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Shakhdaraite-(Y), ideally ScYNb2O8, is a new mineral from the Leskhozovskaya miarolitic granitic pegmatite at the Shakhdara River, southwestern Pamir (Tajikistan). Shakhdaraite-(Y) occurs mainly as grains from 10 to 150 μm in size in a near-miarolitic pegmatite complex in association with quartz, albite, pyrochlore-microlite, fersmite, and an unnamed Sc-Nb oxide; only one large, single, well-shaped crystal 200 μm long was found in a small cavity with quartz, albite, bertrandite, pyrochlore, and jarosite. Shakhdaraite-(Y) is black to dark-brown, streak is brown. Luster is vitreous semi-metallic. It is brittle with conchoidal fracture. Mohs hardness is 5. VHN100 = 436 kg/mm2. Dcalc. = 5.602 g/cm3. In reflected light, it is light gray and its reflective capacity is moderate to low. Anisotropy is distinct, without color effects. Pleochroism was not observed. Internal reflections are red-brown. Reflectance values were measured in air with SiC as reference material [λ(nm), Rmax, Rmin]: 470, 14.6, 13.9; 546, 14.0, 13.4; 589, 13.9, 13.3; 650, 13.8, 13.1. Electron probe microanalysis (WDS mode, 7 points) gives (wt.%): Nb2O5 50.70; Ta2O5 4.52; TiO2 0.08; WO3 0.79; SnO2 1.54; CaO 1.01; Sc2O3 11.35; MnO 1.38; FeO 0.01; Y2O3 12.00; Ce2O3 0.21; Pr2O3 0.04; Nd2O3 0.27; Sm2O3 0.32; Eu2O3 0.07; Gd2O3 0.86; Tb2O3 0.22; Dy2O3 2.07; Ho2O3 0.29; Er2O3 1.33; Tm2O3 0.35; Yb2O3 2.80; Lu2O3 0.32; PbO 0.24; ThO2 1.90; UO2 3.30, total 97.97. The empirical formula of shakhdaraite-(Y) based on O = 8 apfu (atoms per formula unit) is (Nb1.91Sc0.83Y0.53Ta0.10Mn0.10Ca0.09 Yb0.07U4+0.06Dy0.06Sn0.05Th0.04Er0.03Gd0.02W6+0.02Pb0.01Ce0.01Nd0.01Sm0.01Tb0.01Ho0.01Tm0.01Lu0.01Ti0.01)Σ4.00O8, Z = 2. The simplified formula is Sc(Y,Yb)Nb2O8, where Yb is the dominant lanthanoid. Shakhdaraite-(Y) is monoclinic, space group P2/c, a 9.930(2), b 5.6625(11), c 5.2108(10) Å, β 92.38(3)°, V 292.7(5) Å3, Z = 2. The crystal structure was solved by direct methods [R1 = 0.0269, 878 unique reflections (F > 4σF)]. There are three cation M sites: [6]M(1) = Nb2apfu, [6]M(2) = Sc apfu, and [8]M(3) = Y apfu, ideally M = ScYNb2apfu. The M(1) and M(2) octahedra each form a brookite chain along c. The Y-dominant [8]M(3A) polyhedra form a brookite-like kinked chain, and each M(3A) polyhedron of one brookite-like chain shares two edges with the two M(3A) polyhedra from the adjacent brookite-like chain, thus forming a [Y2O8]10– layer. In the structure of shakhdaraite-(Y), M(1A) and M(2) brookite chains and a layer of [8]-coordinated M(3A) polyhedra alternate along a. Shakhdaraite-(Y) is isostructural with samarskite-(Y), ideally YFe3+Nb2O8. Shakhdaraite-(Y) [Russian Cyrillic: шахдараит-(Y)] is named after its type locality: the valley of the Shakhdara River in the southwest of the Pamir Mountains.

