Abstract

Mineral incompatibilities (based on a complete review of natural occurrences), relevant experimental data, and a computer program ("REACTION") have been used to model the probable stabilities of beryllium-bearing minerals in the system CaO-BeO-SiO 2 -P 2 O 5 -F 2 O (sub -1) , in terms of the chemical potentials of the Lewis acid components P 2 O 5 and F 2 O (sub -1) . The somewhat complicated phase relations are greatly simplified if it is assumed that the gangue minerals (in this case, mainly quartz plus fluorite, CaF 2 , or fluorapatite, Ca 5 (PO 4 ) 3 F) are present "in excess," so that only one Be-ore mineral is stable in a given region of the 3mu F 2 O (sub -1) -- mu P 2 O 5 diagram. The resulting diagram shows that gugiaite, Ca 2 BeSi 2 O 7 , and other Be-bearing calc-silicates in skarns, are stable only under relatively high temperature or basic conditions. With increasing activities of the acid components F 2 O (sub -1) and P 2 O 5 , phenakite, Be 2 SiO 4 , becomes stable with fluorite or fluorapatite in typical greisen and vein assemblages (especially in replaced carbonate rocks). Under the very high P 2 O 5 , low F 2 O (sub -1) conditions characteristic of the late stages of certain pegmatites, hurlbutite, CaBe 2 (PO 4 ) 2 , becomes stable, whereas under high activities of both P 2 O 5 and F 2 O (sub -1) , herderite, CaBePO 4 F, becomes stable in very late-stage pegmatites and in greisens. Under conditions more F 2 O (sub -1) -rich than those normally encountered in nature, the phase CaBeF 4 might occur (especially in fluid inclusions).When Al 2 O 3 is added to the model system, the resulting diagrams show the breakdown reactions of beryl, Be 3 Al 2 Si 6 O 18 , to phenakite plus topaz in the fluorite stability field, or to herderite plus topaz in the fluorapatite stability field.

First Page Preview

First page PDF preview
You do not currently have access to this article.