Dmitry Yu. Pushcharovsky

Cupromolybdite, Cu3O(MoO4)2, a new fumarolic mineral from the Tolbachik volcano, Kamchatka Peninsula, Russia

Cupromolybdite is a new fumarolic mineral from the New Tolbachik scoria cones of Tolbachik volcano, Kamchatka Peninsula, Russia. Cupromolybdite forms prismatic crystals up to 150 μm in length or acicular crystals with a thickness and length of 1–5 μm and 100 μm, respectively. The acicular crystals of cupromolybdite are typically combined in radiating aggregates. The mineral is associated with piypite, fedotovite, vergasovaite, hematite, magnetite, aphthitalite, langbeinite, palmierite, As-bearing orthoclase, lammerite, klyuchevskite, alumoklyuchevskite, euchlorine, lyonsite, pseudolyonsite, averievite, rutile and native gold. Cupromolybdite is honey-yellow or brown, and its acicular crystals are bright yellow. The mineral has yellow or light brown streak and an adamantine lustre. Cupromolybdite is brittle, and its fracture ranges from uneven to splintery. The calculated density is 4.512 g/cm 3 . The reflectance values ( R 1 and R 2 , %) in air for the four COM wavelengths are 14.95, 16.3 (470 nm), 13.4, 14.85 (546 nm), 12.73, 14.16 (589 nm), and 12.15, 13.5 (650 nm). The chemical composition (the mean of 7 electron microprobe analyses) of cupromolybdite is: CuO 43.03, FeO 0.08, ZnO 0.53, MoO 3 54.48, SO 3 0.71, V 2 O 5 0.28, SiO 2 0.04, total 99.15 wt%, corresponding, on the basis of 9 O atoms, to (Cu 2.83 Zn 0.03 Fe 0.01 ) ∑2.87 (Mo 1.98 S 0.05 V 0.02 ) ∑2.05 O 9 . The simplified formula of cupromolybdite is Cu 3 O(MoO 4 ) 2 = Cu 3 Mo 2 O 9 . Cupromolybdite is orthorhombic, space group Pnma , a = 7.6638(1), b = 6.8670(1), c = 14.5554(2) A, V = 766.01(2) A 3 and Z = 4. The strongest powder-diffraction lines [ d in A ( I ) ( hkl )] are: 7.312 (67) (002); 3.518 (55) (113); 3.436 (100) (020); 3.301 (99) (210, 104); 3.065 (79) (121); 2.556 (62) (220); 2.506 (66) (301, 024). The crystal structure was solved from single-crystal data and refined to R = 0.0314. Cupromolybdite is a natural analogue of the synthetic compound Cu 3 Mo 2 O 9 and the S-free analogue of vergasovaite, Cu 3 O[(Mo,S)O 4 ][SO 4 ]. The crystal structure of cupromolybdite contains strings of corner-shared Cu(1)O 6 octahedra elongated in the [010] direction. In cupromolybdite, the Cu(2)O 5 square pyramids and the Cu(3)O 6 distorted octahedra are linked to the Cu(1)O 6 octahedra of the strings by edges and to the MoO 4 and (Mo,S)O 4 tetrahedra by vertices. A minor admixture of S is concentrated in one of two independent tetrahedral positions. Cupromolybdite is named for its chemical composition.

Pseudolyonsite, Cu3(VO4)2, a new mineral species from the Tolbachik volcano, Kamchatka Peninsula, Russia

