Abstract

A geochemical association among tin, fluorine, and boron-bearing minerals in skarns and greisens has long been recognized (e.g., cassiterite with fluorite and calcium stanno-and borosilicates in skarns and with fluorite, topaz, and tourmaline in greisens), yet the calcium-tin borate nordenskioeldine, CaSnB 2 O 6 , is extremely rare. The explanation for this apparent anomaly is that nordenskioeldine is incompatible with quartz, the stable assemblage being cassiterite plus danburite (i.e., CaSnB 2 O 6 + 2SiO 2 --> SnO 2 + CaB 2 Si 2 O 8 ). Similarly, cassiterite is incompatible with wollastonite, because of the intermediate phase malayaite (i.e., SnO 2 + CaSi 3 --> CaSnSiO 5 ). The two incompatibilities thus defined are sufficient to fix the topology of the multisystem calcite-fluorite-cassiterite-nordenskioeldine-malayaite-danburite-datolite-wollastonite-quartz on schematic diagrams involving the chemical potentials (mu 's) of H 2 O, B 2 O 3 , CO 2 , and F 2 O (sub -1) .These diagrams show that in the presence of typical gangue minerals, malayaite and other Sn-bearing calc-silicates, are stable only under the realtively low potentials of acid volatiles typical of skarns. At the higher potentials of B 2 O 3 , CO 2 , and F 2 O (sub -1) typical of greisens, cassiterite becomes stable, except under SiO 2 -deficient and B 2 O 3 -rich conditions, when nordenskioeldine may appear. The participation of datolite in the above equilibria is restricted to relatively low-temperature, basic, CO 2 - and F 2 O (sub -1) -depleted environments, and datolite is thus excluded from greisens.

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