Alexander P. Khomyakov

First structure determination of an MDO-2 O mica polytype associated with a 1 M polytype

The structure refinement of associated phlogopite-2 O and phlogopite-1 M from the Khibiny massif (Kola Peninsula, Russia) is reported. Crystal data are: α = 5.2781(5), b = 9.141(1), c = 20.124(4) A, Ccmm (20); a = 5.305(2), b = 9.199(2), c = 10.232(4) A, β = 100.03(2)°, C 2/m (1 M ). Least-squares refinement of single-crystal X-ray diffraction data converged to R1 = 0.034 (2 O , 926 independent reflections) and 0.037 (1 M , 677 independent reflections). This is the first structure refinement of an MDO (standard) mica-2 O : two previous structure reports concerned anandite-2 O , which was not a true polytype, having an oP (primitive orthorhombic) lattice not compatible with the C -centred cell common to all mica polytypes. The two phlogopite polytypes show practically the same chemical composition (K 0 . 95Na 0.01 ) (Mg 2.16 Fe 0.34 Ti 0.04 Mn 0.04 Li 0.40 )[Si 3.40 Al 0.60 O 10 ][(OH) 1.35 F 0 .65] and no cation ordering. Both polytypes are affected by stacking disorder, which broadens the non-family reflections ( k ≠ 3n). As a consequence, the diffracted intensities of the two types are measured at different scales and large residues in the difference Fourier maps were observed at ± b /3 along [010], [310] and [310]. These residues ( uroviceffect ) disappear by carrying out the refinement with separate scale factors for the two types of reflections. On the basis of structural, morphology and zoning considerations the formation of the two associated polytypes is attributed to chemical oscillation in the crystallization mean.

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.

Nechelyustovite, a new heterophyllosilicate mineral, and new data on bykovaite: A comparative TEM study

A new mineral species, nechelyustovite, (Ba 0.75 Sr 0.25 K 0.17 Ce 0.02 Ca 0.01 □ 0.80 ) ∑2.00 {(Na 2.20 Ti 0.94 Mn 0.62 Ca 0.20 Fe 0.04 ) ∑4.00 [(Ti 1.33 Nb 0.67 ) ∑2.00 O 2 Si 4 O 14 ](O 1.30 H 2 O 0.70 ) ∑2.00 }·4.325H 2 O (electron microprobe), was collected from a hydrothermally altered pegmatite body emplaced in the nepheline syenites near their contact with ijolite–urtites in the south-western part of the Khibiny alkaline massif, Kola Peninsula, Russia, in the underground Kirovskii Mine at Mount Kukisvumchorr. It forms rosettes scattered in a natrolite matrix which are up to 1–5 cm in diameter and composed of extremely fine (0.01–0.1 mm) bounded flakes and lamellae. Nechelyustovite is associated with natrolite, belovite-(La), belovite-(Ce), gaidonnayite, nenadkevichite, epididymite, fluorapophyllite, sphalerite and submicrometric barytolamprophyllite. It is creamy with greyish, bluish or yellowish shades; streak is white, lustre vitreous, pearly or silvery; translucent, transparent in fine flakes formed by [010] elongated and (001) flattened lamellae; H = 3 (Mohs); {001} perfect and {100} medium cleavages; brittle; fracture uneven; D meas = 3.32–3.42(2), D calc = 3.22 g/cm 3 . Biaxial (+); at 589 nm α = 1.700(3), β= 1.710(3), γ= 1.734(3); 2V(calc) 66°; X ~ c , Y ~ a , Z ~ b . Nechelyustovite, simplified as (Ba,Sr,K,□) 2 {(Na,Ti,Mn) 4 [(Ti,Nb) 2 O 2 Si 4 O 14 ](O,H 2 O,F) 2 }·4.5H 2 O, is a new heterophyllosilicate member of the mero-plesiotype bafertisite series and a transmission electron microscopy (TEM) study shows that it occurs as two polytypes intergrown at submicrometric scale: polytype 1 M , P 2/ m, a = 5.37, b = 7.00, c = 24.05 A, β= 91.1°, Z = 2; polytype 2 M , A 2/ m , a = 5.38, b = 7.04, c = 48.10 A, β= 91.1°, Z = 4. The spacing (A) and intensities of the most intense X-ray powder diffraction peaks are 24.06 (100), 7.05 (13), 5.95 (36), 3.95 (25), 2.828 (42), 2.712 (19) and 2.155 (13). A TEM study of bykovaite, simplified as (Ba,Na,K,□) 2 {(Na,Ti,Mn) 4 [(Ti,Nb) 2 O 2 Si 4 O 14 ](H 2 O,F, OH) 2 }·3.5H 2 O, shows that also this heterophyllosilicate occurs as two polytypes intergrown at submicrometric scale: polytype 1 M , P 112/ m, a = 5.552, b = 7.179, c = 25.47 A, γ= 91.1°, Z = 2; polytype 2 M, I 112/ m , a = 5.552, b = 7.179, c = 50.94 A, γ= 91.1°, Z = 4. Hypotheses on the crystal structure of the two minerals are discussed.

Decationized and hydrated eudialytes. Oxonium problem

A collection of six decationized eudialytes from alkaline massifs of various regions was investigated by the single-crystal method. These samples differ from typical eudialytes by their low contents of alkaline and alkaline-earth cations and the high degree of hydration. The process of eudialyte decationization is accompanied by hydration, and hydrous species are incorporated into the mineral structure predominantly as H 3 O-groups. Those oxonium groups act as monovalent cations, replacing alkaline and alkaline-earth cations and compensating the positive charge deficiency. In all N -positions, H 3 O-groups replace large cations either partially or completely. Another specific feature of these samples is their low symmetry R 3, which is rare compared to R 3 m in eudialytes.

Structure model of Al,K-substituted tobermorite and structural changes upon heating

A structure model of Al and K-containing tobermorite is proposed based on the results obtained by different methods—powder X-ray diffraction analysis, microdiffraction in an electron microscope, etc. The factors responsible for the stability of the structure modules typical of the specimens of this family are discussed. Most of the microcrystals were demonstrated to consist of two phases characterized by a high degree of silicon-oxygen radical condensation. The examination of two-phase microcrystals in an electron microscope by the method of diffraction contrast allowed us to propose the mechanism of change of the degree of condensation of the tobermorite structures under an electron beam. Heating the starting crystals results in their transformation into an amorphous state with a simultaneous increase in the degree of condensation of the silicon-oxygen ribbons in the structure.