Pseudolyonsite, ideally Cu 3 (VO 4 ) 2 , is a new mineral from the medium-temperature fumaroles of the New Tolbachik scoria cones, Tolbachik volcano, Kamchatka Peninsula, Russia. It occurs as needles that are 5–20 μm across and up to 0.5 mm in the length, which sometimes produce parallel intergrowths, sprays or openwork clusters up to 2 mm. Associated minerals are: piypite, hematite, magnetite, lyonsite, aphthitalite, palmierite, langbeinite, filatovite, lammerite, vergasovaite, rutile and native gold. Pseudolyonsite is dark red with a brownish tint to black, translucent to opaque, with a reddish-brown streak and adamantine to semi-metallic lustre. The mineral is brittle, but thin long needles are flexible. The fracture is conchoidal, and no cleavage was observed. The calculated density is 4.749 g/cm 3 . In reflected light in air the mineral is grey with a weak bluish tint, non-pleochroic, has distinct anisotropy and ubiquitous red to orange internal reflections. The reflectance values (R 1 and R 2 , %) in air for the four COM wavelengths are, respectively, 17.05, 19.6 (470 nm); 16.1, 18.15 (546 nm); 15.85, 17.7 (589 nm); and 15.55, 17.4 (650 nm). Four electron probe (EDS) analyses produced the following mean values: V 2 O 5 40.37, CuO 48.83, ZnO 7.60, MoO 3 1.89, and SiO 2 0.14, total 98.83 wt%, which corresponds, on the basis of 8 O atoms, to (Cu 2.58 Zn 0.44 ) ∑3.02 (V 1.88 Mo 0.06 Si 0.02 ) ∑1.96 O 8 . The idealised formula is Cu 3 (VO 4 ) 2 . Pseudolyonsite is monoclinic: P 2 1 / c , a = 6.2695(4), b = 8.0195(3), c = 6.3620(3) A, β = 111.96(1)°, V = 296.66(3) A 3 , Z = 2. The strongest powder X-ray diffraction lines [ d in A (I) ( hkl )] are: 4.70 (60) (110); 3.30 (79) (021, 120); 3.22 (87) (111); 3.18 (34) (−121, −102); 2.894 (74) (200, −211); 2.761 (100) (012); 2.479 (59) (−212, −122); 2.419 (67) (031, 130). The crystal structure was solved from single-crystal data and refined to R = 0.0444. Pseudolyonsite is isostructural with synthetic monoclinic Cu 3 (VO 4 ) 2 . The crystal structure of pseudolyonsite contains corrugated octahedral layers formed by the chains of edge-shared, distorted Cu(2)-octahedra running along the c axis and connected to each other by distorted Cu(1)-octahedra. The octahedra of both types contain Cu and subordinate Zn, and they are typically Jahn-Teller-distorted. Adjacent octahedral layers are connected to each other by VO 4 tetrahedra. Pseudolyonsite is dimorphous with triclinic mcbirneyite. The name pseudolyonsite comes from its close visual similarity to another vanadate mineral, lyonsite, Cu 3 Fe 3+ 4 (VO 4 ) 6 . Both the mineral and its name have been approved by the IMA CNMNC (IMA No. 2009-062).

Tumchaite Na2(Zr,Sn)Si4O11·2H2O—A new mineral from carbonatites of the Vuoriyarvi alkali-ultrabasic massif, Murmansk Region, Russia

Abstract Tumchaite, Na2(Zr,Sn)Si4O11·2H2O, is a new species from the Vuoriyarvi alkali-ultrabasic massif, Murmansk Region, Russia, where it occurs as colorless to white tabular monoclinic crystals associated with calcite, dolomite, a mineral of the serpentine group and pyrite in the late dolomitecalcite carbonatites. It is transparent to translucent; with vitreous luster; and perfect cleavage on (100). Mohs hardness is 4.5, Dmeas is 2.78 (2) g/cm3. Tumchaite is optically biaxial (-), with α = 1.570 (2), β = 1.588 (2), γ = 1.594 (2), 2Vmeas = 60 (5)°, and elongation positive, Y = b, c ∧ Z = 3°. Pleochroism exists, with Y = Z = colorless, X = greenish-gray. Electron microprobe analysis gave (wt%): Na2O 13.72, CaO 0.15, SiO2 52.71, TiO2 0.35, ZrO2 20.41, SnO2 5.73, HfO2 0.60, H2O (computed assuming 2H2O pfu.) 7.86, total 101.53. The X-ray study pointed to space group P21/c, a = 9.144 (4), b = 8.818 (3), c = 7.537 (3) Å, β = 113.22 (3)°, V = 558.49 Å3, Z = 2. The strongest lines of the powder diffraction pattern [d in Å (I) (hkl)] are: 8.40 (10) (100), 5.38 (9) (111̅), 4.00 (8) (111), 3.401 (9) (202̅), 2.902 (9) (211), 2.691 (9) (131̅). The crystal structure of tumchaite was refined to R = 0.043 for 865 Fo > 4σ(Fo). The mineral is isotypic with penkvilksite-1M. The structure is characterized by silicate sheets parallel (100), formed by alternating clockwise- and counterclockwise-growing spiral chains of corner-sharing SiO4 tetrahedra. The sheets are connected by octahedra occupied by (Zr, Sn) at 0, 1/2, 0. The Zr/Sn ratio in the octahedra is 4. Water molecules and Na cations are placed in the cavities of the polyhedral framework. The ideal crystal-chemical formula is Na2 (Zr0.8Sn0.2)[Si4O11]·2H2O. The mineral is named tumchaite for the river Tumcha near Vuoriyarvi massif.

