Minasgeraisite, a new member of the gadolinite group from Minas Gerais, Brazil

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 (Y_0.72REE_0.41Ca_0.56Bi_0.31)_2.00(Ca_0.45Mn_0.20Mg_0.08-Fe_0.05Zn_0.02Cu_0.01□_0.19)_1.00(Be_1.55B_0.21Si_0.24)_2.00(Si_1.95P_0.08)_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 D_meas is greater than 4.25,D_calc = 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, 2V_Z = 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) ų. Z = 2. Space group is P2₁/a.