The new ettringite-group mineral chiyokoite, ideally Ca3Si(CO3)[B(OH)4]O(OH)5·12H2O, was found in a hydrothermally altered calc-silicate skarn at the Fuka mine, Okayama Prefecture, Japan. Associated minerals are calcite, henmilite, and tacharanite. Chiyokoite occurs as hexagonal prismatic crystals up to 30 μm long and up to 20 μm thick. The major forms are the hexagonal prism {100} and monohedra {0001} and {000}. The crystals are combined in clusters which form friable nests up to 1 cm across. The mineral is pink to colorless with white streak and vitreous luster. The cleavage is parallel to {100} and {0001}, good. The fracture is stepped. Dmeas is 1.85(1) g/cm3, Dcalc is 1.85 g/cm3. Chiyokoite is optically uniaxial (–), ω = 1.523(2) and ε = 1.492(3) (589 nm). The infrared spectrum is reported. The chemical composition (wt.%) is CaO 27.56, B2O3 3.47, Al2O3 3.05, Fe2O3 0.12, As2O3 4.77, MnO2 0.32, SiO2 6.55, SO3 0.76, H2O 46.3, CO2 7.30, total 100.2. The empirical formula calculated on the basis of 3 Ca apfu is H31.37Ca3(Si0.67Al0.37Mn4+0.02Fe3+0.01)Σ1.07(C1.01B0.61As3+0.29S0.06)Σ1.97O24.19. The simplified general formula is Ca3(Si,Al)(CO3,AsO3)[B(OH)4,AsO3](OH)6·12H2O. Chiyokoite is hexagonal, P63, a = 11.0119(5), c = 10.5252(6) Å, and V = 1105.3(1) Å3. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I)(hkl)] are: 9.53(100)(100), 5.50(24)(110), 4.618(11)(102), 3.812(23)(112), 3.412(15)(211), 2.726(14)(302), 2.521(19)(123), and 2.172(13)(320,402,223). The crystal structure, refined from single-crystal X-ray diffraction data [R1(F) = 0.042], is based on [Ca3(Si,Al)(OH)6(H2O)12] columns parallel to the c axis with B(OH)4– and CO32– and admixed AsO33– anionic groups in channels between the columns. The mineral is named in honor of Professor Chiyoko Henmi (1949–2018).

Badakhshanite-(Y), ideally Y2Mn4Al(Si2B7BeO24), is a tetrahedral sheet-structure mineral found in the Dorozhny (Road) miarolitic granitic pegmatite within the Kukurt pegmatite field 45 km E of Murghab, Eastern Pamir, Gorno-Badakhshan Autonomous Oblast, Tajikistan. Badakhshanite-(Y) occurs in medium- to coarse-grained non-graphic albite-microcline-quartz pegmatites in close association with smoky quartz, Sc-bearing spessartine, Sc-bearing tusionite, and schorl. It often grows together with Sc-bearing tusionite and occurs as single columnar crystals ranging from 50 to 400 μm in length, as inclusions in spessartine and tourmaline, and rarely as crystals in blebs along boundaries between garnet, tourmaline, and quartz. Badakhshanite-(Y) is yellow brown and has a white streak and a vitreous luster. It is brittle, with a conchoidal fracture, Mohs hardness of 6.5–7, and calculated density of 4.41 g/cm. In thin section it is transparent and pale yellow, non-pleochroic, biaxial (–), with α = 1.805(2), βcalc = 1.827, γ = 1.835(3) (λ = 590 nm); 2V (meas.) = –60(10)°. Dispersion is weak, r > v. Extinction is straight, elongation is negative. FTIR spectra show the absence of (OH) and H2O groups. Chemical analysis by electron microprobe using WDS (6 points), SIMS, and ICP-OES for B and Be gave SiO2 11.96, ThO2 0.12, Sm2O3 0.17, Gd2O3 0.30, Tb2O3 0.10, Dy2O3 0.73, Ho2O3 0.19, Er2O3 1.34, Tm2O3 0.54, Yb2O3 8.82, Lu2O3 2.32, Y2O3 16.60, Sc2O3 1.57, Al2O3 3.06, B2O3 22.06, FeO 0.94, MnO 23.33, CaO 0.58, BeO 2.84, total 97.57 wt.