Mitridatite from the San Luis granitic pegmatite, La Florida, Argentina

An unusual coarse-grained sample of mitridatite from a granitic pegmatite is described. The mineral was found in three pieces of a fragmented nodule similar to 10-20 cm: in the dumps of the San Luis mine, La Florida, Argentina. Similar nodules of primary triphylite-lithiophilite altered to ferrisicklerite and dufrenite are included in quartz from the core-margin association of the rare-element-enriched, spodumene-bearing granitic pegmatite. Mitridatite is very dark green, almost black, has a dull pale green streak and deep ruby red internal reflections. It has very good {100} cleavage, vitreous luster, a density of 3.221 g/cm(3) and a Mohs hardness of 5 to 5.5. The three strongest X-ray powder-diffraction lines [d in Angstrom(I)(hkl)] are: 8.70(100)(200), 2.901(80)(600), and 2.176(60)(800). The refined unit-cell parameters are: a 17.505(1), b 19.327(3), c 11.261(6) Angstrom, beta 96.05(1)degrees, V 3788(1) Angstrom(3). Optically, mitridatite is transparent, with X pale green, almost colorless, Y = Z dark reddish brown, biaxial (-), 2V similar to 10 degrees, alpha 1.775(5), beta 1.843(5), gamma 1.844(5), and very strong dispersion. The chemical composition gives: CaO 17.16, Mn2O3 1.34, Fe2O3 35.26, P2O5 32.26, H2O+ 8.41, H2O- 0.56, Rem. 4.51. which gives Ca-6.01 (H2O)(6)[Fe8.673+Mn0.33O6.15(PO4)(8.92)].3.17 H2O. The mitridatite is considered to have formed by hydrothermal replacement of primary triphylite.