Whiteite-(MnMnMg), a new jahnsite-group mineral from iron monarch, South Australia: Description and crystal structure

Whiteite-(MnMnMg), is a new jahnsite-group mineral found at the Iron Monarch Quarry, Iron Knob, South Australia, Australia, where it occurs associated with triploidite, rhodochrosite, and an unidentified Ca,Mn phosphate carbonate mineral. The new mineral occurs as a single unterminated, reddish, orange, prismatic crystal 1.2 mm in length and 0.3 mm across. The mineral is uniaxial (-), with α = 1.582(2), β = 1.586(2), γ = 1.613(2). The calculated 2V is 74.5°. The streak is pale orange. The luster is vitreous and the mineral is translucent. The mineral is brittle with an irregular fracture and a good cleavage parallel to {001}; its Mohs hardness is ∼4. The measured specific gravity is 2.61(4) g/cm³. The empirical formula, calculated on the basis of 26 O atoms per formula unit, is (Mn²⁺ _0.60,Ca_0.41,Na_0.03,K_0.01)_σ1.05(Mn²⁺ _0.92,Mg_0.08)_σ1.00Mg_2.00(Al_1.82,Mn³⁺ _0.18)_σ2.00(PO₄)_4.00 (OH)_2.06·7.95H₂O. Whiteite-(MnMnMg) is monoclinic with space group P2/a; its unit-cell parameters are a 15.0357(18), b 6.9408(5), c 9.9431(9) Å, β 110.827(8)°, V 969.86(16) ų, and ℤ; = 2. The eight strongest lines in the X-ray powder diffraction pattern are [d_obs Å (I) (hkl)]: 9.244(100)(001), 5.619(32)(111), 4.839(20)(111, 202), 4.111(16)(112), 3.501(22)(312)(400, 020, 402), 2.936(16)(401), 2.759(30)(022,510), and 2.566(17)(421). The crystal structure has been refined from single-crystal X-ray data to R = 0.0396 for 23,703 unique observed reflections (F_o > 4σF_o). The mineral is named for the chemical composition, in accordance with jahnsite group nomenclature.