Igor V. Pekov

Nomenclature of the apatite supergroup minerals

The apatite supergroup includes minerals with a generic chemical formula IX M12 VII M23( IV TO4)3 X( Z ¼ 2); chemically they can be phosphates, arsenates, vanadates, silicates, and sulphates. The maximum space group symmetry is P63/m, but several members of the supergroup have a lower symmetry due to cation ordering and deviations from the ideal topology, which may result in an increase of the number of the independent sites. The apatite supergroup can be formally divided into five groups, based on crystal-chemical arguments: apatite group, hedyphane group, belovite group, britholite group, and ellestadite group. The abundance of distinct ions which may be hosted at the key-sites (M ¼ Ca 2þ , Pb 2þ , Ba 2þ , Sr 2þ , Mn 2þ , Na þ , Ce 3þ , La 3þ ,Y 3þ , Bi 3þ ;T ¼ P 5þ , As 5þ ,V 5þ , Si 4þ ,S 6þ ,B 3þ ;X ¼ F � , (OH) � , Cl � ) result in a large number of compositions which may have the status of distinct mineral species. Naming of apatite supergroup minerals in the past has resulted in nomenclature inconsistencies and problems. Therefore, an ad hoc IMA-CNMNC Subcommittee was established with the aim of rationalizing the nomenclature within the apatite supergroup and making some order among existing and potentially new mineral species. In addition to general recommendations for the handling of chemical (EPMA) data and for the allocation of ions within the various sites, the main recommendations of this subcommittee are the following: 1. Nomenclature changes to existing minerals. The use of adjectival prefixes for anions is to be preferred instead of modified Levinson suffixes; accordingly, six minerals should be renamed as follows: apatite-(CaF) to fluorapatite, apatite-(CaOH) to hydroxylapatite, apatite-(CaCl) to chlorapatite, ellestadite-(F) to fluorellestadite, ellestadite-(OH) to hydroxylellestadite, phospho- hedyphane-(F) to fluorphosphohedyphane. For the apatite group species these changes return the names that have been used in thousands of scientific paper, treatises and museum catalogues over the last 150 years. The new mineral IMA 2008-009, approved without a name, is here named stronadelphite. Apatite-(SrOH) is renamed fluorstrophite. Deloneite-(Ce) is renamed deloneite. The new mineral IMA 2009-005 is approved with the name fluorbritholite-(Y).

Chukanovite, Fe2(CO3)(OH)2, a new mineral from the weathered iron meteorite Dronino

The new mineral chukanovite, Fe 2 (CO 3 )(OH) 2 , occurs in cavities of weathered fragments of the Dronino ataxite iron eteorite found near the Dronino village, Kasimov district, Ryazan’ Oblast, Russia. It is a product of terrestrial alteration of meteorite iron. Associated minerals are goethite, akaganeite, hematite, hibbingite, reevesite, honessite, etc . Chukanovite forms acicular to fibrous individuals (up to 0.5 mm long and up to 2–3 μm thick) combined in spherulites up to 1 mm in diameter, botryoidal spherulitic clusters and parallel- or radial-columnar aggregates which form crusts up to 1 mm thick. Unaltered chukanovite is transparent, pale-green or colourless. The surface of aggregates is brownish-green. Streak is white. Lustre is vitreous. Cleavage is perfect, probably on {0–21}, fracture is uneven. The mineral is brittle, the Mohs’ hardness is 3.5–4, the calculated density is 3.60 g/cm 3 . It is optically biaxial (−) with α1.673(3), β1.770(5), γ1.780(5), 2 V meas. 10(5)°. Average chemical composition (wt. %; electron probe, H 2 O by modified Penfield method, CO 2 by selective sorption) is: MgO 0.1, FeO 68.8, NiO 0.6, CO 2 19.8, H 2 O 10.9, total 100.2. The empirical formula calculated on the basis of two metal atoms is (Fe 2+ 1.97 Ni 0.02 Mg 0.01 ) ∑2.00 (CO 3 ) 0.93 (OH) 2.14 ·0.18H 2 O, ideally Fe 2 (CO 3 )(OH) 2 . Chukanovite is monoclinic P 2 1 / a , with a = 12.396(1) A, b = 9.407(1) A, c = 3.2152(3) A, β = 97.78°. The strongest lines of the X-ray powder pattern [ d (A), I, ( hkl )] are: 6.14, 40, (200); 5.15, 60, (231); 3.73, 80, (310); 2.645, 100, (230); 2.361, 40, (510); 2.171, 40, (520). The structure of chukanovite was refined on synchrotron data by the Rietveld method up to Rp = 3.43 %, wRp = 4.51 %, R Bragg = 2.48 %. Chukanovite is closely related to the minerals of the malachite-rosasite group. It was named in honour of Nikita V. Chukanov (b. 1953), Russian physicist and mineralogist. The holotype specimen is deposited in the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow.

