An unusual coarse-grained sample of mitridatite from a granitic pegmatite is described. The mineral was found in three pieces of a fragmented nodule approximately 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 Aa(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) Aa, beta 96.05(1) degrees , V 3788(1) Aa 3 . Optically, mitridatite is transparent, with X pale green, almost colorless, Y = Z dark reddish brown, biaxial (-), 2V approximately 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, Mn 2 O 3 1.34, Fe 2 O 3 35.26, P 2 O 5 32.26, H 2 O (super +) 8.41, H 2 O (super -) 0.56, Rem. 4.51, which gives Ca (sub 6.01) (H 2 O) 6 [Fe (super 3+) (sub 8.67) Mn (sub 0.33) O (sub 6.15) (PO 4 ) (sub 8.92) ] . 3.17 H 2 O. The mitridatite is considered to have formed by hydrothermal replacement of primary triphylite.

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