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Jahnsite-(CaMnZn) from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria, and structural flexibility of jahnsite-group minerals

Published online by Cambridge University Press:  08 May 2020

Ian E. Grey*
Affiliation:
CSIRO Mineral Resources, Private Bag 10, Clayton South, Victoria3169, Australia
Erich Keck
Affiliation:
Algunderweg 3, D-92694Etzenricht, Germany
Anthony R. Kampf
Affiliation:
Mineral Sciences Dept., Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA90007, USA
Colin M. MacRae
Affiliation:
CSIRO Mineral Resources, Private Bag 10, Clayton South, Victoria3169, Australia
John D. Cashion
Affiliation:
Monash University, School of Physics and Astronomy, Victoria3800, Australia
A. Matt Glenn
Affiliation:
CSIRO Mineral Resources, Private Bag 10, Clayton South, Victoria3169, Australia
*
*Author for correspondence: Ian E. Grey, Email: [email protected]

Abstract

Jahnsite-(CaMnZn), CaMn2+Zn2Fe3+2(PO4)4(OH)2⋅8H2O, is a new jahnsite-group mineral associated with alteration of phosphophyllite at the Hagendorf-Süd pegmatite, Bavaria. It forms as thin yellow crusts and brown epitactic growths on altered phosphophyllite, both of which comprise lath-like crystals in orthogonal orientation, up to 100 μm long. The crystals contain intergrowths of jahnsite-(CaMnZn) and jahnsite-(CaMnMn) on a scale of ~50 μm. The calculated density is 2.87 g cm−3 based on the empirical formula. Optically it is biaxial (–), with α = 1.675(2), β = 1.686(2) and γ = 1.691(2) (white light). The calculated 2V is 68°. Dispersion could not be observed, and the optical orientation is Z = b. Pleochroism was imperceptible. Electron microprobe analyses together with results from Mössbauer spectroscopy gives the formula (Ca0.59Mn0.24)Σ0.83Mn(Zn0.74Mn2+0.48Mg0.18Fe2+0.13Fe3+0.47)Σ2Fe3+2(P0.995O4)4(OH)2.03(H2O)7.97.

Jahnsite-(CaMnZn) is monoclinic, P2/a, with a = 15.059(1), b = 7.1885(6), c = 10.031(2) Å, β = 111.239(8)° and V = 1012.1(2) Å3. The recent International Mineralogical Association approved nomenclature system for jahnsite-group minerals was applied to establish jahnsite-(CaMnZn) from the empirical formula. The structural flexibility of jahnsite-group minerals to accommodate cations of quite different sizes in the X and M1 sites is discussed in terms of rotations about the 7 Å axis of two independent octahedra centred at the M3 sites.

Type
Article
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2020

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Footnotes

Associate Editor: Charles A. Geiger

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