2024 Volume 119 Issue 1 Article ID: 240722
Miyawakiite-(Y), a newly discovered mineral having the ideal formula □Y4Fe2(Si8O20)(CO3)4(H2O)3 of a new structure type, was found in a granitic pegmatite from Suishoyama, Kawamata Town, Fukushima Prefecture, Japan. It is composed of tiny crystals of granular, plate-like, or columnar shape up to 0.8 mm with a pale-yellow color. It is transparent with a vitreous luster, occurring as a secondary mineral in a cavity in an aggregate composed of allanite-(Y) and britholite-(Y). The Mohs hardness is 3-4. Its tenacity is brittle, and its calculated density is 2.95 g·cm−3. Under plane-polarized light, the mineral is pleochroic with O = colorless and E = pale yellow. It is uniaxial (+), with refractive indices of ω = 1.593(3) and ε = 1.654(2) (white light). Dispersion is weak. The empirical formula, calculated on the basis of 4 C and 8 Si atoms per formula unit, is (□0.70K0.30)Σ1.00(Y2.95Dy0.21Yb0.12Er0.10Gd0.08Sm0.02Tb0.01Ca0.33)Σ3.82(Fe2+1.14Mg0.48Mn0.24)Σ1.86Si8C4O31.42[(H2O)2.70(OH)0.30] after electron microprobe, Raman spectroscopic, and crystal structure analyses. The refined unit-cell parameters determined by single-crystal X-ray diffraction are a = 17.53637(9), c = 9.55702(8) Å, V = 2939.02(4) Å3, and Z = 4 in conjunction with the I4/mcm (#140) tetragonal space group. The structure is quite unique: the Y- and Fe-centered polyhedral arrangement with CO3 triangles forms a prismatic framework, with channels leading in the c-axis direction. The SiO4 tetrahedral networks are developed in this channel, forming a zeolite-like framework with K sites inside. The correlation between the OH/H2O and K contents results in a solid-solution series of □Y4Fe2(Si8O20)(CO3)4(H2O)3-KY4Fe2(Si8O20)(CO3)4[(H2O)2(OH)].
The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists
The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists
