Epifanovite, NaCaCu₅(PO₄)₄[AsO₂(OH)₂] · 7H₂O: a New Mineral from the Kester Deposit, Sakha (Yakutia) Republic, Russia

Epifanovite, NaCaCu₅(PO₄)₄[AsO₂(OH)₂] · 7H₂O, a new natural copper, sodium and calcium arsenate–phosphate, has been found in a quartz–phosphate pocket within greisenized cassiterite-bearing granodiorite of the Kester tin deposit, Sakha (Yakutia) Republic, Russia. The mineral occurs as crusts of tabular pseudotetragonal crystals up to 50 μm across and 10 μm thick. Associated minerals are fluorapatite, pseudomalachite, malachite, a Na-analogue of batagayite, tobermorite, libethenite, arsenolite, native copper and unknown Mg–Zn phosphate. Epifanovite is turquoise-blue with pale blue streak, vitreous luster (dull in crusts), and a Mohs hardness of 3. The mineral is brittle. Cleavage is perfect on (001) and good on (100) and (010). Density measured in the Clerici solution is 3.65(3) g/cm³; the calculated density is 3.73 g/cm³. Epifanovite is optically biaxial (–), α = 1.708(5), β = 1.730(5), γ = 1.735(5). 2Vobs = 40°–45°, 2V_calc = 50°. Optical orientation: X = a, Y = b. The empirical formula calculated on the basis of P + As = 5 is (Na_0.94K_0.06)Σ_1.00(Ca_0.82Na_0.08)Σ_0.90(Cu_5.04Zn_0.06)Σ_5.10(PO₄)₄[(As_0.81P_0.19)Σ_1.00(O_1.92OH_2.06Cl_0.02)Σ_4.00] · 7.37H₂O. The idealized formula is NaCaCu₅(PO₄)₄[AsO₂(OH)₂] · 7H₂O. The Raman spectrum contains the following bands, cm^–1: 293, 359 (ν_1–2, CuO₅); 455, 556, 594, 640, 921, 962, 1002, 1086, 1153 (ν_1–4, PO4), 77, 121, 161, 183, 730, 828, 858 (ν_1–3, AsO₄), 2900, 3200, 3410 (ν₁, OH). The mineral is monoclinic, P2₁/m, a = 9.6912(9), b = 9.7440(9), c = 9.9561(9) Å, β = 102.23 (1)°, V = 918.7(1) ų, Z = 2. The strongest reflections in the powder X-ray diffraction pattern are, I–dÅ–hkl: 100–9.73–001, 35–6.79–110, 12–4.355–021, 43–3.072–130, 24–3.061–221, 24–3.003–\(\bar 222\), 11–2.698–023, 10–1.6775–504. The mineral was named in honor of the Russian geologist Porphyry Prokop’evich Epifanov, who discovered the Ege-Khaya and Kester tin deposits. Epifanovite is structurally close to the lavendulan-group minerals and related species: andyrobertsite, calcioandyrobertsite, mahnertite and richelsdorfite.