%.The empirical formula based on 24 O apfu is (Y1.21REE0.78Th0.01)Σ2(Mn3.47Y0.34Ca0.11Fe2+0.08)Σ4(Al0.63Sc0.24Fe2+0.06□0.07)Σ1[(Si2.10B6.69Be1.20)Σ9.99O24], where REE = (Yb0.47Lu0.12Dy0.04Er0.07Tm0.03 Ho0.01Gd0.02Sm0.01Tb0.01)Σ0.78. Badakhshanite-(Y) is orthorhombic, space group Pnma, a 12.852(1), b 4.5848(5), c 12.8539(8) Å, V 757.38(7) Å3, Z = 2. The crystal structure was refined to R1 = 4.31% based on 1431 unique [F > 4σF] reflections. In the crystal structure of badakhshanite-(Y), a layer of tetrahedra parallel to (010) is composed of four different tetrahedrally coordinated sites: Si, B(1), B(2), and T ( = 1.623 Å, = 1.485 Å, = 1.479 Å, = 1.557 Å), which form four-, five-, and eight-membered rings, having the composition (Si2B7BeO24). Between the sheets of tetrahedra, there are three cation sites: M(1), M(2), and M(3) ( = 2.346 Å, = 2.356 Å, = 2.016 Å) occupied by Y(REE), Mn2+(Y, Ca, Fe2+), and Al(Sc), respectively. The M(1,2) sites ideally give Y2Mn4apfu; the M(3) site ideally gives Al apfu. Badakhshanite-(Y) is an Al- and Be-analogue of perettiite-(Y).

Fluorapophyllite-(Cs) (IMA 2018-108a), ideally CsCa4(Si8O20)F(H2O)8, is an apophyllite-group mineral from the moraine of the Darai-Pioz glacier, Tien-Shan, Northern Tajikistan. Associated minerals are quartz, pectolite, baratovite, aegirine, leucosphenite, pyrochlore, neptunite, fluorapophyllite-(K), and reedmergnerite. Fluorapophyllite-(Cs) is a hydrothermal mineral. It is colorless and has a vitreous luster and a white streak. Cleavage is perfect; it is brittle and has a stepped fracture. Mohs hardness is 4.5–5. Dmeas. = 2.54(2) g/cm3, Dcalc. = 2.513 g/cm3. Fluorapophyllite-(Cs) is unixial (+) with refractive indices (λ = 589 nm) ω = 1.540(2), ε = 1.544(2). It is non-pleochroic. Chemical analysis by electron microprobe gave SiO2 48.78, Al2O3 0.05, CaO 22.69, Cs2O 10.71, K2O 1.13, Na2O 0.04, F 1.86, H2Ocalc. 14.61, –O=F2 –0.78, sum 99.09 wt.%; H2O was calculated from crystal-structure analysis. The empirical formula based on 29 (O + F) apfu, H2O = 8 pfu, is (Cs0.75K0.24)Σ0.99(Ca3.99Na0.01)Σ4(Si8.01Al0.01)Σ8.02O20.03F0.97(H2O)8, Z = 2. The simplified formula is (Cs,K)(Ca,Na)4(Si,Al)8O20F(H2O)8. Fluorapophyllite-(Cs) is tetragonal, space group P4/mnc, a 9.060(6), c 15.741(11) Å, V 1292.10(19) Å3. The crystal structure has been refined to R1 = 4.31% based on 498 unique (Fo > 4σF) reflections. In the crystal structure of fluorapophyllite-(Cs), there is one [4]T site occupied solely by Si, = 1.615 Å. SiO4 tetrahedra link to form a (Si8O20)8– sheet perpendicular to [001]. Between the Si–O sheets, there are two cation sites: A and B. The A site is coordinated by eight H2O groups [O(4) site], A–O(4) = 3.152(4) Å; the A site contains Cs0.75K0.24□0.01, ideally Cs apfu. The Cs–O bond length of 3.152 Å is definitely larger than the K–O bond length of 2.966–2.971 Å in fluorapophyllite-(K), KCa4(Si8O20)F(H2O)8. The [7]B site contains Ca3.99Na0.01, ideally Ca4apfu; = 2.417 Å (φ = O, F, H2O). The Si–O sheets connect via A and B polyhedra and hydrogen bonding; two H atoms have been included in the refinement. Fluorapophyllite-(Cs) is isostructural with fluorapophyllite-(K). Fluorapophyllite-(Cs) is a Cs-analogue of fluorapophyllite-(K).

