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

Abstract

<jats:title>Abstract</jats:title> <jats:p>Jahnsite-(CaMnZn), CaMn<jats:sup>2+</jats:sup>Zn<jats:sub>2</jats:sub>Fe<jats:sup>3+</jats:sup><jats:sub>2</jats:sub>(PO<jats:sub>4</jats:sub>)<jats:sub>4</jats:sub>(OH)<jats:sub>2</jats:sub>⋅8H<jats:sub>2</jats:sub>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<jats:sup>−3</jats:sup> 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 <jats:italic>Z</jats:italic> = <jats:bold>b</jats:bold>. Pleochroism was imperceptible. Electron microprobe analyses together with results from Mössbauer spectroscopy gives the formula (Ca<jats:sub>0.59</jats:sub>Mn<jats:sub>0.24</jats:sub>)<jats:sub>Σ0.83</jats:sub>Mn(Zn<jats:sub>0.74</jats:sub>Mn<jats:sup>2+</jats:sup><jats:sub>0.48</jats:sub>Mg<jats:sub>0.18</jats:sub>Fe<jats:sup>2+</jats:sup><jats:sub>0.13</jats:sub>Fe<jats:sup>3+</jats:sup><jats:sub>0.47</jats:sub>)<jats:sub>Σ2</jats:sub>Fe<jats:sup>3+</jats:sup><jats:sub>2</jats:sub>(P<jats:sub>0.995</jats:sub>O<jats:sub>4</jats:sub>)<jats:sub>4</jats:sub>(OH)<jats:sub>2.03</jats:sub>(H<jats:sub>2</jats:sub>O)<jats:sub>7.97</jats:sub>.</jats:p> <jats:p>Jahnsite-(CaMnZn) is monoclinic, <jats:italic>P</jats:italic>2/<jats:italic>a</jats:italic>, with <jats:italic>a</jats:italic> = 15.059(1), <jats:italic>b</jats:italic> = 7.1885(6), <jats:italic>c</jats:italic> = 10.031(2) Å, β = 111.239(8)° and <jats:italic>V</jats:italic> = 1012.1(2) Å<jats:sup>3</jats:sup>. 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 <jats:italic>X</jats:italic> and <jats:italic>M</jats:italic>1 sites is discussed in terms of rotations about the 7 Å axis of two independent octahedra centred at the <jats:italic>M</jats:italic>3 sites.</jats:p>

Whiteite-(MnMnMn), a new jahnsite-group mineral species from the Foote mine, North Carolina, USA, and chemical pressure effects in jahnsite-group minerals.

“…For jahnsite-subgroup minerals, we have reported previously an unusual negative correlation between the mean ionic radius (r) of the X + M1 cations and the unit-cell parameter a (Grey et al, 2020). That is, with increasing size of the cations substituting at the X and M1 sites, the unit cell contracts along [100].…”

“…The negative correlation between a and <r X + r M1 > was explained in terms of rotations of the M3 centred octahedra about [010] to change the coordination number of the X and M1 cations from 6 for small cations towards 8 for large cations (Grey et al, 2020). This is illustrated in Fig.…”

“…Within the two subgroups, different species are distinguished by the dominant cations in the X, M1 and M2 sites, represented by a suffix -(XM1M2) in the mineral name. It has been generally observed that the mean sizes of these cations are in the order X > M1 > M2, (Moore and Ito, 1978;Kampf et al, 2019) and this observation has been used to define and name new species where site assignments cannot be made unambiguously from crystal structure refinements (Grey et al, 2020). To date, 12 jahnsite-subgroup members have been officially approved by the IMA but only seven whiteite-subgroup members have been approved.…”

“…Plots of mean ionic radius for X + M1 site cations vs. a. The upper plot is for jahnsitesubgroup species (fromGrey et al, 2020) and lower plot is for whiteite-subgroup species (data in Table6).…”

On the occurrence of jahnsite/whiteite phases on Mars: A thermodynamic study

“…The monoclinic symmetry, space group P2/a, was assessed for all samples analyzed. The structure may accommodate a variety of substituting cations, hence the occurrence of several end-member compositions (often involving manganese) (Grey et al 2020) and possibly solid solutions. Among reported substituting elements are Ca 2+ , Mn 2+ , Na + in X sites, Mg 2+ , Mn 2+ , Fe 2+ or Fe 3+ in M1 sites and Mg 2+ , Mn 2+ , Fe 2+ (Fe 3+ ) or Zn 2+ in M2 sites.…”