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Ch. 7: Garnet

Ch. 7: Garnet Page of 149 Ch. 7: Garnet Text size:minus plus Restore normal size   Mail page  Print this page
Besides the bases indicated in the above list as characteristic of the several kinds of garnet there are in all of them minor con­stituents as to which a few words are required. Amongst these special mention must be made of chromium sesquioxide to which the emerald hue of some demantoid has been attributed and which is also found in much pyrope. Traces of other oxides likewise occur in several of the remaining varieties. So also in those garnets in which iron is a large and essential constituent, this metal is found to exist, at least generally, in two states of oxidation indicated by the terms ferrous for the protoxide and ferric for the sesquioxide. Thus, almandine, along with 32 or 33 per cent, of ferrous oxide, generally contains 2 or 3 per cent, of ferric oxide, while in demantoid a little ferrous oxide is associated with about 30 per cent, of ferric oxide. Then, too, manganous oxide, which is the chief protoxide in spessartite, occurs in small proportion in all or nearly all the remaining varieties of garnet. These and other minor peculiarities of composition are all covered by the chemical expression of " garnet formula." This formula contains three molecules of silicacombined with cne molecule of a sesquioxide
such asorand with three molecules of a mon-
oxide, such as FeO, MnO, CaO or MgO. If any of the above sesquioxides be represented by the formula and the above
monoxides by MO, then the general expression for garnet becomes One character common to all garnets save uvarovite is their fusibility before the blow pipe ; they thus yield a vitreous mass which is of much lower density than that of the original garnet hefore fusion. As the members of the garnet group differ so widely from each other in appearance, hardness,
Ch. 7: Garnet Page of 149 Ch. 7: Garnet
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