The crystal structure of polylithionite-1M from Darai-Pioz, (K0.97Na0.03Rb0.01)Σ1.01(Li2.04Al0.84 Ti4+0.09Fe3+0.03)Σ3.00(Si3.98Al0.02)O10[F1.68(OH)0.33]Σ2, a 5.1974(4), b 8.9753(6), c 10.0556(7) Å, β 100.454(1)°, V 461.30(6) Å3, space group C2, Z = 2, was refined to R1 = 1.99% using MoKα X-radiation. In the space group C2, there are three octahedrally coordinated M sites in the 1M mica structure: the M(1) site is occupied by Li+ and minor vacancy that is likely locally associated with Ti4+ at the M(2) site; the M(2) site is occupied dominantly by Al3+, with other minor divalent to tetravalent cations; the M(3) site is completely occupied by Li+. In the space group C2, the structure is completely ordered. Each non-bridging O2– ion is surrounded by an ordered arrangement of 2Li+ + Al3+ + Si4+ with an incident bond-valence sum of 1.95 vu (valence units). The F– ion is coordinated by Li+ + Li+ + Al3+ with an incident bond-valence sum of 0.84 vu (values around F– generally tend to be lower than ideal). Thus, the valence-sum rule is satisfied, both long range and short range. In the space group C2/m, there is long-range order but not short-range order. There are three different short-range arrangements, one of which has bond-valence deficiencies of 0.38 and 0.49 vu around the non-bridging O2– ion and the F– ion, destabilizing the structure relative to the more ordered arrangement of the C2 structure, which conforms more closely to the valence-sum rule. The drive to lower the symmetry in polylithionite-1M from C2/m to C2 comes from the short-range bond-valence requirements of the structure.

Cesiokenopyrochlore is a new mineral belonging to the pyrochlore group. It was discovered in a specimen of granitic pegmatite collected at Tetezantsio, Betafo region, Madagascar. The mineral forms rough equant crystals up to 0.05 mm in size intergrown with béhierite and rynersonite. Cesiokenopyrochlore is light-brown, translucent, with resinous luster. Dcalc. = 5.984 g/cm3. In reflected light it is light gray, isotropic, with strong light-brown internal reflections. The crystal structure was refined to R1 = 0.0212. The mineral is cubic, , a = 10.444(1) Å, V = 1139.5(2) Å3, and Z = 8. The strongest lines of the powder X-ray diffraction pattern [d, Å, (I, %) (hkl)] are: 6.03 (37) (111), 3.70 (9) (220), 3.15 (100) (311), 3.02 (36) (222), 2.012 (17) (511, 333), 1.848 (19) (440), 1.576 (11) (622). The chemical composition is (wt.%; electron microprobe): Cs2O 22.66, Na2O 1.74, CaO 0.64, Nb2O5 20.87, Ta2O5 21.27, WO3 30.67, H2O (calc) 0.12, total 97.97. The empirical formula of the holotype specimen calculated on the basis of (Nb+Ta+W) = 2 apfu and (O+OH) = 6 apfu and written according to the pyrochlore-supergroup nomenclature is Na0.29Ca0.06(Nb0.81W0.69Ta0.50)Σ2[O5.93(OH)0.07]Σ6Cs0.83. The simplified formula of the holotype specimen is □2(Nb,W,Ta)Σ2O6Cs. Cesiokenopyrochlore is the first natural niobate to adopt the inverse pyrochlore structure.