Fluorcalciobritholite, Ca,REE)5[(Si,P)O4]3F, a new mineral: description and crystal chemistry

The new mineral fluorcalciobritholite, ideally Ca 3 Ce 2 (SiO 4 ) 2 (PO 4 )F, has been found at Mount Kukisvumchorr, Khibiny alkaline complex, Kola Peninsula, Russia, in veinlets which contains aggregates of orthoclase, nepheline, sodalite and biotite in association with grains of fayalite, gadolinite-(Ce), zircon, monazite-(Ce), zirconolite (“polymignite”), fluorapatite, fluorite, molybdenite, lollingite and graphite. Fluorcalciobritholite forms long-prismatic hexagonal crystals up to 0.5 x 10 mm; the main crystal form is the hexagonal prism {10–10}. The mineral is transparent, with a pale pinkish to brown colour and a white streak. The hardness (Mohs) is 5.5, and the observed density is 4.2(1) g/cm 3 . Optically, it is uniaxial (−) with ω 1.735(5), e 1.730(5). Electron microprobe gave the following empirical formula based on [Si+P+S] = 3 apfu : [Ca 2.80 (Ce 0.93 La 0.54 Nd 0.26 Y 0.18 Pr 0.08 Sm 0.03 Gd 0.03 Dy 0.02 Yb 0.02 Er 0.01 ) ∑2.12 Th 0.04 Mn 0.03 Sr 0.02 ] ∑4.99 [(Si 1.94 P 1.06 ) ∑3 O 12 ] [F 0.76 O 0.22 Cl 0.01 ] ∑0.99 (Z = 2). The IR spectrum of metamict fluorcalciobritholite from Siberia showed a marked similarity with those of hydroxylbritholite-(Ce) and hydroxylbritholite-(Y). The strongest lines of the X-ray powder pattern [ d in A ( I ) ( hkl )] are: 3.51 (45) 002, 3.15 (70) 102, 2.85 (100) 211, 121, 2.78 (60) 300. The mineral is hexagonal, space group P 6 3 / m , with a = 9.580(7), c = 6.985(4) A, V = 555.2(7) A 3 . The crystal structure was refined from single-crystal X-ray diffraction data to R F = 0.029. Fluorcalciobritholite, whose simplified formula is (Ca,REE) 5 [(Si,P)O 4 ] 3 F, differs from fluorbritholite in having Ca > ∑REE, and differs from fluorapatite in having Si > P. Its compositional field falls within the limits Ca 2.5 REE 2.5 (SiO 4 ) 2.5 (PO 4 ) 0.5 F (boundary with fluorbritholite) and Ca 3.5 REE 1.5 (SiO 4 ) 1.5 (PO 4 ) 1.5 F (boundary with fluorapatite). Both the mineral and its name have been approved by the IMA Commission on New Minerals and Mineral Names.

