232 A TREATISE ON METAMORPHISM. 



DIASPORE. 



occurrence. — Diaspore is especially found in the serpentine- or chlorite- 

 bearing schists and gneisses and in dolomites. In these rocks it is frequently 

 associated with corundum. Diaspore has been recorded as a constituent 

 of granite, nepheline-syenite, and basaltic rocks. As noted on other pages, 

 for the alterations of various minerals in the zone of katamorphism, espe- 

 cially the belt of weathering, it may be produced as one of the alteration 

 products of the following minerals: Biotite, corundum, gibbsite, haliynite, 

 muscovite, nephelite, noselite, phlogopite, scapolites, sodalite. 



Alterations. — No alterations of diaspore are recorded. However, it is 

 probable that where diaspore is deposited in sedimentary rocks and is deeply 

 buried, so as to undergo alteration in the zone of anamorphism, it passes 

 into corundum (rhombohedral, sp. gr. 4.025); or, like corundum, unites 

 with other bases to produce such minerals as spinel (isometric, sp. gr. 3.8) 

 sillimanite (orthorhombic, sp. gr. 3.235), and cyanite (triclinic, sp. gr. 3.615) 

 muscovite (monoclinic, sp. gr. 2.88); margarite (monoclinic, sp. gr. 3.035) 

 and zoisite (orthorhombic, sp. gr. 3 31). So far as the hydrous minerals 

 muscovite, margarite, and zoisite are formed, the water may have been 

 derived from the diaspore, which contains more water, and thus their forma- 

 tion be really a process of dehydration and silication. For these supposed 

 reactions the equations may be : 



(1) 2A10(OH)=AU0 3 +H 2 0-k. 



(2) 2A10(OH) +MgC0 3 =MgAl 2 4 +C0 2 +H 2 0-k. 



(3) 2A10(OH)+Si0 2 =Al 2 Si0 5 +H 2 0-k. 



(■i) 6A10(OH)+6Si0 2 +ICC0 3 =2H 2 KAl 3 Si 3 12 +C0 2 +H 2 0-k. 



(5) 4A10(OH) +2Si0 2 +CaC0 3 =H 2 CaAl 4 Si 2 12 +C0 2 +H 2 0-k. 



(6) 6A10(OH)+6Si0 2 +4CaC0 3 =H 2 Ca 4 Al 6 Si 6 6 26 -r-4C0 2 +2H 2 0-k. 



Supposing that all the compounds in the first member of the equation are 

 solids, and that the liberated C0 2 and H 2 escape, the decrease in volume 

 for corundum (1) would be 28.18 per cent; for spinel (2), 40.39 per cent; 

 for sillimanite (3), 13.52 per cent; for cyanite (3), 22.61 per cent; for 

 muscovite (4), 54.21 per cent; for margarite (5), 14.08 per cent; for zoisite 

 (6), 29.44 per cent. All take place with absorption of heat. Therefore 

 all of these reactions are characteristic of the zone of anamorphism. 



-Limonite as a mineral is produced either as an original 

 chemical precipitate or by the alteration of other minerals. It is not, 



