Aristarainite is a new hydrous sodium magnesium borate found at the Tincalayu borax deposit, Salta, Argentina. It occurs as small crystals in a matrix of borax, kernite, and tincalconite.

The new mineral is monoclinic, 2/m; space group, P21a; a 18.869(2)Å, b 7.531(1)Å, c 7.810(1)Å, β 97°43.8(5)'; a:b:c = 2.506:1:1.037; cell volume 1099.8(2)Å3 cell content 2[Na2O · MgO · 6B2O3 · 10H2O]. The strongest lines in the X-ray powder photograph are (in Å): 7.74; I = 100; 5.40, 11; 3.869, 12; 3.037,13; 2.579,19; 2.400,10. The mineral occurs in euhedral crystals elongated on [010] averaging 0.3 mm in length, and 0.05 by 0.05 mm in cross section. It also occurs in tabular crystals and as aggregates up to 1 mm. Common forms are c{001}, a{lOO}, m{l1O}; less common are n{21O}, x{2O1}, r{{3O1}, s{{4O1}, p{211}, o{{211}.

Aristarainite is colorless, with vitreous luster. In short-wave U. V. it shows cream-white fluorescence andslight phosphorescence. Cleavage: {001} and {100} perfect, {110} fair. The hardness is 3 1/2, the specific gravity 2.027(5) (meas), 2.102(calc). Optically biaxial ( - ), α 1.484(1), α 1.498(1), γ 1.523(1), 2V 70μ, r > v weak, X = b, Y Λ c = 38μ in μ, Z Λ a = 46° in β.

A chemical analysis yields: Na2O 7.3, K2O 1.05, MgO 5.65, B2O3 59.5, H2O 25.7, insol. 0.5, total 99.7 percent. The mineral is insoluble in water (66°C). The infrared spectrum shows broad absorption in regions 1100-1300 and 800-1100 cm−1, indicating both 3− and 4− coordinated B; it also indicates both OH and H2O.

The name is for Lorenzo Francisco Aristarain, Professor of Economic Geology, Universidad Nacional de la Plata, Argentina, in recognition of his contributions to borate mineralogy.

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