The crystal structure and refined formula of mountainite, KNa2Ca2[Si8O19(OH)] · 6 H2O

Abstract Mountainite was described as a new mineral in 1957 with formula (Ca,Na2,K2)16Si32O80 · 24 H2O; its crystal structure was not solved up to now. We studied the structure of mountainite from the Yubileinaya pegmatite, Lovozero alkaline complex, Kola Peninsula, Russia. Mountainite is monoclinic, P2/c, a = 13.704(2), b = 6.5760(10), c = 13.751(2) Å, β = 105.752(10)°, V = 1192.7(3) Å3, Z = 2, Dcalc = 2.28 g/cm3. The crystal structure was solved by direct methods and refined to R(F) = 0.0639 for 1186 unique reflections with I > 3σ(I). Rietveld refinement on powder data completely confirmed the model obtained using a single crystal. Mountainite is a phyllosilicate, representative of a new structure type. The most specific feature of the mountainite structure is a TOT block formed by two SiO-layers [Si8O18(O,OH)2] (T-layers) and zig-zag columns of edge-sharing CaO5(H2O) octahedra sandwiched between them (O-layer). K cations occupy 10-fold polyhedra and are located between the columns of Ca-centered octahedra. The interlayer space between the neighboring TOT blocks is filled by Na cations and H2O molecules. The crystal-chemical formula is: KNa2Ca2{Si8O18[O(OH)]} · 6 H2O, the simplified formula is: KNa2Ca2Si8O19(OH) · 6 H2O.

Koksharovite, CaMg2Fe3+ 4(VO4)6, and grigorievite, Cu3Fe3+ 2Al2(VO4)6, two new howardevansite-group minerals from volcanic exhalations

Two new howardevansite-group minerals were discovered in the exhalations of fumaroles related to two volcanoes in Kamchatka, Russia. Koksharovite, CaMg2Fe 3þ 4(VO4)6, is found at the Bezymyannyi volcano in association with bannermanite. Grigorievite, Cu3Fe 3þ 2Al2(VO4)6, associated with bannermanite, ziesite, hematite, etc., was found at the Second scoria cone of the NorthernBreakthroughoftheGreatTolbachikFissureEruption,Tolbachikvolcano.Koksharoviteoccursasequanttoprismaticcrystals upto30 � 70mm.Itistranslucent,yellowish-browntoreddish-brownwithanadamantinelustre.Grigorieviteformsprismatic totabular crystals up to 40 � 100 mm. It is opaque, black with a semi-metallic lustre. Both minerals are brittle. The VHN hardness is 368 and 489 kg mm � 2 , the calculated density (Dcalc) 3.39 and 3.67 g cm � 3 for koksharovite and grigorievite, respectively. In reflected light, koksharovite is light grey, grigorievite is grey; both minerals are weakly anisotropic. Reflectance values (koksharovite//grigorievite: Rmax-Rmin ,%( l, nm)) are: 15.3-14.4//16.8-16.4 (470), 14.1-13.2//15.9-15.5 (546), 13.8-13.0//15.3-14.9 (589), 13.4-12.7//14.8-14.1 (650). Chemical data (wt%, electron-microprobe analysis; first value is for koksharovite, second for grigorievite) are: Na2O 0.76, 0.00; K2O 0.05, 0.00; MgO 9.43,2.78; CaO 3.57, 0.95; MnO 0.46,0.04;CuO 0.00,17.70; NiO 0.11, 0.00;ZnO 0.00,0.14; Al2O3 3.04,11.76; Fe2O3 23.88, 10.10; TiO2 2.42, 1.47; SiO2 0.20, 0.00; P2O5 0.98, 0.13; V2O5 53.86, 54.97; total 98.76, 100.04. The empirical formulae, based on 24 O atoms per formula unit, are: Na0.24K0.01Ca0.63Mg2.30Mn0.06Ni0.01Al0.59Fe 3þ 2.94Ti0.30Si0.03P0.14V5.83O24 (koksharovite); Ca0.17Mg0.69Mn0.01Cu2.23Zn0.02Al2.31Fe 3þ 1.27Ti0.18P0.02V6.05O24(grigorievite).Bothmineralsaretriclinic,spacegroupP-1,Z ¼1.Unit-