The new sulfate mineral ferroefremovite, ideally (NH4)2Fe2+2(SO4)3, was discovered at the “Bocca Grande” fumarole, Solfatara di Pozzuoli, Flegrean Volcanic Complex, Naples Province, Campania, Italy. Associated minerals are adranosite, adranosite-(Fe), godovikovite, huizingite-(Al), mascagnite, and opal. Ferroefremovite forms cubic crystals up to 0.1 mm across. The new mineral is colorless and has a vitreous luster and white streak. It is brittle and has an uneven fracture. No cleavage or parting are observed. Mohs hardness is 2. Dmeas. = 2.69(1) g/cm3. Dcalc. = 2.700 g/cm3. Optically, ferroefremovite is isotropic, n = 1.574(3) (λ = 589 nm). It is non-pleochroic. The presence of the NH4+ cation is confirmed by the Raman spectrum. The chemical composition of ferroefremovite is (wt.%; electron microprobe, N determined by CNH analysis) (NH4)2O 11.51, Na2O 0.11, K2O 1.35, MgO 8.38, MnO 0.98, FeO 18.94, SO3 57.08, total 98.35. The empirical formula based on 12 O apfu is [(NH4)1.85K0.12Na0.01]Σ1.98(Fe2+1.11Mg0.87Mn0.06)Σ2.04S2.99O12. Ferroefremovite is cubic, space group P213, with a = 10.0484(9) Å, V = 1014.59(16) Å3, and Z = 4. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are 5.80 (40) (111), 4.50 (20) (201, 210), 4.11 (30) (211), 3.17 (100) (310, 301), 3.02 (20) (311), 2.68 (50) (312, 321), 1.86 (18) (502, 432), 1.62 (18) (523, 532, 611).

Wernerbaurite (IMA 2012–064) and schindlerite (IMA 2012–063) from the St. Jude mine, Slick Rock district, San Miguel County, Colorado, USA, were described as hydronium-bearing decavanadate minerals with the formulae {[Ca(H 2 O) 7 ] 2 (H 2 O) 2 (H 3 O) 2 }{V 10 O 28 } and {[Na 2 (H 2 O) 10 ](H 3 O) 4 }{V 10 O 28 }, respectively. Because these phases correspond to known synthetic phases with these formulae, the presence of NH 4 in these minerals was not considered. Recent investigations of a similar phase discovered at a vanadium locality in the Fergana Valley of Kyrgyzstan showed it to contain NH 4 , leading us to reanalyze the original electron-microprobe mounts of wernerbaurite and schindlerite, specifically seeking N; those analyses confirmed the presence of sufficient NH 4 to replace the originally assigned H 3 O. With the additional H sites included and O replaced by N at the NH 4 sites, the structure refinement residual improved for wernerbaurite from R 1 = 3.41% to R 1 = 3.26% and for schindlerite from R 1 = 3.99% to R 1 = 3.70%. The newly assigned NH 4 sites exhibited normal NH 4 –O bond distances and coordination and reasonable bond-valence sums. The H 3 O synthetic equivalents of both phases had been reported, as well as the NH 4 synthetic equivalent of schindlerite. Subsequent to the publication of the original description of the minerals, the NH 4 equivalent of wernerbaurite has also been reported.