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.

New arsenate minerals from the Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. I. Yurmarinite, Na7(Fe3+,Mg,Cu)4(AsO4)6

Abstract A new mineral, yurmarinite, Na7(Fe3+,Mg,Cu)4(AsO4)6, occurs in sublimates of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with hatertite, bradaczekite, johillerite, hematite, tenorite, tilasite and aphthitalite. Yurmarinite occurs as well-shaped, equant crystals up to 0.3 mm in size, their clusters up to 0.5 mm and thin, interrupted crystal crusts up to 3 mm × 3 mm on volcanic scoria. Crystal forms are {101}, {011}, {100}, {110} and {001}. Yurmarinite is transparent, pale green or pale yellowish green to colourless. The lustre is vitreous and the mineral is brittle. The Mohs hardness is ~4½. One direction of imperfect cleavage was observed, the fracture is uneven. D(calc.) is 4.00 g cm-3. Yurmarinite is optically uniaxial (-), ω = 1.748(5), ε = 1.720(3). The Raman spectrum is given. The chemical composition (wt.%, electron microprobe data) is Na2O 16.85, K2O 0.97, CaO 1.28, MgO 2.33, MnO 0.05, CuO 3.17, ZnO 0.97, Al2 O3 0.99, Fe2O3 16.44, TiO2 0.06, P2O5 0.12, V2O5 0.08, As2O5 56.68, total 99.89. The empirical formula, calculated on the basis of 24 O atoms per formula unit, is (Na6.55Ca0.28K0.22)∑7.05(Fe3+2.48Mg0.70Cu0.48Al0.23Zn0.14 Ti0.01Mn0.01)∑4.05(As5.94P0.02V0.01)∑5.97O24. Yurmarinite is rhombohedral, R3̅ c, a = 13.7444(2), c = 18.3077(3) Å, V = 2995.13(8) Å3, Z = 6. The strongest reflections in the X-ray powder pattern [d,Å (I)(hkl)] are: 7.28(45)(012); 4.375(33)(211); 3.440(35)(220); 3.217(36)(131,214); 2.999(30)(223); 2.841(100)(125); 2.598(43)(410). The crystal structure was solved from single-crystal X-ray diffraction data to R = 0.0230. The structure is based on a 3D heteropolyhedral framework formed by M4O18 clusters (M = Fe3+ > Mg,Cu) linked with AsO4 tetrahedra. Sodium atoms occupy two octahedrally coordinated sites in the voids of the framework. In terms of structure, yurmarinite is unique among minerals but isotypic with several synthetic compounds with the general formula (Na7-x⃞x)(M3+3+xM2+1-x)(T5+O4)2 in which T = As or P, M3+ = Fe or Al, M2+ = Fe and 0 ≤ x ≤ 1. The mineral is named in honour of the Russian mineralogist, petrologist and specialist in studies of ore deposits, Professor Yuriy B. Marin (b. 1939). The paper also contains a description of the Arsenathaya fumarole and an overview of arsenate minerals formed in volcanic exhalations.

