Saccoite, Ca₂Mn⁺³₂F(OH)₈⋅0.5(SO₄), a new, microporous mineral from the Kalahari Manganese Field, South Africa

Saccoite, Ca₂Mn³⁺₂F(OH)₈⋅0.5(SO₄ ), is a new mineral found at the N’Chwaning III mine, Kalahari Manganese Field, Northern Cape Province, Republic of South Africa. It occurs as fillings of voids in hydrothermally altered manganese ore (comprising mostly of bixbyite and baryte). Further associated minor minerals are braunite, gypsum, chlorite, sturmanite and ettringite. Saccoite forms small needles, felted crystal masses or crusts. The new mineral is olive green, transparent, with white streak and vitreous lustre. No luminescence is observed. Saccoite is uniaxial (–) with refractive indices at 589(1) nm of ω = 1.705(5) and ε = 1.684(2). Pleochroism is distinct, i.e. bluish green (ω) and yellowish green (ε). The chemical composition was studied by means of an electron probe micro-analyser (EPMA) using wavelength-dispersive X-ray spectrometry (WDS). The empirical mineral formula is Ca_2.06Mn³⁺_1.78Cu_0.10Mg_0.07F_0.97(OH)_8.02(SO₄)_0.39. The unit-cell dimensions of saccoite (space group P4/ncc) are a = 12.834(3) Å, c = 5.622(2) Å, V = 926.0(4) Å3 ), and the calculated mass dens-
ity is 2.73 g⋅cm^–3 . Saccoite exhibits a heteropolyhedral framework structure that is composed of edge- and corner sharing CaF₂(OH)₆ and M(OH)₆ polyhedra (M = Mn³⁺ and Cu²⁺ ) with large channels along [001], which host disordered and only partially occupied groups, especially SO₄ ^2–. The hydrogen atoms of the OH groups point into the channel to form hydrogen bonds with the channel anions. Ca–F distances are ∼2.3 Å, the Ca–OH distances in the range of 2.44–2.58 Ǻ, and the M(OH)₆ octahedron is strongly 4+2 Jahn-Teller distorted (4 × ∼1.92 Å, 2 × 2.27 Å). The F atom is tetrahedrally coordinated to calcium atoms. The strongest lines in the powder X-ray diffraction pattern [d in Å (relative intensity) (hkl)] are: 9.0735 (35) (110), 4.5370 (95) (220), 4.0644 (20) (310), 3.0105 (100) (321), 2.8117 (20) (002), 2.7242 (75) (411), 1.9755 (35) (611), and 1.8142 (20) (550).