Bulgakite, ideally Li 2 (Ca,Na)Fe 2+ 7 Ti 2 (Si 4 O 12 ) 2 O 2 (OH) 4 (O,F)(H 2 O) 2 , and nalivkinite, ideally Li 2 NaFe 2+ 7 Ti 2 (Si 4 O 12 ) 2 O 2 (OH) 4 F(H 2 O) 2 , are astrophyllite-supergroup minerals. Bulgakite is a new mineral from the Darai-Pioz alkaline massif in the upper reaches of the Darai-Pioz river, in the area of the joint Turkestansky, Zeravshansky, and Alaisky ridges, Tajikistan. Bulgakite was found in fenitized amphibole–quartz–feldspar rock with brannockite, sogdianite, bafertisite, albite, and titanite. Bulgakite is brownish orange, transparent in thin grains, and has a vitreous luster. Mohs hardness is 3, D meas. = 3.30(2) g/cm 3 , D calc. = 3.326 g/cm 3 . Bulgakite is biaxial (+) with refractive indices (λ = 589 nm) α = 1.695(3), β = 1.711(2), γ = 1.750(3); 2 V meas. = 70(5)°, 2 V calc. = 67°, strong dispersion: r > v . Cleavage is perfect parallel to {001} and moderate parallel to {010}. Chemical analysis by electron microprobe gave SiO 2 35.63, Al 2 O 3 0.95, Na 2 O 1.04, K 2 O 3.27, Cs 2 O 0.31, CaO 2.56, MgO 0.16, ZnO 0.15, FeO 29.24, MnO 7.14, TiO 2 11.07, Nb 2 O 5 0.49, ZrO 2 0.37, SnO 2 1.18, F 1.01, Li 2 O 1.36 (AAS), Rb 2 O 0.85 (AAS), (H 2 O) calc. 4.04, sum 100.38 wt.%, H 2 O was calculated from crystal-structure analysis. The empirical formula based on 31.94 (O + OH + F + H 2 O) pfu is (Li 0.94 K 0.91 Rb 0.12 Cs 0.03 ) Σ2 (Ca 0.60 Na 0.40 ) Σ1 (Fe 2+ 5.34 Mn 1.32 Li 0.25 Mg 0.05 Na 0.04 Zn 0.02 ) Σ7.02 (Ti 1.82 Sn 0.10 Nb 0.05 Zr 0.04 ) Σ2.01 [(Si 7.78 Al 0.24 ) Σ8.02 O 24 ]O 2 (OH) 4 (F 0.70 O 0.30 )[(H 2 O) 0.94 □ 1.06 ] Σ2 , Z = 1. Bulgakite is triclinic, space group P , a 5.374(1), b 11.965(2), c 11.65(3) A, α 113.457(8), β 94.533(8), γ 103.08(1)°, V 657.5(8) A 3 . The six strongest reflections in the X-ray powder diffraction data [ d (A), I, ( hkl )] are: 10.54, 100, (001); 3.50, 100, (003); 2.578, 100, (130); 2.783, 90, (1 2); 1.576, 68, (3 1, 2); 2.647, 55, ( 11). The crystal structure has been refined to R 1 = 2.6% for 3592 unique F o > 4σ F ) reflections. In the crystal structure of bulgakite, there are four [4] T sites, with T –O> = 1.626 A, occupied mainly by Si, with minor Al. The TO 4 tetrahedra constitute the T 4 O 12 astrophyllite ribbon. The [6] D site is occupied mainly by Ti, with D –φ> = 1.965 A (φ = O, F). The T 4 O 12 astrophyllite ribbons and D octahedra constitute the H (Heteropolyhedral) sheet. In the O (Octahedral) sheet, there are four Fe 2+ -dominant [6] M (1–4) sites, with = 2.159 A (φ = O, OH). Two H and the central O sheets form the HOH block, and adjacent HOH blocks link via a common anion (X P D ) of two D octahedra. In the I (Intermediate) block between adjacent HOH blocks, there are two interstitial cation sites, A and B , and a W site, partly occupied by H 2 O. The A site splits into two partly occupied sites, [13] A (1) and [6] A (2), with A(1)–A(2) = 1.16 A. The [6] A (2) site is occupied by Li with = 2.285 A (φ = O, F, H 2 O), and the [13] A (1) site is occupied by K, Rb, and Cs with = 3.298 A. The aggregate content of the A site is (Li 0.94 K 0.91 Rb 0.12 Cs 0.03 ) Σ2 , ideally Li 2 apfu . The [10] B site is occupied by (Ca 0.60 Na 0.40 ) with B –φ> = 2.593 A. The W