New arsenate minerals from the Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. IV. Shchurovskyite, K2CaCu6O2(AsO4)4 and dmisokolovite, K3Cu5AlO2(AsO4)4

Abstract Two new minerals shchurovskyite, ideally K2CaCu6O2(AsO4)4, and dmisokolovite, ideally K3Cu5AlO2(AsO4)4, are found in sublimates of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka Peninsula, Russia. They are associated with one another and with johillerite, bradaczekite, tilasite, melanarsite, tenorite, hematite, aphthitalite, langbeinite, orthoclase, etc. Shchurovskyite occurs as coarse tabular or prismatic crystals up to 0.15 mm in size or anhedral grains forming parallel aggregates and crusts up to 1.5 cm × 2 cm across. Dmisokolovite forms tabular, prismatic or dipyramidal crystals up to 0.2 mm in size, commonly combined in clusters or crusts up to 0.7 cm × 1.5 cm across. Both minerals are transparent with a vitreous lustre. They are brittle, with Mohs’ hardness ≈ 3. Shchurovskyite is olive-green or olive drab. Dmisokolovite is bright emerald-green to light green. Dcalc = 4.28 (shchurovskyite) and 4.26 (dmisokolovite) g cm−3. Both are optically biaxial; shchurovskyite: (+), α = 1.795(5), β = 1.800(5), γ = 1.810(6), 2Vmeas = 70(15)°; dmisokolovite: (−), α = 1.758(7), β = 1.782(7), γ = 1.805(8), 2Vmeas = 85(5)°. The Raman spectra are given. Chemical data (wt.%, electron-microprobe; first value is for shchurovskyite, second for dmisokolovite):Na2O 0.00, 0.83;K2O 8.85, 10.71; Rb2O 0.11, 0.00;MgO 0.00, 0.35; CaO 4.94, 0.21; CuO 43.19, 38.67; ZnO 0.42, 0.20; Al2O3 0.04, 4.68; Fe2O3 0.00, 0.36; P2O5 0.59, 0.78; V2O5 0.01, 0.04; As2O5 40.72, 43.01; SO3 0.35, 0.00; total 99.22, 99.84. The empirical formulae, based on 18 O a.p.f.u., are shchurovskyite: K2.05Rb0.01Ca0.96Cu5.92Zn0.06Al0.01P0.09S0.05As3.86O18; dmisokolovite: Na0.28K2.36Mg0.09C a0.04Cu5.04Zn0.04 Al0.95Fe0.053+P0.11As3.88O18. The strongest reflections of X-ray powder patterns [d,Å(I)(hkl)] are shchurovskyite: 8.61(100)(200, 001), 5.400(32)(110), 2.974(32)(3̅12, 510), 2.842(47)(003, 020), 2.757(63) (6̅01, 511), 2.373(36)(512, 420) and 2.297(31)( 4̅21, 2̅22, 313); dmisokolovite: 8.34(95)(002), 5.433(84)(110), 2.921(66)(510, 3̅14), 2.853(58)(511, 020) and 2.733(100)(006, 512, 6̅02). Shchurovskyite is monoclinic, C2, a = 17.2856(9), b = 5.6705(4), c = 8.5734(6) Å, β = 92.953(6)°, V = 839.24(9) Å3 and Z = 2. Dmisokolovite is monoclinic, C2/c, a = 17.0848(12), b = 5.7188(4), c = 16.5332(12) Å, β = 91.716(6)°, V = 1614.7(2) Å3 and Z = 4. Their crystal structures [single-crystal X-ray diffraction data, R = 0.0746 (shchurovskyite) and 0.1345 (dmisokolovite: model)] are closely related in the topology of the main building units. They are based on a quasi-framework consisting of AsO4 tetrahedra and polyhedra centred by Cu in shchurovskyite or by Cu and Al in dmisokolovite. K and Ca are located in channels of the quasi-framework. The minerals are named in honour of outstanding Russian geologists and mineralogists Grigory Efimovich Shchurovsky (1803−1884) and Dmitry Ivanovich Sokolov (1788-1852).

