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

Published online by Cambridge University Press:  01 June 2020

Martin Števko Open the ORCID record for Martin Števko [Opens in a new window] ,
Jiří Sejkora ,
Jakub Plášil Open the ORCID record for Jakub Plášil [Opens in a new window] ,
Zdeněk Dolníček  and
Radek Škoda
Martin Števko*
Affiliation:
Earth Science Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 840 05Bratislava, Slovakia Department of Mineralogy and Petrology, National Museum, Cirkusová 1740, 193 00 Prague 9 - Horní Počernice, Czech Republic
Jiří Sejkora
Affiliation:
Department of Mineralogy and Petrology, National Museum, Cirkusová 1740, 193 00 Prague 9 - Horní Počernice, Czech Republic
Jakub Plášil
Affiliation:
Institute of Physics, Academy of Sciences of the Czech Republic v.v.i, Na Slovance 2, 182 21 Prague 8, Czech Republic
Zdeněk Dolníček
Affiliation:
Department of Mineralogy and Petrology, National Museum, Cirkusová 1740, 193 00 Prague 9 - Horní Počernice, Czech Republic
Radek Škoda
Affiliation:
Department of Geological Sciences, Faculty of Science, Masaryk University, Kotlářská 2, 611 37Brno, Czech Republic
*
*Author for correspondence: Martin Števko, Email: msminerals@gmail.com
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Abstract

The new mineral fluorapophyllite-(NH4), ideally NH4Ca4(Si8O20)F⋅8H2O, was found at the Vechec andesite quarry located near Vechec village, Vranov nad Topľou Co., Prešov 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 non-fluorescent under ultraviolet radiation. The Mohs hardness is ~4½ 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 ω = 1.5414(5) and ɛ = 1.5393(8) (λ = 589 nm). It is non-pleochroic. The empirical formula (based on 29 O + F apfu) is [(NH4)0.55K0.32Na0.07Ca0.06]Σ1.00(Ca4.01Mg0.02)Σ4.03Si7.97O20[F0.84(OH)0.16]Σ1.00⋅8H2O. Fluorapophyllite-(NH4) is tetragonal, space group P4/mnc, a = 8.99336(9) Å, c = 15.7910(3) Å, V = 1277.18(3) Å3 and Z = 2. The seven strongest X-ray powder diffraction lines are [dobs in Å,(I,%)(hkl)]: 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 R1 = 0.0299 for 743 unique (I > 3σI) 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).

Keywords

fluorapophyllite-(NH4)new mineralapophyllite groupphyllosilicatesRaman spectroscopyxenolithsVechecSlovakia
Type
Article
Information
Mineralogical Magazine , Volume 84 , Issue 4 , August 2020 , pp. 533 - 539
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2020

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Footnotes

Associate Editor: G. Diego Gatta

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