Original paper

Jahnsite-(CaFeMg), a new mineral from Tom's quarry, South Australia: description and crystal structure

Elliott, Peter

European Journal of Mineralogy Volume 28 Number 5 (2016), p. 991 - 996

published: Dec 31, 2016

DOI: 10.1127/ejm/2016/0028-2562

BibTeX file

ArtNo. ESP147052805010, Price: 29.00 €

Download preview PDF Buy as PDF

Abstract

Jahnsite-(CaFeMg) is a new member of the whiteite–jahnsite group from Tom's Quarry, Kapunda, South Australia, Australia where it occurs as a low-temperature secondary mineral. Jahnsite-(CaFeMg) forms brownish orange prismatic crystals to 0.2 mm in length. Crystals are transparent, have a vitreous lustre and a good cleavage on {001}. The streak is white. The Mohs hardness is ∼4, the tenacity is brittle and the fracture is splintery. The measured density is 2.76(4) g/cm3 and the calculated density is 2.772 g/cm3. Optically jahnsite-(CaFeMg) is biaxial negative with α = 1.629(4), β = 1.658(4), γ = 1.677(4) and 2V calc = 76.8°. Electron-microprobe analyses provided: Na2O 0.65, CaO 4.74, Al2O3 0.06, Fe2O3 20.18, FeO 9.85, MgO 6.47, MnO 5.02, P2O5 34.41, H2Ocalc 19.46 (from crystal-structure analysis), total 100.84 wt.%. The empirical formula (based on 26 oxygen atoms) is: (Ca0.70Na0.17Mn2+ 0.16)Σ1.03Fe2+ 1.00(Mg1.33Mn2+ 0.43Fe3+ 0.24)Σ2.00(Fe3+ 1.99Al0.01)Σ;2.00(PO4)4.01(OH)2.10H2O7.88. Jahnsite-(CaFeMg) is monoclinic, P2/a, a = 14.975(5), b = 7.1645(14), c = 9.928(2) Å, β = 110.65(3)°, V = 996.7(5), Å3 and Z = 2. The eight strongest lines in the powder diffraction pattern are [d(Å)(I)(hkl)]: 9.339(100)(001); 4.923(20)(21–1, 111, 310); 3.562(20)(400); 3.518(20)(112, 40–2); 3.453(20)(202); 2.965(20)(221); 2.839(35)(5-1-1); 2.592(20)(203). The crystal structure has been refined using a single crystal twinned on {001} to R1 = 0.0517 on the basis of 1616 observed reflections with F o > 4σ(F o) and confirms a whiteite–jahnsite-type structure.

Keywords

phosphatewhiteite–jahnsite groupkapundajahnsitenew mineralcrystal structuretom's quarry