Severe structural damage in Cr- and V-bearing clinozoisite (Czo) from the Outokumpu copper mine, Finland, was discovered by Nagashima et al. (Eur J Miner 23:731–743, 2011). Single-crystal X-ray and electron diffraction patterns indicated moderate-to-very-poor structural coherency, suggesting a high density of faults in the translational symmetry. However, the poor crystallinity cannot be attributed to self-radiation due to negligible concentrations of radioactive elements. Annealing of Cr- and V-bearing Czo up to 750 °C only slightly improved crystallinity. To solve this enigma, polarized Raman spectroscopy was applied to gain further insights into the structural state of the Cr + V-bearing Finnish Czo. According to the parallel-polarized Raman spectra of Cr + V-rich (Czo-ts3), Cr + V-bearing (Czo-ts2), and Cr + V-free (Czo-ts1) Czo, the peaks broaden with the Cr + V content, indicating increasing density of structural defects. Spectra from a euhedral Cr + V-bearing Czo single-crystal evidence intact structural domains by strong polarization and orientation dependence. Thus, the structural state of Finnish Czo should be described in terms of defect-rich segmented domains with common crystallographic orientation. The incorporation of V and Cr into the clinozoisite structure strongly affects the Raman-active O–H bond stretching modes near 3343 cm −1 [(OH)a] and 3435 cm −1 [(OH)b], assigned to O10-H···O4 and O10-H···O2 hydrogen bridges, respectively. The H···O2 hydrogen bonding is strongly promoted when M3 is partially occupied by Cr + V, and as a result, the linkages between the M2O 6 - and (M1, M3)O 6 -chains are disturbed, leading to periodicity faults and size reduction of coherent structural domains. In the case of M3 Fe 3+ substitution for M3 Al, the local stress imposed by the larger M3-site cation is relaxed within the distorted M3O 6 octahedron. In the case of M3 V 3+ or M3 Cr 3+ substitution for M3 Al, M3O 6 octahedra are less distorted and local stress fields propagate farther than only to the first coordination sphere, thus disturbing the transitional symmetry. New high-resolution transmission electron microscopy (HRTEM) images indeed display nanocrystals of ca. 10 nm with common crystallographic orientation surrounded by amorphous regions. It is suggested that the segmentation of large coherent crystalline areas in Cr + V-rich Czo to nanocrystals may be triggered by a potential miscibility gap in combination with the sluggish diffusion behavior of Cr 3+ . Moreover, this study implies that nanostructures as commonly observed for metamict materials may also be caused by strain effects and/or exsolution phenomena precluded by tardy diffusion.

