Fluorapophyllite-(NH4), NH4Ca4(Si8O20)F•8H2O, a new member of the apophyllite group from the Vechec quarry, eastern Slovakia

The new mineral fluorapophyllite-(NH4), ideally NH4Ca4(Si8O20)F center dot 8H(2)O, was found at the Vechec andesite quarry located near Vechec village, Vranov nad Topl'ou Co., Pregov Region, Slovak Republic. It occurs in cavities of quartz-illite-saponite-tobelite xenolith embedded in pyroxene andesite. Fluorapophyllite-(NH4) is associated with calcite, tridymite, pyrite, chabazite-Ca and heulandite-Ca. It forms clusters, aggregates or crystalline crusts consisting of individual, well-developed crystals up to 4 mm in size, exhibiting the forms {110}, {101} and {001}. The mineral is colourless to light pink and translucent with white streak and vitreous to pearly lustre; it is nonfluorescent under ultraviolet radiation. The Mohs hardness is similar to 4% to 5, tenacity is brittle, fracture is irregular, and there is perfect cleavage on {001}. The calculated density is 2.325 g cm(-3). Fluorapophyllite-(NH4) is optically uniaxial (+) with omega = 1.5414(5) and epsilon = 1.5393(8) (lambda = 589 nm). It is non-pleochroic. The empirical formula (based on 29 O + F apfu) is [(NH4)(0.55)K0.32Na0.07Ca0.06](Sigma 1.00)(Ca4.01Mg0.02)(Sigma 4.03) Si7.97O20[F-0.84(OH)(0.16)](Sigma 1.00)center dot 8H(2)O. Fluorapophyllite-(NH4) is tetragonal, space group P4/mnc, a = 8.99336(9) angstrom, c = 15.7910(3) angstrom, V= 1277.18(3) angstrom(3) and Z = 2. The seven strongest X-ray powder diffraction lines are [d(obs) in angstrom,(I,%)(hkl)1: 7.897(32)(002), 7.812(13) (101), 4.547(14)(103), 3.946(100)(004), 2.985(39)(105), 2.4841(11)(215) and 1.5788(12)(00.10). The crystal structure of fluorapophyllite-(NH4), refined to R-1 = 0.0299 for 743 unique (I > 3 sigma I) observed reflections, confirmed that the atomic arrangement is very similar to that of the other members of the apophyllite group. The new mineral is named according to the current nomenclature scheme for apophyllite-group minerals and is an NH4 dominant analogue of fluorapophyllite-(K), fluorapophyllite-(Na) and fluorapophyllite-(Cs).