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).

Shirokshinite, K(NaMg2)Si4O10F2, a new mica with octahedral Na from Khibiny massif, Kola Peninsula: descriptive data and structural disorder

Shirokshinite, K(NaMg 2 )Si 4 O 10 F 2 , is the analogue of tainiolite, K(LiMg 2 )Si 4 O 10 F 2 , with the M 1 octahedron fully occupied by Na instead of Li. It was found in the Kirovskii underground apatite mine (Kukisvumchorr Mountain, Khibiny massif, Kola Peninsula, Russia) as a late hydrothermal mineral in a small hyperalkaline pegmatite embedded in ristschorrite. Shirokshinite is associated with microcline, kupletskite, aegirine, natrolite, lorenzenite, calcite, remondite-(Ce), donnayite-(Y), mckelveyite-(Y) and galena. Crystals are usually skeletal and coarse hexagonal [001] prismatic. Shirokshinite is transparent to translucent, colourless to pale greyish, hardness Mohs9 ∼2.5; D(calc) = 2.922 g/cm 3 . Optically biaxial (-), α = 1.526(1), β = 1.553(2), γ = 1.553(2); 2V meas = −5(5)°, 2V calc = −0°; Y = b , Z ∼ a , Xc = 3(2)°. The IR spectrum of shirokshinite is unique even if close to that of tainiolite: in particular, the presence of Na + instead of Li + shifts some bands towards low-frequencies. Single-crystal diffraction data (Mo K α-radiation) gave a = 5.269(2), b = 9.092(9), c = 10.198(3) A, β = 100.12(7)°, Z = 2, 1 M -polytype, space group C 2/ m. Structure anisotropic refinement converged R = 0.13 for 715 observed reflections. Evidence of stacking faults in the structure is discussed and compared with the so called Ďurovic effect. The very little ditrigonal distortion in spite of the large dimension of the Na octahedron is discussed in comparison with tainiolite. A critical revision of old published data indicating octahedral Na in micas shows that this hypothesis was biased by the low quality of the chemical analyses.

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.

Starovaite, KCu5O(VO4)3, a new mineral from fumarole sublimates of the Tolbachik volcano, Kamchatka, Russia

The new mineral species starovaite, ideally KCu 5 O(VO 4 )3, has been found in the sublimates of the Yadovitaya fumarole at the Second scoria cone of the Northern Breach of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. The mineral is associated with lammerite, hematite, palmierite, tenorite, piypite, rutile, orthoclase, lyonsite, pseudolyonsite, lammerite-β, langbeinite, calciolangbeinite, and cupromolybdite. Starovaite occurs as prismatic crystals up to 3 × 6 × 20 μm or divergent long-prismatic crystals up to 1 × 3 × 70 μm. The crystals are combined in sprays, sheaf-like aggregates or crusts up to 0.3 × 0.5 mm overgrowing lammerite. Starovaite is golden brown to reddish brown with a semi-metallic luster. The mineral is brittle, VHN is 182 (range 165–195) kg mm −2 . Cleavage and parting were not observed, fracture is uneven. D (calc.) = 4.54 g cm −3 . In reflected light, starovaite is grey with a brownish hue. Bireflectance is weak, internal reflections are distinct red-brown, anisotropy is weak. The reflectance values [ R 1 −R 2 , % (λ, nm)] are: 14.2–12.45 (470), 13.2–11.6 (546), 13.0–11.4 (589), 12.6–11.35 (650). The chemical composition (wt%, electron microprobe data) is: K 2 O 4.90, CaO 0.04, PbO 1.29, CuO 48.20, ZnO 5.59, Al 2 O 3 0.08, Fe 2 O 3 0.10, P 2 O 5 0.05, As 2 O 5 4.49, V 2 O 5 31.89, SO 3 0.19, MoO 3 2.34, total 99.16. The empirical formula calculated on the basis of 13 O apfu is: (K 0.76 Pb 0.04 Ca 0.01 ) Σ0.81 (Cu 4.45 Zn 0.51 Al 0.01 Fe 0.01 ) Σ4.98 (V 2.58 As 0.29 Mo 0.12 S 0.02 P 0.01 ) Σ3.02 O 13 . Starovaite is triclinic, P –1, a = 6.08(4), b = 8.26(5), c = 10.71(6) A, α = 97.8(1), β = 92.4(1), γ = 90.4(1)°, V = 532(2) A 3 , and Z= 2. The strongest reflections of the X-ray powder diffraction pattern [ d , A ( I , %) (hkl)] are: 10.62 (32) (001); 8.18 (46) (010); 3.047 (41) (022, 200,12-2); 2.745 (47) (2-1-1, 03-1); 2.526 (100) (031, 2-12, 2-1-2, 023, 1-31,13-1); 2.322 (98) (03-3, 21-3, 221, 22-2); 1.867 (23) (302); 1.410 (23) (24-5, 053,41-3, 03-7). Starovaite is a natural analogue of synthetic KCu 5 O(VO 4 )3. The mineral is named in honour of the Russian crystallographer and crystal chemist Galina L. Starova (b. 1946) for her contributions to the crystal chemistry of minerals from the Tolbachik fumaroles.

