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Fluoro-sodic-ferropedrizite, NaLi₂(Al₂Li)Si₈O₂₂F₂, a new mineral of the amphibole group from the Sutlug River, Tuva Republic, Russia: description and crystal structure

¹ CNR-Istituto di Geoscienze e Georisorse, unità di Pavia, via Ferrata 1, I-27100 Pavia, Italy
² Centro Grandi Strumenti, Università di Pavia, via Bassi 21, I-27100 Pavia, Italy
³ Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada

^* E-mail: oberti{at}crystal.unipv.it

Fluoro-sodic-ferropedrizite, ideally ^ANa^BLi₂^C(Al₂Li)^TSi₈O₂₂^WF₂, is a new mineral of the amphibole group from the Sutlug River, Tuva Republic, Russia. It occurs at the endogenic contact of a Li-pegmatite with country rocks near to a diabase dyke and formed by reaction of the pegmatitic melt with the country rock. Fluoro-sodic-ferropedrizite occurs as prismatic to acicular crystals, ranging in length from 0.1-3 cm and widths of up to 50 µm. Crystals occur in parallel to sub-parallel aggregates up to 5 mm across in a matrix of calcite and plagioclase feldspar. Crystals are pale bluish-grey with a greyish-white streak. Fluoro-sodic-ferropedrizite is brittle, has a Mohs hardness of 6 and a splintery fracture; it is non-fluorescent with perfect {110} cleavage, no observable parting, and has a calculated density of 3.116 g cm^-3. In plane-polarized light, it is pleochroic, X = pale purple-grey, Y = light grey, Z = colourless; X ^ a = 71.2° (in β acute), Y || b, Z ^ c = 83.4° (in β obtuse). Fluoro-sodic-ferropedrizite is biaxial positive, = 1.642(1), β = 1.644(1), = 1.652(1); 2V_(obs) = 68.0(3)°, 2V_(calc) = 56.4°. Fluoro-sodic-ferropedrizite is monoclinic, space group C2/m, a = 9.3720(4) Å, b = 17.6312(8) Å, c = 5.2732(3) Å, β = 102.247(4)°, V = 851.5(2) ų, Z = 2. The strongest ten X-ray diffraction lines in the powder pattern are (d in Å,(I),(hkl)): 8.146,(10),(110); 2.686,(9),(151); 3.008,(8),(310); 4.430,(7),(021); 2.485,(6),(02); 3.383,(4),(131); 2.876,(3),(51, 11); 2.199,(3),(12); 4.030,(2),(111) and 3.795,(2),(31). Analysis by a combination of electron microprobe and crystal-structure refinement gives SiO₂ 59.81, Al₂O₃ 12.66, TiO₂ 0.09, FeO 10.32, MgO 5.56, MnO 0.73, ZnO 0.17, CaO 0.20, Na₂O 2.81, Li₂O 4.80, F 2.43, H₂O_calc 1.10, sum = 99.65 wt.%. The formula unit, calculated on the basis of 24(O,OH,F) is ^A(Na_0.68)^B(Li_1.92Na_0.05Ca_0.03)^C(Mg_1.10Zn_0.02Al_1.97Ti_0.01Li_0.66)^T(Si_7.98Al_0.02)O₂₂^W(F_1.03OH_0.97). Crystal-structure refinement shows Li to be completely ordered at the M(3) and M(4) sites. Fluoro-sodic-ferropedrizite, ideally ^ANa^BLi₂^C(Al₂Li)^TSi₈O₂₂^WF₂, is related to the theoretical end-member `sodic-pedrizite', ^ANa^BLi₂^C(Mg₂Al₂Li)^TSi₈O₂₂^W(OH)₂, by the substitutions ^CFe^{2+ C}Mg and ^WF ^W(OH).

KEYWORDS: Fluoro-sodic-ferropedrizite, new amphibole, electron microprobe analysis, optical properties, crystal-structure refinement, Sutlug river, Tuva Republic, Russia