Minasgeraisite, ideally Y2CaBe2Si2O10, is a new member of the gadolinite group, occurring in the form of 0.2- to 1.0-mm-diameter rosettes coating and intergrown with milarite, albite, quartz, and muscovite in a late-stage complex granitic pegmatite. The empirical formula, on the basis of 10 oxygen atoms, is (Y0.72REE0.41Ca0.56Bi0.31)2.00(Ca0.45Mn0.20Mg0.08-Fe0.05Zn0.02Cu0.010.19)1.00(Be1.55B0.21Si0.24)2.00(Si1.95P0.08)2.03O10. Only several hundred milligrams are known to exist. The mineral is lavender to lilac purple and has a sheaflike habit, with a grain size of less than 3-5 μm. It is not magnetic and not fluorescent under either SW or LW ultraviolet radiations; Mohs hardness is estimated at 6 to 7 Dmeas is greater than 4.25,Dcalc = 4.90 g/cm3, luster 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 (+), with α = 1.740(4), β = 1.754(4), γ = 1.786(4), γ - α = 0.046, 2VZ = 68° (average), X = colorless, Y = pale grayish yellow, Z = lavender purple, with Z > Y > X. Dispersion is r > v, very weak. Least-squares refinement of 114-mm-diameter Gandolfi film data gives a = 9.833(2) Å,b= 7.562(1) Å, c = 4.702(1) Å, β = 90.46(6)°, V = 349.60(14) Å3. Z = 2. Space group is P21/a.

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