of Mineral Species. 1 03 



IX. Changes in some of the so-called Earthy Minerals, and others. 



The explanation of many of the cases enumerated above, de- 

 pends upon the ordinary laws, active in our chemical laborato- 

 ries. Carbonates are changed into sulphates, metallic substances 

 are oxidized, copper is replaced by iron : in general weaker affi- 

 nities give way to stronger ones. The conversion of sulphates 

 into carbonates, and other cases, may perhaps depend upon some 

 process of mutual decomposition, in which one of the products 

 has been subsequently removed ; but the specimens preserved in 

 collections do not usually present any explanations of the facts 

 which they furnish. We must endeavour to ascertain the causes 

 which have contributed towards successive alterations in the 

 chemical composition of minerals, by observing their natural re- 

 positories, veins and beds, and mountain masses, exposed to the 

 action of the atmosphere and of water, and to the mutual re- 

 action of the mineral species of which they are constituted. 



One of these examples, where the cause of a change in ap- 

 pearance is not so palpable, is the well-known one of the substance 

 usually named the Grey Andalusite. Its specific gravity alone, 

 being above 3.5, while that of the real andalusite never exceeds 

 3.2, would be sufficient to prove them to belong to different 

 species. But Professor MOHS has found the grey crystals actual- 

 ly to consist of a great number of small individuals of disthene, 

 with an easy cleavage, whenever they are large enough to be dis- 

 tinguished from others, and lying in different directions through- 

 out the mass. Both minerals are found in nodules of quartz en- 

 gaged in mica-slate. From the analysis by ARFVEDSON, it ap- 

 pears that disthene is a compound of one atom of silica and two 

 of alumina, or AF Si. Andalusite contains about 83 per cent. 



