Jahnsite-(CaMnZn) from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria, and structural flexibility of jahnsite-group minerals. (8th August 2020)

• Record Type: Journal Article
• Authors: Grey, Ian E.; Keck, Erich; Kampf, Anthony R.; MacRae, Colin M.; Cashion, John D.; Glenn, A. Matt
• Is Part Of: Mineralogical magazine. Volume 84:Number 4(2020)
• Page Start: 547
• Page End: 553
• Publication Date: 2020-08-08
• Subjects: jahnsite-(CaMnZn), -- jahnsite group, -- new mineral, -- Hagendorf Süd, -- structural flexibility in jahnsite; Mineralogy -- Periodicals; Mineralogy -- Great Britain -- Periodicals; 549.05
• DOI: 10.1180/mgm.2020.40
• Languages: English
• ISSNs: 0026-461X

Abstract

Abstract: Jahnsite-(CaMnZn), CaMn 2+ Zn2 Fe 3+ 2 (PO4 )4 (OH)2 ⋅8H2 O, 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 (Ca0.59 Mn0.24 )Σ0.83 Mn(Zn0.74 Mn 2+ 0.48 Mg0.18 Fe 2+ 0.13 Fe 3+ 0.47 )Σ2 Fe 3+ 2 (P0.995 O4 )4 (OH)2.03 (H2 O)7.97 . Jahnsite-(CaMnZn) is monoclinic, P 2/ a, with a = 15.059(1), b = 7.1885(6), c = 10.031(2) Å, β = 111.239(8)° and V = 1012.1(2) Å 3 . 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 M 1 sites is discussed in terms of rotations about the 7 Å axis of two independent octahedra centred at the M 3 sites.