site is occupied by [(H 2 O) 0.94 □ 1.06 ] Σ2 pfu . The mineral is named bulgakite after Lev Vasil9evich Bulgak (born 1955), Russian mineralogist, gemologist, and discoverer of several new minerals. The crystal structure of nalivkinite has been revised and refined to R 1 = 4.52% for 3546 unique ( F o > 4σ F ) reflections: space group P , a 5.374(3), b 11.948(5), c 11.676(5) A, α 113.360(6), β 94.538(8), γ 103.01(1)°, V 658.7(9) A 3 , Z = 1, D calc. = 3.347 g/cm 3 . The revised empirical formula of nalivkinite is based on 32.14 (O + OH + F + H 2 O) pfu : (Li 1.14 K 0.75 Cs 0.09 Pb 0.02 ) Σ2 (Na 0.71 Ca 0.29 ) Σ1 (Fe 2+ 5.62 Mn 0.90 Zr 0.08 Na 0.08 Mg 0.04 Zn 0.04 ) Σ6.76 (Ti 1.56 Nb 0.24 Sn 0.09 Zr 0.08 Ta 0.04 ) Σ2 [(Si 7.86 Al 0.15 ) Σ8.01 O 24 ]O 2 (OH) 4 F[(H 2 O) 1.14 □ 0.86 ] Σ2 . The presence of H 2 O groups in the bulgakite and nalivkinite structures was confirmed by infrared spectroscopy. Bulgakite is a Ca-analogue of nalivkinite. Bulgakite and nalivkinite are related by the following substitution: 0.3 B Ca 2+ + 0.3 X O 2– ↔ 0.3 B Na + + 0.3 X F – .

Kirchhoffite, CsBSi 2 O 6 , is a new species of silicate mineral from the Darai-Pioz alkaline massif in the upper reaches of the Darai-Pioz River, in the area of the joint Turkestansky, Zeravshansky and Alaisky ridges, Tajikistan. It occurs as equant grains 10 to 80 μm in diameter, associated with quartz, pectolite, baratovite, polylithionite, aegirine, fluorite, leucosphenite, pyrochlore, neptunite and reedmergnerite. Kirchhoffite is colorless, transparent with a white streak, and a vitreous luster; it does not fluoresce under ultraviolet light. No cleavage or parting was observed. The Mohs hardness is 6 to 6½, and it is brittle with a conchoidal fracture. The measured and calculated densities are 3.62(2) and 3.639 g/cm 3 , respectively. Kirchhoffite is uniaxial (+) with indices of refraction ω = 1.592(2), ɛ = 1.600(2). The mineral is tetragonal, space group I 4 1 / acd , a 13.019(2), c 12.900(3) A, V 2186.3(1) A 3 , Z = 16, a : c = 1 : 0.9909. The six strongest lines in the X-ray powder diffraction pattern are [ d in A( I )( hkl )] are 3.26(100) (040), 3.48(82)(132), 2.770(67)(332, 233), 2.294(41)(044), 2.109(34)(352, 253), 5.32(32)(121). Chemical analysis by electron and ion microprobes gave SiO 2 40.47, B 2 O 3 11.27, K 2 O 0.11, Cs 2 O 48.16, Rb 2 O 0.09, for a sum of 100.10 wt.%. The resulting empirical formula on the basis of six atoms of oxygen is (Cs 1.02 K 0.01 ) ∑1.03 B 0.96 Si 2.02 O 6 , ideally CsBSi 2 O 6 . The crystal structure of kirchhoffite was refined to an R 1 index of 3.1% based on 487 observed reflections collected on a four-circle diffractometer with Mo K α X-radiation. In the structure, there are two tetrahedrally coordinated sites: Si is occupied by silicon, Si –O> = 1.610 A, B is occupied by boron, B –O> = 1.465 A, and Cs is [12]-coordinated, = 3.301 A. Tetrahedra form a [BSi 2 O 6 ] framework, the topology of which is identical to that of tetragonal pollucite, CsAlSi 2 O 6 . The [12]-coordinated Cs atoms occupy the channels along [111] formed by six-membered rings of tetrahedra. Kirchhoffite, CsBSi 2 O 6 , is a B analogue of the tetragonal modification of pollucite.

