Khvorovite, Pb²⁺₄Ca₂[Si₈B₂(SiB)O₂₈]F, a new hyalotekite-group mineral from the Darai-Pioz alkaline massif, Tajikistan: Description and crystal structure

Abstract Khvorovite, ideally Pb 4 2+ Ca 2 [Si 8 B 2 (SiB)O 28 ]F, is a new borosilicate mineral of the hyalotekite group from the Darai-Pioz alkaline massif in the upper reaches of the Darai-Pioz river, Tajikistan. Khvorovite was found in a pectolite aggregate in silexites (quartz-rich rocks). The pectolite aggregate consists mainly of pectolite, quartz and fluorite, with minor aegirine, polylithionite, turkestanite and baratovite; accessory minerals are calcite, pyrochlore-group minerals, reedmergnerite, stillwellite-(Ce), pekovite, zeravshanite, senkevichite, sokolovaite, mendeleevite-(Ce), alamosite, orlovite, leucosphenite and several unknown Cs-silicates. Khvorovite occurs as irregular grains, rarely with square or rectangular sections up to 150 μm, and grain aggregates up to 0.5 mm. Khvorovite is colourless, rarely white, transparent with a white streak, has a vitreous lustre and does not fluoresce under ultraviolet light. Cleavage and parting were not observed. Mohs hardness is 5–5.5, and khvorovite is brittle with an uneven fracture. The measured and calculated densities are 3.96(2) and 3.968 g/cm 3 , respectively. Khvorovite is biaxial (+) with refractive indices (λ = 589 nm) α = 1.659(3), β calc. = 1.671(2), γ = 1.676(3); 2V meas. = 64(3)°, medium dispersion: r < v . Khvorovite is triclinic, space group I 1¯, a = 11.354(2), b = 10.960(2), c = 10.271(2) Å, α = 90.32(3), β = 90.00(3), γ = 90.00(3)°, V = 1278(1) Å3, Z = 2. The six strongest lines in the powder X-ray diffraction pattern [ d (Å), I, ( hkl )] are: 7.86, 100, (110); 7.65, 90, (101); 7.55, 90, (011); 3.81, 90, (202); 3.55, 90, (301); 2.934, 90, (312, 312). Chemical analysis by electron microprobe gave SiO 2 36.98, B 2 O 3 6.01, Y 2 O 3 0.26, PbO 40.08, BaO 6.18, SrO 0.43, CaO 6.77, K 2 O 1.72, Na 2 O 0.41, F 0.88, O=F –0.37, sum 99.35 wt.%. The empirical formula based on 29 (O+F) a.p.f.u. is (Pb 2.76 2+ Ba 0.62 K 0.56 Na 0.16 ) Σ4.10 (Ca 1.86 Sr 0.06 Y 0.04 Na 0.04 ) Σ2 [Si 8 B 2 (Si 1.46 B 0.65 ) Σ2.11 O 28 ](F 0.71 O 0.29 ), Z = 2 , and the simplified formula is (Pb 2+ , Ba, K) 4 Ca 2 [Si 8 B 2 (Si,B) 2 O 28 ]F. The crystal structure of khvorovite was refined to R 1 = 2.89% based on 3680 observed reflections collected on a four-circle diffractometer with Mo K α radiation. In the crystal structure of khvorovite, there are four [4]-coordinated Si sites occupied solely by Si with <Si–O>= 1.617 Å. The [4]-coordinated B site is occupied solely by B, with <B–O> = 1.478 Å. The [4]-coordinated T site is occupied by Si and B (Si 1.46 B 0.54 ), with <T–O> = 1.605 Å; it ideally gives (SiB) a.p.f.u. The Si, B and T tetrahedra form an interrupted framework of ideal composition [Si 8 B 2 (SiB)O 28 ] 11– . The interstitial cations are Pb 2+ , Ba and K (minor Na) [ A (11–22) sites] and Ca [ M site]. The two A sites are each split into two subsites ∼0.5 Å apart and occupied by Pb 2+ and Ba + K. The [8]-coordinated M site is occupied mainly by Ca, with minor Sr, Y and Na. Khvorovite is a Pb 2+ analogue of hyalotekite, (Ba,Pb 2+ ,K) 4 (Ca,Y) 2 [Si 8 (B,Be) 2 (Si,B) 2 O 28 ]F and a Pb 2+ -, Ca-analogue of kapitsaite-(Y), (Ba,K) 4 (Y,Ca) 2 [Si 8 B 2 (B,Si) 2 O 28 ]F. It is named after Pavel V. Khvorov (b. 1965), a Russian mineralogist, to honour his contribution to the study of the mineralogy of the Darai-Pioz massif.

Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.