Falsterite, Ca₂MgMn²⁺₂(Fe²⁺_0.5Fe³⁺_0.5)₄Zn₄(PO₄)₈(OH)₄(H₂O)₁₄, a new secondary phosphate mineral from the Palermo No. 1 pegmatite, North Groton, New Hampshire

Falsterite, ideally Ca₂MgMn²⁺₂(Fe²⁺_0.5Fe³⁺_0.5)₄Zn₄(PO₄)₈(OH)₄(H₂O)₁₄, is a new mineral from the Palermo No. 1 pegmatite in North Groton, Grafton County, New Hampshire, U.S.A., and also occurs at the Estes pegmatite quarry, Baldwin, Cumberland County, Maine, U.S.A. It formed as the result of secondary alteration of primary triphylite and associated sphalerite. The crystals occur as very thin greenish-blue plates and rectangular laths, up to 0.7 mm in length, but no more than a few micrometers thick. Laths are flattened on {010}, elongate along [100], and exhibit lamellar twinning. The mineral is transparent and has a very pale greenish-blue streak, vitreous luster, Mohs hardness of about 2, flexible tenacity, irregular fracture, and perfect cleavage on {010}. The measured and calculated densities are 2.78(3) and 2.837 g/cm³, respectively. It is optically biaxial (–), a = 1.575(10), b = 1.600(5), g = 1.610(5) (white light), 2V_meas = 60(10), and 2V_calc = 63.8. Falsterite exhibits strong dispersion, r > v. The optical orientation is X = b, Y ≈ a, Z ≈ c. Pleochroism is pronounced: X, Z = colorless to very pale yellow, Y = blue green; Y >> X ≈ Z. Electron-microprobe analyses (average of 7), with FeO and Fe₂O₃ apportioned and H₂O calculated on structural grounds, provided: CaO 6.36, MgO 2.13, MnO 8.10, ZnO 18.49, FeO 8.02, Fe₂O₃ 8.90, Al₂O₃ 0.02, P₂O₅ 31.81, H₂O 16.17, total 100.00 wt%. The empirical formula (based on 50 O atoms) is Ca_2.02Mg_0.94Mn²⁺_2.04Fe²⁺_1.99Fe³⁺_1.99Zn_4.05P_7.99 O₃₂(OH)₄(H₂O)₁₄. The mineral dissolves very easily in cold, dilute HCl. Falsterite is monoclinic, P2₁/c, with the unit-cell parameters: a = 6.3868(18), b = 21.260(7), c = 15.365(5) Å, b = 90.564(6), V = 2086.2(1.1) ų, and Z = 2. The eight strongest lines in the X-ray powder diffraction pattern are [d_obs in Å(I)(hkl)]: 12.86(34)(011); 10.675(100)(020); 4.834(12)(102, 1̄12); 4.043(18)(1̄32); 3.220(25)(1̄52); 3.107(14)(044); 2.846(19)(2̄22); 1.596(14)(0·12·4). The structure of falsterite (R₁ = 6.42% for 714 F_o > 4sF) contains edge-sharing chains of Fe²⁺/Fe³⁺O₆ octahedra and corner-sharing chains of ZnO₄ tetrahedra along [100]. These chains are linked to one another by PO₄ tetrahedra, forming a sheet parallel to {010}. Mn²⁺O₆ octahedra and CaO₇ polyhedra also link to this sheet, resulting in a thick slab. The slabs are bridged in the [010] direction by edge-sharing dimers of MgO₆ octahedra, which link to the slabs by sharing edges with ZnO₄ tetrahedra in adjacent slabs. The structures of falsterite and schoonerite, while topologically quite different, share similar components and structural features.