Three non-metamict chevkinite-subgroup minerals, space group P21/a, from Cape Ashizuri, Japan, (No. 1), Tangir Valley, Diamar District, Pakistan (No. 2) and Haramosh Mts., Skardu district, Pakistan, (No. 3) were studied by crystal chemical techniques. Powder X-ray diffraction and transmission electron microscopic observations confirmed well crystalline samples. Electron-microprobe analyses indicated the general composition [(REE, Ca)4Fe2+(Fe2+, Fe3+, Ti)2Ti2(Si2O7)2O8] known for chevkinite-(Ce). Site scattering values determined by single-crystal X-ray structure refinements suggested assignment of subordinate Nb to the octahedral M3 and M4 sites, minor Th to M1 for the Ashizuri sample and minor Mg to M1 for both samples from Pakistan. Neutron time-of-flight powder diffraction studies were applied to determine the Ti/Fe distribution among octahedral sites for all samples and Mössbauer spectroscopy served for the Fe valence assignment at the four octahedral sites. Combined results gave the simplified formulas with Ce as dominant REE: No. 1 A(REE,Ca)4M1(Fe2+,Ti)M2(Ti,Fe3+)2M3,M4(Ti,Fe2+,Nb)2(Si2O7)2O8, No. 2 A(REE,Ca)4M1(Fe2+,Fe3+,Mg)M2(Ti,Fe3+)2M3,M4(Ti,Fe2+)2(Si2O7)2O8 and No. 3 A(REE,Ca)4M1(Fe3+,Fe2+,Mg,Ti)M2(Ti,Fe3+)2M3,M4(Ti,Fe2+)2(Si2O7)2O8. The dominant iron valence at M1 of the Haramosh sample (No. 3) is ferric whereas for samples Nos. 1 and 2 iron is ferrous. The structure of chevkinite-subgroup minerals consists of octahedral layers (M2, M3 and M4) alternating with an intermediate layer containing disilicate groups, REE polyhedra and strongly distorted M1 octahedra. Increased O atomic displacement parameters within the intermediate layers indicate disorder and/or strain to fit the extension of the embracing octahedral layers. A statistical analysis of chevkinite-subgroup structures (space groups P21/a, C2/m and P21/m) indicates a positive correlation between < M1–O > bond length and M1 bond angle variance. A negative correlation is found between < M1–O > bond length and the disilicate bending angle Si–O–Si, varying between ca. 160° and 175°. Outliers of this trend are delhuyarite-(Ce) Ce4Mg(Fe3+2W)□(Si2O7)2O6(OH)2, mainly due to octahedral W6+ and vacancies in the octahedral layers associated with OH groups, and synthetic Nd end-members with strong bonds between Nd and the intersecting O of the disilicate group. The observed trends reflect the main mechanisms in the intermediate layer to adopt to the size of the embracing rigid octahedral layers formed by M2, M3 and M4 octahedra.

new mineral was discovered in Cr–V-bearing marbles of the Sludyanka Complex from the Pereval marble quarry, Sludyanka district, southern Baikal region, Russia. It was named vanadio-pargasite as vanadium-bearing analog of pargasite according to the amphibole supergroup classification and CNMNC recommendations. Black Cr–V-spinel (magnesiocoulsonite–magnesiochromite), red Cr–V-bearing spinel, calcite, dolomite, Cr–V-bearing diopside and chlorite, phlogopite, and forsterite are associated minerals. Vanadio-pargasite occurs as subhedral long- and short-prismatic crystals 0.10–0.8 × 0.05–0.10 mm in size, with (110) and (010) faces and perfect cleavage by (110). Macroscopically, the new mineral is bright green to emerald green with vitreous luster; in thin sections and powder, it is pale green, without pleochroism. The new mineral is biaxial, positive, 2 V = 86° ± 2°, γ = 1.659(2), β = 1.651(2), α = 1.643(2). The Mohs hardness is 6, average VHN50;100 is 795; range 752–824 kg/mm2. The measured and calculated density is 3.05(5) and 3.112 g/cm3, respectively. In a thermogram over the range 654–1081°С, H2O is released with a endothermic effect. Over the range 900–1183°C, the main endothermic effect is caused by water and, possibly, F release, as well as melting of the mineral (1020°C). The absorption bands in the IR spectrum are, cm–1: 3445, 1633, 980, and 469. Vanadio-pargasite is monoclinic, space group 2C/m; the unit cell parameters are: a = 9.914(3), b = 18.003(2), c = 5.300(2) Å, β = 105.69(3)°, V = 910.7(5) Å 3 , Z = 2. The strongest reflections in the X-ray diffraction pattern are [d, Å (I) (hkl)]: 8.98 (15) (020), 8.43 (40) (110), 3.27 (30) (240), 3.14 (100) (310), 2.82 (35) (330), 2.70 (18) (151), 2.34 (15) ( 4.101̅ ), 1.898 (15) (510), 1.445 (25) (4.101). The average chemical composition (528 point analyses) is, wt

