination, since it is not expelled below 

 a red heat, while the carbon is probably derived from the admixture 

 of small particles of the schist in which the mineral occurs, and from 

 which not even the finest crystals are altogether free. On the whole, 

 Boussingault agrees with Wohler, G. Bose, and Vauquelin, in at- 

 tributing the colour of the emerald to the presence of oxide of 

 chromium. 



It is now several years since Tschermak first enunciated his cele- 

 brated theory on the constitution of the felspars, in which he showed 

 that the several species may be regarded as isomorphous mixtures of 

 the two extreme types — albite and anorthite. Some difficulty has 

 lately arisen through Vom Bath's analysis of a Norwegian felspar, 

 regarded as Labradorite, whose composition did not fit in with the 

 theory. Tschermak thereupon called the analysis in question, and 

 asserted that no labradorite could exist free from soda. This pro- 

 position has called forth some ingenious observations on isomorphism 

 by Dr. Kenngott.f 



* " Analyse de l'Emeraude Morallon des Mines de Muso, Nouvelle- Grenade." — 

 1 Annales de Chiniie et de Physique,' 1870, p. 328. 



t "Ueber den Isomorphisoms verschieden zusammengesetzter Korper." — 

 1 Journ. f. prakt. Chemie/ 1870, p. 77. 



