Minasgeraisite, Y2CaBe2Si2O10, a 9.833(2), b 7.562(1), c 4.702(1) A, beta 90.46(6)o, P21/a, Z = 2, is a new member of the gadolinite group. Found in quantities of only several hundred mg in the Jaguaracu pegmatite, Minas Gerais, Brazil, it occurs as 0.2 to 1.0 mm-diameter rosettes, coating and intergrown with milarite, albite, quartz and muscovite. The mineral is lavender to lilac purple and has a sheaf-like habit, with a grain size of <3-5 mu m. It is not magnetic and not fluorescent under either short-wave or long-wave ultraviolet radiations; H. approx 6-7, D(meas.) >4.25, D(calc.) 4.90 g/cm3, lustre is earthy to subvitreous and the streak is pale purple. Minasgeraisite is slowly soluble in common acids. The mineral has one excellent cleavage, (100) by analogy with gadolinite, and another good cleavage (001). Minasgeraisite is biaxial positive with alpha (colourless) 1.740(4), beta (pale greyish yellow) 1.754(4), gamma (lavender purple) 1.786(4), 2Vgamma 68o. Strongest powder XRD lines include 3.11(100), 2.830(100), 2.540(90), 1.768(35) A.-J.A.Z.
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Minasgeraisite, a new member of the gadolinite group from Minas Gerais, Brazil.