Kiryuite and gunmaite, two new minerals from Tsukubara, Kiryu City, Gunma Prefecture, Japan

抄録

Kiryuite and gunmaite were found as new minerals from Tsukubara, Kiryu City, Gunma Prefecture, Japan. Kiryuite is a Mn-rich analogue of viitaniemiite with an ideal formula of NaMnAl(PO₄)F₃ that mainly occurs as a white powder in cracks in triplite aggregate and occasionally forms porous plate-like aggregates up to 5 mm in size, while the grains are several micrometers in size. In most cases, the aggregates are also accompanied by small amounts of other minerals such as goyazite, gorceixite, and fluorite. Kiryuite shows white streaks with a vitreous luster, and its Mohs hardness is estimated to be 5 by analogy with viitaniemiite. The calculated density is 3.32 g·cm⁻³ based on the empirical formula and unit cell volume refined from powder X-ray diffraction (XRD) data. The empirical formula for kiryuite calculated on the basis of O = 4 and F + OH = 3 is Na_0.97(Mn_0.56Ca_0.38Fe_0.04Mg_0.02)_Σ1.00Al_0.98P_1.02O₄(F_2.29OH_0.71)_Σ3. Kiryuite is monoclinic (P2₁/m) with a = 5.425(4) Å, b = 7.128(4) Å, c = 6.817(6) Å, β = 109.41(7)°, and V = 248.7(3) ų (Z = 2). The parameters [d in Å (I/I₀) hkl] for the six strongest lines associated with kiryuite in the powder XRD pattern are 3.123 (57) 002, 2.923 (53) 012 and 120, 2.877 (100) 121, 2.560 (27) 200, 2.263 (43) 103, and 2.155 (76) 221. Gunmaite is a new alunite-related mineral with a new structure type that has an ideal formula of (Na₂Sr)Sr₂Al₁₀(PO₄)₄F₁₄(OH)₁₂. Gunmaite mainly occurs as a core in hexagonal tablet crystals that coexist with gorceixite and goyazite-like minerals. Gunmaite is colorless and transparent with a greasy to resinous luster and is non-fluorescent. As a bulk crystal including gunmaite, the Mohs hardness is 5, cleavage is perfect on {001} planes, and the tenacity is brittle. The calculated density based on the empirical formula is 3.38 g·cm⁻³ using the empirical formula and single-crystal XRD data. The empirical formula for gunmaite calculated on the basis of O = 16 and F + OH = 26 is (Na_1.72Sr_0.70Mg_0.56Ca_0.01)_Σ2.99(Sr_1.32Ba_0.68)_Σ2(Al_9.82Mg_0.18)_Σ10P_3.99O₁₆F_16.21(OH)_9.79. Gunmaite is trigonal (R3m) with a = 6.9972(2) Å, c = 50.270(2) Å, and V = 2131.51(13) ų (Z = 3). The structure consists of two PO₄, AlO₂(OH)₄, AlF₆, AlOF₃(OH)₂, SrO₆(OH)₆, and NaF₈ polyhedra. Kiryuite and gunmaite are products of the final stage of hydrothermal activity in association with greisenization.