Crystal structures of shlykovite and cryptophyllite: comparative crystal chemistry of phyllosilicate minerals of the mountainite family

We solved the crystal structures of the phyllosilicates shlykovite, KCaSi 4O 9(OH)·3H 2O, and cryptophyllite, K 2CaSi 4O 10 ·5H 2O, two minerals recently discovered in the Khibiny alkaline complex (Kola Peninsula, Russia). Both minerals are monoclinic with close values of the a, b and s parameters but significantly different c parameters: shlykovite a -6.4897(4), b = 6.9969(5), c = 26.714(2) A, β = 94.597(8)°, V = 1209.12(15) A 3, sp. gr. P2 1/c, Z = 4; cryptophyllite a = 6.4934(14), b = 6.9919(5), c = 32.087(3) A, β = 94.680(12)°, V= 1451.9(4) A 3, sp. gr. P2 1/n, Z = 4. The structures were solved from synchrotron diffraction data collected on the same intergrown crystal and refined anisotropically to R(F) = 0.0960 for 1147 unique reflections with I > 2σ(I) (shlykovite) and R(F) = 0.0856 for 1667 unique reflections with I > 2σ(I) (cryptophyllite). Shlykovite and cryptophyllite are representatives of two new, closely related structure types. The main structural units of both minerals are TOT blocks consisting of tetrahedral Si layers (T) and an octahedral component (O), sandwiched between them. Each T-layer consists of 4- and 8-membered rings of SiO 4-tetrahedra and can be considered as a half of a double layer described in the structures of the members of the rhodesite mero-plesiotype series. A topologically closely related Si layer also forming T-fragments of TOT blocks was recently discovered in mountainite, KNa 2Ca 2[Si 8O 19(OH)]·OH 2O. The O-fragment of the TOT blocks in shlykovite and cryptophyllite is formed by columns of edge-sharing Ca-centred octahedra CaO 5(H 2O). In both minerals K cations are located in the voids of the Si layer. The content of the interlayer space is different in each mineral: only H 2O molecules in shlykovite, K atoms and H 2O molecules in cryptophyllite. Mountainite, shlykovite and cryptophyllite are closely related in configuration of the TOT blocks, their main structure unit, and could be used to define the mountainite structural family. ©2010 E. Schweizerbartsehe Verlagsbuchhandlung, D-70176 Stuttgart.

Biraite-(Ce), Ce2Fe 2+(CO3)(Si2O7), a new mineral from Siberia with a novel structure type

Biraite-(Ce), ideally Ce 2 Fe 2+ (CO 3 )(Si 2 O 7 ), has been found in the Biraia deposit (Irkutsk district, Russia), associated with cordylite-(Ce) and -(La), aragonite, strontianite, Sr,Fe- bearing dolomite, ancylite-(Ce) and -(La), hydroxy 1 -bastnasite-(Ce), daqingshanite-(Ce) and -(La), tremolite, winchite, ferriallanite-(Ce), tornebohmite-(Ce), cerite, chevkinite-(Ce), belkovite, humite, fergusonite-(Ce) and -(Nd), pyrochlore, barite, monazite-(Ce) and other unknown minerals. Biraite-(Ce) occurs as irregular to well-shaped grains from 0.1 to 3 mm in length, has brown colour with a white streak, is semi-transparent with a vitreous luster and brittle.The hardness (Mohs) is 5, and the calculated density is 4.76 g/cm 3 . Optically, biraite-(Ce) is biaxial (-), with α 1.785(1),β 1.810(2),γ 1.820(1), 2V 66°(1). Electron microprobe and wet chemical analyses gave the following empirical formula based on 10 O+F:(Ce 1.01 La 0.57 Nd 0.25 Pr 0.09 Sm 0.02 Ca 0.07 Na 0.02 Ba 0.01 )Σ= 2.04 (Fe 0.60 Mg 0.25 Mn 0.11 Ti 0.01 )Σ=0.97 (CO 3 ) 0.99 [Si 1.97 (O 6.87 F 0.17 )Σ=7.04].The simplified formula is Ce 2 Fe 2+ (CO 3 )(Si 2 O 7 ). The IR spectrum confirmed the presence of [CO 3 ] groups. The strongest lines of the X-ray powder pattern [ d in A ( I ) ( hkl )] are: 3.30 (5) (021), 2.92 (10) (006, 21–2), 2.65 (5) (202, 12–4), 2.23 (5) (116, 031). Biraite-(Ce) is monoclinic, space group P 2 1 / c , with a 6.505(7), b 6.744(2), c 18.561(4) A,β 108.75(2)°. Its crystal structure was refined from single-crystal X-ray diffraction data to R(F) = 0.033. Biraite-(Ce) displays a new structure type, based on polyhedral (001) sheets composed of pairs of edge-sharing [FeO 6 ] octahedra, [Si 2 O 7 ] groups, and [CO 3 ] triangles. Ce 3+ cations in ten-fold coordination provide the linkage between neighbour polyhedral sheets. Both the mineral and its name have been approved by the IMA Commission on New Minerals and Mineral Names.