A new mineral was discovered in Cr–V-bearing marbles of the Sludyanka Complex from the Pereval marble quarry, Sludyanka district, southern Baikal region, Russia. It was named vanadio-pargasite as vanadium-bearing analog of pargasite according to the amphibole supergroup classification and CNMNC recommendations. Black Cr–V-spinel (magnesiocoulsonite–magnesiochromite), red Cr–V-bearing spinel, calcite, dolomite, Cr–V-bearing diopside and chlorite, phlogopite, and forsterite are associated minerals. Vanadio-pargasite occurs as subhedral long- and short-prismatic crystals 0.10–0.8 × 0.05–0.10 mm in size, with (110) and (010) faces and perfect cleavage by (110). Macroscopically, the new mineral is bright green to emerald green with vitreous luster; in thin sections and powder, it is pale green, without pleochroism. The new mineral is biaxial, positive, 2V = 86° ± 2°, γ = 1.659(2), β = 1.651(2), α = 1.643(2). The Mohs hardness is ~ 6, average VHN50;100 is 795; range 752–824 kg/mm2. The measured and calculated density is 3.05(5) and 3.112 g/cm3, respectively. In a thermogram over the range 654–1081°С, H2O is released with a endothermic effect. Over the range 900–1183°C, the main endothermic effect is caused by water and, possibly, F release, as well as melting of the mineral (1020°C). The absorption bands in the IR spectrum are, cm–1: 3445, 1633, 980, and 469. Vanadio-pargasite is monoclinic, space group 2C/m; the unit cell parameters are: a = 9.914(3), b = 18.003(2), c = 5.300(2) Å, β = 105.69(3)°, V = 910.7(5) Å3, Z = 2. The strongest reflections in the X-ray diffraction pattern are [d, Å (I) (hkl)]: 8.98 (15) (020), 8.43 (40) (110), 3.27 (30) (240), 3.14 (100) (310), 2.82 (35) (330), 2.70 (18) (151), 2.34 (15) ( $$4.10\bar 1$$ ), 1.898 (15) (510), 1.445 (25) (4.101). The average chemical composition (528 point analyses) is, wt %: 42.75 SiO2, 0.14 TiO2, 12.75 A12O3, 0.44 Cr2O3, 5.92 V2O3, 19.15 MgO, 0.03 FeO, 0.01 MnO, 12.52 CaO, 3.45 Na2O, 0.41 K2O, 0.74 F (wet chem.) 1.75 H2O (calc.); the total is 99.91. The simplified formula is K0.1Na0.9Ca2.0Mg4.0V0.7Al0.3(Si6.1Al1.9)8.0O22(OH1.7F0.3)2.0. Holotype material has been deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia (registration nos. 5035/1, 5035/2, and 5035/3).

The crystal structure of a new member of the calcium amphibole subgroup, vanadio-pargasite ideally NaCa2(Mg4V3+) (SiAlO22)-Al-6-O-2(OH)(2) with empirical formula (K0.07Na0.90)(Na0.05Ca1.91Mg0.04)(Sigma 2.00)(Mg4.02Cr0.05V0.68 Al0.23Ti0.02)(Sigma 5.00)(Si6.09Al1.91)(Sigma 8.00)O-22(OH1.67F0.33)(Sigma 2.00,)was studied by single-crystal X-ray diffraction and refined to R1 of 0.0181. It is monoclinic, space group C2/m, unit-cell parameters alpha=9.8956(1), b = 17.9970(2), c = 5.2970(1) angstrom, beta = 105.391(1)degrees, and V = 909.52 angstrom(3)The mineral is isostructural with the amphiboles pargasite, magnesio-hastingsite, chromio-pargasite and Mn3+-rich pargasite. Site populations were derived from the structure refinement and electron-microbe analysis, and validated on the basis of OH-stretching FTIR spectroscopy. Accordingly, V is ordered at M(2) together with minor Al-[6], while a low amount of Al-[6] is present at M(3)..

