Abstract

Beusite, (Mn, Fe, Ca, Mg)3(PO4)2 is the manganese analogue of graftonite and with it forms a continuous series. The new mineral was found in granite pegmatites in three localities in San Luis Province, Argentina: Los Aleros, Amanda and San Salvador. The mineral from Los Aleros has the following properties: It occurs in rough prismatic crystals up to 30 cm long, interlaminated with lithiophilite. Beusite is monoclinic, 2/m; space group P21/c, a = 8.78, b= 11.52 and c = 6.15 Å,β = 99°25′, a: b:c = 0.7621:1:0.5338; volume 613.7 Å3; Z = 4. Strongest lines in X-ray powder photograph are in Å:3.50 (100), 2.863 (100), 2.708(60), 3.13(40), 1.924(40), 3.01(35), 2.89(30), 2.402(30).

The new mineral is optically biaxial, positive, α= 1.702, β= 1.703, γ = 1.722, 2V = 25°, r>v strong, X = b Zc = 37°. The color is reddish-brown, cleavage: {110} good, {100} fair. The hardness is 5; density 3.702 (meas), 3.715 (calc).

A wet chemical analysis recalculated to 100 percent is: CaO, 4.78; MgO, 2.64; FeO, 14.62; MnO, 36.56: P2O5, 41.40. DTA shows one endothermic peak at 960°C resulting from fusion.

Optical properties, specific gravity and analyses by X-ray fluorescence and atomic absorption show beusite from San Salvador and Amanda to be similar to that from Los Aleros.

The differences in the properties that result from changes in FeO:MnO ratio are offset by MgO and CaO making a chemical analysis necessary to distinguish between graftonite and beusite.

The interlaminated lithiophilite is interpreted as exsolved from a high temperature disordered phase, A wet chemical analysis of this mineral from Los Aleros recalculated to 100 percent is: Li2O, 9.58; Na2O, 0.03; FeO, 13.04; MnO, 23.02; MgO, 6.45; P2O5, 47.88. Optical properties of triphylite-lithiophilite from five localities and of graftonite from Ranquel and Cacique Canchuleta, all from San Luis, are presented.

Beusite is named in honor of Professor Alexey Beus, formerly Professor of Mineralogy and Geochemistry, Moscow Polytechnical Institute.

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