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 Pb42+Ca2[Si8B2(SiB)O28]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/cm3, respectively. Khvorovite is biaxial (+) with refractive indices (λ = 589 nm) α = 1.659(3), βcalc. = 1.671(2), γ = 1.676(3); 2Vmeas. = 64(3)°, medium dispersion: r < v. Khvorovite is triclinic, space group I1¯, 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 SiO2 36.98, B2O3 6.01, Y2O3 0.26, PbO 40.08, BaO 6.18, SrO 0.43, CaO 6.77, K2O 1.72, Na2O 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 (Pb2.762+Ba0.62K0.56Na0.16)Σ4.10(Ca1.86Sr0.06Y0.04Na0.04)Σ2[Si8B2(Si1.46B0.65)Σ2.11O28](F0.71O0.29), Z = 2 , and the simplified formula is (Pb2+, Ba, K)4Ca2[Si8B2(Si,B)2O28]F. The crystal structure of khvorovite was refined to R1 = 2.89% based on 3680 observed reflections collected on a four-circle diffractometer with MoKα 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 (Si1.46B0.54), 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 [Si8B2(SiB)O28]11–. The interstitial cations are Pb2+, 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 Pb2+ and Ba + K. The [8]-coordinated M site is occupied mainly by Ca, with minor Sr, Y and Na. Khvorovite is a Pb2+ analogue of hyalotekite, (Ba,Pb2+,K)4(Ca,Y)2[Si8(B,Be)2(Si,B)2O28]F and a Pb2+-, Ca-analogue of kapitsaite-(Y), (Ba,K)4(Y,Ca)2[Si8B2(B,Si)2O28]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.

Chemographic exploration of the hyalotekite structure-type

“…Previous crystal-structure work on hyalotekite-group minerals (e.g. Sokolova et al , 2000; Pautov et al , 2015) identified two distinct tetrahedrally coordinated sites, B occupied solely by B 3+ , and T occupied by B 3+ and Si 4+ . As we shall show later, the B site in other potential hyalotekite-group minerals may be occupied by cations other than B 3+ , and hence we relabel the B and T sites as T (1) and T (2).…”

“…(2000) for kapitsaite-(Y), <Si–O> = 1.617 Å, and by Pautov et al . (2015) for khvorovite, <Si–O> = 1.617 Å, indicate that the Si sites are occupied only by Si 4+ and that there is negligible substitution of B 3+ at the Si sites. The < T (1)–O > distances are 1.480 and 1.478 Å in these two minerals, the same within one standard deviation as the grand < B 3+ –O > distance of 1.476 Å reported for borate minerals by Hawthorne et al .…”

“…The utility of this approach is immediately seen from inspection of Fig. 2 (modified from Pautov et al ., 2015). End-member compositions 3 and 4 are also shown on Fig.…”

“…Based on the chemical analysis of Lindström (1887) and calculated on 29 (O + F) anions, the empirical formula of hyalotekite from Långban is (Ba 1.96 Pb 2+ 1.68 K 0.28 Na 0.08 ) Σ4 (Ca 2.08 Mn 0.06 Mg 0.03 Cu 0.02 ) Σ2.19 (Si 9.82 B 1.60 Be 0.45 Al 0.05 Fe 3+ 0.01 ) Σ11.93 O 28.20 F 0.78 Cl 0.03 for Z = 2. The general formula has been written in various ways (Moore et al ., 1982; Grew et al ., 1994, Christy et al ., 1998), and the currently accepted form is as follows: (Ba,Pb 2+ ,K) 4 (Ca,Y) 2 [Si 8 (B,Be) 2 (Si,B) 2 O 28 ]F (Pautov et al ., 2015). Hyalotekite has been reported from the Darai-Pioz alkaline massif, Tajikistan (Grew et al ., 1994).…”

“…Hyalotekite has been reported from the Darai-Pioz alkaline massif, Tajikistan (Grew et al ., 1994). Two other minerals isostructural with hyalotekite have also been reported from the Darai-Pioz massif: kapitsaite-(Y), ideally Ba 4 (YCa)[Si 8 B 2 B 2 O 28 ]F (Pautov et al ., 2000; Sokolova et al ., 2000) and khvorovite, ideally Pb 2+ 4 Ca 2 [Si 8 B 2 (SiB)O 28 ]F (Pautov et al ., 2015). The degree of complication of the hyalotekite structure, combined with the complexity of the parageneses in which it has been found, suggest the possibility of: (1) the occurrence of additional hitherto-undiscovered minerals; and (2) the possibility of synthesizing a wide variety of chemical variants.…”