Lileyite, Ba2(Na,Fe,Ca)3MgTi2(Si2O7)2O2F2, a new lamprophyllite-group mineral from the Eifel volcanic area, Germany

The new Mg- and F-dominant lamprophyllite-group mineral lileyite (IMA 2011-021) was found at the Lohley quarry, Udersdorf, near Daun, Eifel Mountains, Rhineland-Palatinate (Rheinland-Pfalz), Germany, and named for the old name of the type locality, Liley. Associated minerals are nepheline, leucite, augite, magnetite, fluorapatite, perovskite, gotzenite. Lileyite is brown, translucent; streak is white. It forms platy crystals up to 0.1 × 0.3 × 0.5 mm in size and their clusters up to 1 mm across on the walls of cavities in an alkaline basalt. Lileyite is brittle, with Mohs hardness of 3–4 and perfect cleavage on (001). D calc is 3.776 g/cm 3 . The new mineral is biaxial (+), α = 1.718(5), β = 1.735(5), γ = 1.755(5), 2V (meas.) = 75(15)°, 2V (calc.) = 86°. The IR spectrum is given. The chemical composition is (EDS-mode electron microprobe, mean of 5 analyses, wt%): SiO 2 28.05, BaO 26.39, TiO 2 18.53, Na 2 O 6.75, MgO 4.58, FeO 4.48, CaO 2.30, SrO 2.23, MnO 1.44, K 2 O 1.41, Nb 2 O 5 0.95, F 3.88, –O=F 2 -1.63; total 99.36. The empirical formula based on 18 anions is: Ba 1.50 Sr 0.19 K 0.26 Na 1.89 Ca 0.36 Mn 0.18 Mg 0.99 Fe 0.54 Ti 2.01 Nb 0.06 Si 4.06 O 16.23 F 1.77 . The simplified formula is: Ba 2 (Na,Fe,Ca) 3 MgTi 2 (Si 2 O 7 ) 2 O 2 F 2 . The crystal structure was solved using single-crystal X-ray diffraction data ( R = 0.024). Lileyite is monoclinic, space group C 2/ m , a = 19.905(1), b = 7.098(1), c = 5.405(1) A, β = 96.349(5)°, V = 758.93(6) A 3 , Z = 2. The strongest lines of the powder diffraction pattern [ d , A ( I , %) ( hkl )] are: 3.749 (45) (31–1), 3.464 (76) (510, 311, 401), 3.045 (37) (51–1), 2.792 (100) (221, 511), 2.672 (54) (002, 601, 20-2), 2.624 (43) (710, 42–1). Type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, registration number 4106/1.

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.