Witzkeite: a new rare nitrate-sulphate mineral from a guano deposit at Punta de Lobos, Chile

Witzkeite, ideally Na4K4Ca(NO3)2(SO4)4·2H2O, is a new mineral found in the oxidation zone of the guano mining field at Punta de Lobos, Tarapacá region, Chile. It occurs as colorless, tabular crystals up to 140 μm in length, associated with dittmanite and nitratine. Witzkeite is colorless and transparent, with a white streak and a vitreous luster. It is brittle, with Mohs hardness 2 and distinct cleavage on {001}. Measured density is 2.40(2) g/cm3, calculated density is 2.403 g/cm3. Witzkeite is biaxial (−) with refractive indexes α = 1.470(5), β = 1.495(5), γ = 1.510(5), measured 2V = 50–70°. The empirical composition is (electron microprobe, mean of five analyses, H2O, CO2, and N2O5 by gas chromatography; wt%): Na2O 12.83, K2O 22.64, CaO 7.57, FeO 0.44, SO3 39.96, N2O5 12.7, H2O 4.5, total 100.64; CO2 was not detected. The chemical formula, calculated based on 24 O, is: Na3.40K3.95Ca1.11Fe0.05(NO3)1.93(SO4)4.10(H4.10O1.81). Witzkeite is monoclinic, space group C2/c, with unit-cell parameters: a = 24.902(2), b = 5.3323(4), c = 17.246(1) Å, β = 94.281(7)°, V = 2283.6(3) Å3 (Z = 4). The crystal structure was solved using single-crystal X-ray diffraction data and refined to R1(F) = 0.043. Witzkeite belongs to a new structure type and is noteworthy for the very rare simultaneous presence of sulfate and nitrate groups. The eight strongest X-ray powder-diffraction lines [d in Å (I in %) (h k l)] are: 12.38 (100) (2 0 0), 4.13 (19) (6 0 0), 3.10 (24) (8 0 0), 2.99 (7) (8 0 2), 2.85 (6) (8 0 2), 2.69 (9) (7 1 3), 2.48 (12) (10 0 0), and 2.07 (54) (12 0 0). The IR spectrum of witzkeite was collected in the range 390–4000 cm−1. The spectrum shows the typical bands of SO4 2− ions (1192, 1154, 1116, 1101, 1084, 993, 634, and 617 cm−1) and of NO3− ions (1385, 1354, 830, 716, and 2775 cm−1). Moreover, a complex pattern of bands related to H2O is visible (bands at 3565, 3419, 3260, 2405, 2110, 1638, and 499 cm−1). The IR spectrum is discussed in detail.