Umbrianite, K7Na2Ca2[Al3Si10O29]F2Cl2, a new mineral species from melilitolite of the Pian di Celle volcano, Umbria, Italy

The new mineral umbrianite, ideally K 7 Na 2 Ca 2 [Al 3 Si 10 O 29 ]F 2 Cl 2 , was discovered as an essential groundmass mineral in melilitolite of the Pian di Celle volcano, Umbria, Italy. It forms rectangular, lamellar or lath-shaped crystals (up to 25 × 30 × 200 μm), typically flattened on {010}, and sheaf-like aggregates (up to 200–500 μm across). Umbrianite is commonly associated with kalsilite, leucite, fluorophlogopite, melilite, olivine (Fo >60 ), diopside, nepheline, Ti-rich magnetite, fluorapatite, cuspidine–hiortdahlite series minerals, gotzenite, khibinskite, monticellite–kirschsteinite series minerals, westerveldite, various sulphides and peralkaline silicate glass. The empirical formula (based on Si + Al + Fe 3+ = 13) of the holotype umbrianite (mean of 58 analyses) is (K 6.45 Na 0.35 (Sr,Ba) 0.01 ) ∑6.81 (Na 1.22 Ca 0.78 ) ∑2.00 (Ca 1.85 Mg 0.13 Mn 0.01 Ti 0.01 ) ∑2.00 [(Fe 3+ 0.34 Al 3.06 Si 9.60 ) ∑13.00 O 29.00 ]F 2.05 Cl 1.91 (OH) 0.04 . The strongest lines of the X-ray diffraction powder pattern {d[A] ( I obs )} are: 9.65(100), 6.59(97), 3.296(77), 3.118(70), 2.819(53), 2.903(52), 6.91(43). The strong bands in the Raman spectrum of umbrianite are at 525, 593, 735 and 1036 cm −1 . The mineral is orthorhombic, space group Pmmn, unit-cell parameters are: a = 7.0618(5), b = 38.420(2), c = 6.5734(4) A, V = 1783.5(2) A 3 , Z = 2. The calculated density is 2.49 g/cm 3 . The crystal structure of umbrianite has been refined from X-ray single-crystal data to R = 0.0941 for 1372 independent reflections with I > 2σ( I ). Umbrianite is a representative of a new structure type. Its crystal structure contains the triple-layer tetrahedral blocks [Al 4 (Si,Al) 2 (Si,Al,Fe) 4 Si 16 O 58 ] ∞ connected to each other via the columns of edge-shared octahedra CaO 5 F to form a 3D quasi-framework with channels filled by Cl − , K + (inside the tetrahedral blocks) and Na + (between the Ca octahedral columns). Umbrianite, gunterblassite and hillesheimite, containing topologically identical triple-layer tetrahedral blocks, form the gunterblassite group. Umbrianite is unstable under postmagmatic hydrothermal conditions and alters to Ba-rich hydrated phases.