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Mg3(BO3)(F,OH)3
Hexagonal
Fluoborite was first reported from Sterling Hill, the second occurrence of the species, by Bauer and Berman (1929b, 1929d); their results were summarized by Palache (1935).
Fluoborite is white to colorless, occurring in subparallel fibrous masses, porcelaneous aggregates, and white fluffy aggregates and, in the Franklin Marble, as euhedral crystals. Much fluoborite is soft and flexible, and some is pulverent. The density is 2.92 g/cm3; it is uniaxial, negative, with w = 1.547 - 1.548 and e = 1.522 - 1.518 (Bauer and Berman, 1929b). Fluoborite is fluorescent in shortwave ultraviolet with a light yellowish-white color. It has physical similarity to sussexite and some fibrous amphiboles.
Fluoborite is a magnesium boron fluorine mineral. Some hydroxyl substitutes for fluorine. The substitution of octahedral cations for Mg is ambiguous; modern analyses are lacking, and those of Bauer were done using admittedly impure material. Several of those analyses are given by Palache (1935). Little is known of the exact composition of local fluoborite.
Fluoborite was first reported from Sterling Hill by Bauer and Berman (1929b) in two types of occurrences. It was found associated with rhodochrosite, pyrochroite, and zincite in a vein in franklinite-willemite-calcite ore. Here it occurs as capillary fluffy aggregates, occurring with mooreite, and is the last-formed mineral in the assemblage. Fluoborite was also found as subparallel fibrous aggregates, 3-5 centimeters in length, intimately associated with carbonates and bright orange zincite from Sterling Hill.
Fluoborite was found in small 1-3 mm crystals, associated with calcite, graphite, and possible chondrodite at the Franklin Quarry in Franklin.
Fluoborite is also found elsewhere in the Franklin Marble, specifically at the Lime Crest Quarry in Sparta and in l-cm crystals at the Bodnar Quarries in Hamburg (Kearns, 1975), both outside of the Franklin-Sterling Hill Area as defined herein.
Although specimens labeled as being from the Franklin Mine are found in many collections, their provenance is not beyond question; such material has not yet been shown to be unambiguously from the Franklin Mine.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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