Three old specimens, collected in the 19 th century and now deposited in Fersman Mineralogical Museum, Moscow, are labeled as rhodizite from the Sarapulka, Shaitanka (both cotype localities) and Alabashka granitic pegmatite fields, Central Urals, Russia. All are Cs-dominant (Cs > K) and must now be considered londonite. The crystal structure of londonite from Sarapulka was solved from single-crystal data collected at 193 K and refined to R = 0.0203. The mineral is cubic, space group P 43 m, a 7.3149(7) A. Its structure is based on a microporous quasi-framework formed by clusters of four edge-sharing AlO 6 octahedra linked by BO 4 and BeO 4 tetrahedra. Both Cs + and K + are ordered in the cages of the quasi-framework. The very short Cs–K distance, 0.51(3) A, prevents simultaneous occupancy of these positions in the same cage. The Be and K atoms are also separated by an unallowable short distance of 2.76(3) A, and thus their contents are coupled. The solid-solution system between rhodizite (K-dominant), londonite (Cs-dominant) and the hypothetical K- and Cs-free analogue, (□,H 2 O){Al 4 [Be 4 B 12 O 28 ]}, is complicated, with numerous coupled heterovalent substitutions. Taking into consideration chemical, structural and IR data, it can be presented as: ( A ,□,H 2 O) 1 ( Al ,Li) 4 ( Be ,Li,Al,□ ) 4 ( B ,Be) 12 [ O 28− x (OH,F) x ], where A = K , Cs and x 4 O 4 ] clusters and a sodalite-type [B 12 O 24 ] framework results in the formation of an original complex, [Al 4 O 4 B 12 O 24 ], in rhodizite and londonite.

Nous avons affine la structure cristalline de la senkevichite, de composition ideale Cs K Na Ca 2 T1 4+ O [Si 7 O 18 (OH)], triclinique, PI, a 10.4191(4), b 12.2408(5), c 7.0569(3) A, a 90.857(1), β 99.193(1). γ 91.895(1)°, V 887.8(1) A 3 , Z= 2. D calc 3.125 g/cm 3 , provenant du complexe de Dara-i-Pioz, dans les montagnes Tien-Shan. au Tajikistan, jusqu'a un residu R 1 de 4.5% en utilisant 4872 reflexions uniques (F o > 4σF) prelevees avec un diffractometre a monocristal Bruker P4 muni d'un detecteur CCD 4K et rayonnement MoKα. Les analyses effectuees avec une microsonde electronique ont donne SiO 2 51.08 TiO 2 8.94, FeO 0.50, MnO 2.59, CaO 10.98, K 2 O 6.13, Na 2 O 3.76, Nb 2 O 5 0.64, Cs 2 O 15.28, (H 2 O) calc 1.09, pour un total de 100.99% (poids), et la proportion de (H 2 O) a ete etablie par analyse de la structure. La senkevichite est isostructurale avec la tinaksite et la tokkoite. La structure contient sept sites Si a coordinence tetraedrique, avec une distance moyenne de 1.623 A. Six de ces sites sont coordonnes par des atomes d'oxygene, et le site Si(7) est coordonne par trois atomes d'oxygene et un groupe (OH). Il s'agit donc d'un groupe silicate acide. Il y a trois sites M a coordinence [6]. Le site M(1) contient surtout T1 4+ (et un peu de Nb), avec = 1.985 A. Le site M(2) contient uniquement Ca, avec = 2.382 A, et le site M(3) contient Ca (et des quantites mineures de Fe 2+ et Mn 2+ ). avec = 2.317 A. Il y a un site Na a coordinence [7], avec = 2.504 A. Il y a deux sites A, le site A(1) a coordinence [12], contenant surtout le cesium (et un peu de K), avec = 3.318 A. et le site A(2), a coordinence [10], contenant uniquement le K, avec = 2.987A. La senkevichite est l'analoque a Cs de la tinaksite, K 2 Na Ca 2 T1 4+ O [S1 7 O 18 (OH)], et l'oxyanalogue a Cs-Na-Ti 4+ de la tokkoite, K 2 Ca 4 F |Si 7 O 18 (OH)].