Pekov I. V., Britvin S. N., Yapaskurt V. O., Koshlyakova N. N., Polekhovsky Yu. S., Gottlicher J., Chukanov N. V., Vigasina M. F., Krivovichev S. V., Turchkova A. G., Sidorov E. G.

Arsmirandite, Na₁₈Cu₁₂Fe³⁺O₈₍AsO₄)₈Cl₅, and lehmannite, Na₁₈Cu₁₂TiO₈(AsO₄)₈FCl₅, new mineral species from fumarole exhalations of the Tolbachik volcano, Kamchatka, Russia

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2020. V. 149. N 3. P. 1-17

https://doi.org/10.31857/S0869605520030077

Full text is available on eLIBRARY.RU

Language: English 

Abstract

Two closely related new minerals arsmirandite Na₁₈Cu₁₂Fe³⁺O₈(AsO₄)₈Cl₅ and lehmannite Na₁₈Cu₁₂Ti⁴⁺O₈(AsO₄)₈FCl₅ were discovered in sublimates of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. They are associated with one another and with hematite, sanidine, sylvite, halite, tenorite, cassiterite, rutile, and various arsenates and sulfates. Arsmirandite and lehmannite are visually indistinguishable and occur as equant crystals up to 20 . 20 . 30 μm3, typically combined in thin crusts up to 2 . 3 cm. The minerals are dark greyish green to olive-greenish black and have strong vitreous lustre.Dcalc = 3.715 (arsmirandite) and 3.676 (lehmannite) g cm–3. The empirical formula of arsmirandite is (Na_17.06K_0.51Pb_0.08Ca_0.06)_Σ17.71(Cu_11.73Mg_0.11Zn_0.08Mn_0.01)_Σ12.93(Ti_0.10Al_0.02)_Σ1.04(As_7.91S_0.08P_0.03Si_0.02V_0.01)_Σ8.05O_40.23Cl_4.77. The empirical formula of lehmannite is (Na_17.92K_0.18Ca_0.24)_Σ18.34(Cu_11.59 )_Σ11.80(Ti_0.85Sn_0.11)_Σ0.96 (As_7.74S_0.14P_0.09Si_0.03)_Σ8O_40.10F_0.75Cl_5.42. Both minerals are monoclinic, space group C2/m, Z = 2. Unit-cell parameters (arsmirandite/lehmannite) are: a = 10.742(2)/10.8236(15), b = 21.019(3)/21.1077(17), c = 11.787(2)/11.8561(11) A, β = 117.06(3)/117.195(8)°, and V = 2370.0(7)/2409.2(5) A³. The crystal structures of arsmirandite and lehmannite were solved by means of single-crystal X-ray diffraction analysis. The minerals have two unique structural features: (1) they contain Fe³⁺ and Ti⁴⁺ in cubic coordination for arsmirandite and lehmannite, respectively; (2) their structures are built up by packing of unusual nanoscale (~1.5 nm across) clusters with the composition {[MCu₁₂O₈](AsO₄)₈} (M = Fe³⁺ in arsmirandite and Ti⁴⁺ in lehmannite). Each nanocluster contains (MO₈) cube surrounded by twelve flat squares (CuO₄) linked with eight (AsO₄) tetrahedra. Sodium and halogen atoms are located in between the nanoclusters. The name arsmirandite reflects the presence of arsenic and the unusual crystal structure (from the Latin mirandus, marvellous). Lehmannite is named in honour of the outstanding German and Russian mineralogist and geologist Johann Gottlob Lehmann (1719–1767).

Keywords: arsmirandite, lehmannite, new mineral, arsenate, crystal structure, nanocluster, polyoxometalate, polyoxocuprate, iron in cubic coordination, titanium in cubic coordination, fumarole, Tolbachik volcano, Kamchatka