New Mexico Mineral Symposium — Abstracts

Bixbyite, (Fe,Mn)³⁺₂O₃, occurs in Tertiary lithophysal rhyolite at a number of localities in the Thomas Range, Juab County, Utah and in the Black Range, Sierra and Catron Counties, New Mexico. Associated minerals may include hematite, quartz, sanidine, beryl, topaz, pseudobrookite, and rare amounts of other minerals.

Bixbyite is an example of an ion-deficient fluorite structure, with one-fourth of the anion sites vacant: pure a-Mn₂O₃ (`partridgeite') is orthorhombic, but all natural bixbyite is cubic because of the stabilizing effects of even a small amount of ferric iron (approximately >0.75 mol% Fe).

Bixbyite from two localities in the north end of the Thomas Range (Cubic Claim and Pismire Knolls) and from five localities in the Black Range (Alexander, 'the clearing' near Boiler Peak, Easter Canyon, Inman South, and Paramount Canyon) were examined in this study. Bixbyite from the Thomas Range (TR) is Fe-dominant and shows only minor compositional variation with a small but distinct Mn-enrichment at the cessation of crystallization. Substitution of other elements is relatively minor (Table 1). Bixbyite crystals from the Black Range tin district (BRTD), however, generally are Mn-dominant and often are intergrown with braunite, Mn²⁺Mn³⁺₆SiO₁₂. A crystal of bixbyite from `the clearing' contains <0.1 wt % SiO₂ except for the presence of a few small growth zones with up to 2.1 wt% SiO₂. The crystal shows no Mn-enrichment trend. A crystal from Easter Canyon contains small amounts of braunite and also shows no Mn-enrichment. Three crystals from Alexander Cienega show discrete and major amounts of intergrown braunite (to 9.9 wt% SiO₂). Three crystals from Inman South show variable amounts of intergrown Fe-dominant bixbyite and braunite. Five crystals of bixbyite from Paramount Canyon show discrete and well-resolved alternating growth zones of bixbyite and braunite. Zoning patterns are intricate and show periods of rapid alteration as well as areas of single-phase braunite or bixbyite. Compositions of bixbyites from the literature as well as from this study are shown in Fig. 1.

Braunite previously had not been reported from the BRTD. The presence of braunite in most of the BRTD samples and its absence in the TR samples is interpreted to indicate significant differences in the degree of oxidation during crystallization and also in the activity of SiO₂. It should be noted that some minor elements such as Cu, Zn, and REEs have not previously been reported from bixbyite. The bixbyite from the TR contains more Al (average of 1.9 wt % Al₂O₃) than that from the BRTD (average of 0.4 wt% Al₂O₃). TiO₂ contents in bixbyite are comparable between the two areas. According to the published phase-equilibria diagrams of the system Fe₂O₃-Mn₂O₃, bixbyite from the TR is stable below about 970°C whereas that of the BRTD is stable at a slightly lower temperature (960°C). Both are stable above 650°C.

The nomenclature of bixbyite needs to be addressed by the CNNMMN IMA. Type material from Utah is Fe-dominant whereas most of that from New Mexico is Mn-dominant. Extremely Mn-rich bixbyite occurs in manganese deposits elsewhere in the world, and the name partridgeite has been used unofficially for the orthorhombic end-member Mn₂O₃.

×