Hainite from Pocos de Caldas, Minas Gerais, Brazil

Hainite occurs in evolved alkaline rocks (nepheline syenites and tinguaites ) of the Pocos de Caldas massif, Minas Gerais, Brazil. It forms pale browni sh yellow anhedral, elongate or tabular crystals. The crystals display a pe rfect {100} cleavage, indistinct and irregular {010} and {001} cleavages, a nd twinning on {100} that may be either simple or lamellar. Its streak is p ale yellow. Hainite is generally poikilitic (except inside the vugs), enclo sing alkali feldspar, nepheline and other minerals. Optically, the mineral is biaxial (+), with alpha 1.662(1), beta 1.663(1), gamma 1.675(1), 2V(z) ( obs.) 30 to 45 degrees, 2V(z) (calc.) 32.4 degrees. Dispersion r < v crosse d, very strong. Its pleochroic scheme is: X colorless, Y colorless to pale yellow, and Z golden yellow. The mineral is triclinic, space group P1, a 9. 584(2), b 7.267(2), c 5.708(1) Angstrom, alpha 89.85(2), beta 101.22(2), ga mma 101.03(1)degrees V 382.50 Angstrom(3), Z = 1. The calculated density is 3.170 g/cm(3). The strongest seven lines of the observed X-ray powder-diff raction pattern [d in Angstrom(I)(hkl)] are 3.081(100)(300), 2.970(43)(021) , 2.635(11)(20 (2) over bar), 1.904(10)(4 (2) over bar 1), 2.312(9)(30 (2) over bar), 2.496(8)(301) and 3.966(7)(20 (1) over bar). The empirical formu lae, derived from results of electron-microprobe analyses, are in good agre ement with the structural formula {Na2Ca[Ti(OH)(2)(Si2O7)2]} {Ca-3(Ca0.7Mn0 .1Fe0.1Ce0.1)}F-2. The simplified formula is Na2Ca5Ti(Si2O7)(2)(OH)(2)F-2.