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

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/cm 3 and the calculated density is 2.772 g/cm 3 . Optically jahnsite-(CaFeMg) is biaxial negative with α = 1.629(4), β = 1.658(4), γ = 1.677(4) and 2 V calc = 76.8°. Electron-microprobe analyses provided: Na 2 O 0.65, CaO 4.74, Al 2 O 3 0.06, Fe 2 O 3 20.18, FeO 9.85, MgO 6.47, MnO 5.02, P 2 O 5 34.41, H 2 O calc 19.46 (from crystal-structure analysis), total 100.84 wt.%. The empirical formula (based on 26 oxygen atoms) is: (Ca 0.70 Na 0.17 Mn 2+ 0.16 ) Σ 1.03 Fe 2+ 1.00 (Mg 1.33 Mn 2+ 0.43 Fe 3+ 0.24 ) Σ 2.00 (Fe 3+ 1.99 Al 0.01 ) Σ 2.00 (PO 4 ) 4.01 (OH) 2.10 H 2 O 7.88 . Jahnsite-(CaFeMg) is monoclinic, P 2/a, a = 14.975(5), b = 7.1645(14), c = 9.928(2) A, β = 110.65(3)°, V = 996.7(5), A 3 and Z = 2. The eight strongest lines in the powder diffraction pattern are [ d (A)( 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 R 1 = 0.0517 on the basis of 1616 observed reflections with F o > 4σ( F o ) and confirms a whiteite–jahnsite-type structure.