Dargaite, ideally BaCa12(SiO4)(4)(SO4)(2)O-3, is an additional member of the arctite group belonging to minerals with a modular intercalated antiperovskite structure derived from hatrurite. The holotype specimen was found at a small outcrop of larnite pseudoconglomerates in the Judean Mts, West Bank, Palestinian Autonomy. Larnite, fluorellestadite-fluorapatite, brownmillerite, fluormayenite-fluorkyuygenite and ye'elimite are the main minerals of the holotype specimen; ternesite, shulamitite and periclase are noted rarely. Dargaite, nabimusaite and gazeevite occur in linear zones with higher porosity within larnite rocks. Pores are filled with ettringite and Ca-hydrosilicates, less commonly with gibbsite, brucite, baryte, katoite and calciolangbeinite. Dargaite is colourless, transparent with a white streak and has a vitreous lustre. It exhibits pronounced parting and imperfect cleavage along (001). Mohs' hardness is similar to 4.5-5.5. The empirical formula is (Ba0.72K0.24Na0.04)(Sigma 1)(Ca11.95Mg0.04Na0.01)(Sigma 12)([SiO4](0.91) [PO4](0.05)[AlO4](0.03)[Ti4+O4](0.01))(Sigma 4)([SO4](0.84)[PO4](0.14)[CO3](0.02))(Sigma 2)(O2.54F0.46)(Sigma 3). Dargaite is trigonal R (3) over barm, the unit-cell parameters are: a = 7.1874(4) angstrom, c = 41.292(3) angstrom, V = 1847.32(19) angstrom(3) and Z = 3. The crystal structure of dargaite was refined from X-ray single-crystal data to R-1 = 3.79%. The calculated density is 3.235 g cm(-3). The following main Raman bands are distinguished on the holotype dargaite (cm(-1)): 122, 263, 323, 464, 523, 563, 641 and 644, 829 and 869, 947, 991 and 1116. The formation conditions of dargaite are linked to the local occurrence of pyrometamorphic by-products (gases, fluids and melts) transforming earlier mineral associations at similar to 900 degrees C.

Ferrites of K and Ba close in composition to the known synthetic compounds KFe22+Fe153+O25 and BaMg2Fe163+O27 were found in a thin vein, filled with magnesioferrite and khesinite, in pyrometamorphic flamite-gehlenite hornfels of the Hatrurim Complex, Palestinian Autonomy. Both ferrites are characterized by a modular structure composed of 5-layered spinel (S) blocks interstratified with R-blocks. The R-modules of K-ferrite are of the beta-alumina type whereas those in Ba-ferrite are of the magnetoplumbite type. Rare grains of K-Ba-ferrite of intermediate composition with the empirical crystal chemical formula (EPMA): (K0.57Ba0.38Na0.05)(Sigma 1)(Fe14.083+Mg1.42Al0.80Zn0.59Ni0.16 Ca0.14Cu0.09Mn0.03Ti0.03Si0.02)Sigma O-17.36(25.54), were also discovered. The structure of the natural mixed potassium-barium ferrite was investigated by single-crystal X-ray diffraction. The hexagonal space-group symmetry (Y (6) over bar2, Z= 2) with a = 5.9137(2) and c = 33.1450(15)angstrom is different to that (P6(3)/mmc) of the end-members and yields alternate stacking of beta-alumina type KMg2Fe15O25 and magnetoplumbite type BaMg2Fe16O27 "supermodules", each extending 1/2 of the unit cell along c. Raman spectroscopic study confirms the mixed character of K-Ba-ferrite structure. K-Baferrite formed at non-equilibrium conditions and may be interpreted as an example of a nano-dissipative structure.

