Abstract
Two new minerals have been found in San Benito County, California, that are structurally and chemically related to joaquinite. A combination of electron and ion microprobe analyses of strontiojoaquinite yields the formula (Na2.28Li0.11Fe1.44☐0.17)☐2Ba4Ti4[O1.27(OH)2.73]Sr4.01REE0.01[Si4O12]4·1.62H2O. The virtual absence of REE indicates that this is the Sr analog of joaquinite.
X-ray single crystal study showed the mineral to be monoclinic with the same submicroscopic twinning and one dimensional disorder exhibited by joaquinite. Several reflections in the X-ray powder pattern show that the space group must be P2, Pm, or P2/m, as compared to the C2 of joaquinite. Refined cell dimensions are a 10.516(6), b 9.764(5), c 11.87(1)Å, β 109°17(4)′ Z = 1. The strongest lines in the X-ray powder pattern are 2.801(100)(004,311), 2.967(72)(023), 3.001(48)(113), 2.923(45)(301), 2.611(42)(401,402), 2.432(40)(312), and 4.30(35)(121).
The pseudo-orthorhombic, bipyramidal crystals have the forms {110}, {111}, and {001}. All crystals are zoned with a joaquinite core. Strontiojoaquinite is green, yellow-green, and less commonly yellow-brown. Hardness (Mohs) is 5½ cleavage {001}, good. Calculated density is 3.68 g/cm3. It is optically biaxial, positive, 2V 35° to 45° with strong dispersion of the optic axes r > v. Refractive indices are α 1.710(2), β 1.718(2), γ 1.780(3). Optical orientation and axial colors: X (colorless):a = 19°, Y (colorless) = b, Z (yellow) = c; absorption X = Y < Z.
A combination of electron and ion microprobe analyses of bario-orthojoaquinite yields the formula (Na0.10Fe3.61Mn0.24)☐2Ba4(Ti3.88Al0.12)(Ba2.88Sr0.88Ca0.08Al0.19O4[Si4O12]·2H2O. The absence of REE and the excess Ba indicates that this mineral is the Ba analog of joaquinite.
X-ray single crystal study showed it to be orthorhombic, space group Ccmm, CC2m, or Ccm21, a 10.477(5), b 9.599(1), c 22.59(1)Å, Z = 2. The strongest lines in the X-ray powder pattern are 2.997(100)(224), 2.953(95)(132), 2.824(90)(008), 5.64(70)(004), 2.935(70)(117), 4.30(62)(203), 3.203(50)(223), and 2.602(50)(401). Crystals are up to 8 mm in diameter, in the form of steep bipyramids {111} truncated by the basal pinacoid, and are yellow-brown with a vitreous luster. Hardness (Mohs) is 5½. Cleavage {001}, good. Measured density is 3.959(2) g/cm3 (calculated, 3.962 g/cm3). It is optically biaxial, positive, 2V 10° to 15°, with strong dispersion of the optic axes r > v. Refractive indices are α 1.735(2), β 1.737(2), γ 1.80(1). Optic orientation and axial colors: X(very pale yellow) = a, Y (pale yellow) = b, and Z (yellow) = c, absorption X < Y << Z.
These two new minerals, along with others previously described, allow names of the joaquinite group minerals to be based on the symmetry and occupancy of the REE position: REE-bearing members are joaquinite and orthojoaquinite; Sr-bearing, strontiojoaquinite and strontio-orthojoaquinite; and Ba-bearing, bario-orthojoaquinite.