Microporous VSH-13Na of composition Na2(VO)(Si4O10)·3H2O was synthesized under mild hydrothermal conditions and studied by single-crystal X-ray diffraction at room temperature and 398 K. Its vanadosilicate framework, consisting of sheets of silicate tetrahedra connected by vanadyl-type square-based pyramids, closely resembles that of the mineral cavansite, Ca(VO)(Si4O10)·4H2O. Due to the disorder in the orientation of the short apical vanadyl groups, the topological symmetry of VSH-13Na was originally described in space group Imma. However, when analysing the systematic absences in our dataset, only the 21 screw axis along b was strictly fulfilled suggesting monoclinic space group P1211. The resulting structure in P21 with a = 14.364 (4), b = 9.134 (2), c = 10.373 (3) Å, β = 90.056 (7)°, V = 1360.9 (7) Å3 was interpreted as a case of allotwinning of two polytypes with topologically idealized orthorhombic symmetry: A (∼62%) with antiparallel orientation of the vanadyl groups in adjacent (100) layers and B (∼38%) with all vanadyl groups in adjacent layers oriented in the same way. At 398 K, the structure of VSH-13Na became fully dehydrated and adopted the unit-cell parameters a = 12.584 (16), b = 9.525 (13), c = 9.696 (14) Å, β = 90.10 (4)°, V = 1162 (3) Å3 (space group P21). Release of H2O caused severe contraction of T—O—T angles and the unit-cell volume decreased by ∼15%. Despite their structural similarity, the VSH-13Na framework seems to be more flexible upon dehydration compared with cavansite, whose structure collapsed before removal of the last H2O molecule. Thus, the presence of monovalent or divalent extraframework cations plays a key role in the dehydration process of natural and synthetic vanadosilicates.

Single-crystal X-ray crystal-structure refinements and electron-microprobe analyses of two julgoldite samples, in one case complemented by Mossbauer spectroscopy to determine the Fe valence distribution, yielded the compositions Ca2.00X(Fe0.442+Fe0.293+Mg0.17Al0.10)(Y)(Fe0.823+Al0.18)(2)Si3O10.39(OH)(3.61 )(Bombay) and Ca-2.00(X)(Fe0.67Mg0.16Al0.17)(Y)(Fe0.813+Al0.19)(Si3O14 - n)-Si-2(OH)(n) (4 >= n >= 3) (Kreimbach/Kaulbach), respectively. Structure refinements also allowed specification of the system of hydrogen bonds, which concur with those known for pumpellyite. A review of structural and chemical data for pumpellyite-group minerals including six julgoldite samples indicated that the length of the b axis is governed by the average size of ionic radii at the Y site, except for one sample of julgoldite-(Fe3+). Size effects of the concentration of "large" divalent cations (Mg, Fe) at the X site can be seen in the lateral extension of the (010) plane that acts as a buffer to accomodate the variable size of the X octahedra. Using unit-cell plots, the data suggest that five of the six tested julgoldite samples are julgoldite-(Fe2+) and only the one sample of the Bombay area studied by Artioli et al. (2003) is julgoldite-(Fe3+). Both samples studied in this paper were classified into julgoldite-(Fe2+). In contrast to pumpellyite samples sensu stricto, which exhibit a Me2+:Me3+ ratio of approximately 1:1, julgoldite samples may cover the whole range between Ca-2(X)(Fe3+)(Y)(Fe3+)(2)Si3O11(OH)(3) and Ca-2(X)(Fe2+)(Y)(Fe3+)(2)Si3O10(OH)(4).

The crystal structure of cafarsite from Wanni glacier, Monte Cervandone, was re-investigated by single-crystal X-ray diffraction (space group Pn (3) over bar, R1= 0.022) and new electron-microprobe analyses. The REE (mainly Ce and Y) were found to substitute Ca. A new anion site, four-coordinated by Ca, was located by difference Fourier analysis and refined with F scattering factors. Two octahedral sites (Ti1 and Fe2) are occupied by mainly Ti and Fe3+, replacing each other whereas the octahedral site Mnl hosts Mn2+ and Fe2+. The new structure refinements and chemical analyses lead to the simplified chemical formula (Ca7.8Na0.8Mn0.5REE0.4)(Sigma)(9.)(5)(Ti3.9Fe2.13+Fe0.92+Mn0.12+)(Sigma 7)(AsO3)(14)F-0.5. According to our findings, there is no evidence for either H2O or OH in the structure. In contrast to the original study of 1977, the structure has no significant cation vacancies and is based on 14 AsO3 groups per formula unit, not on 12 AsO3 as previously suggested.

Non-equilibrium dehydration dynamics of cavansite (Ca(VO)(Si4O10) center dot 4H(2)O), a zeolite-like material, were investigated with fast time-resolved high-temperature (HT) synchrotron powder X-ray diffraction (XRD). In-situ dehydration experiments were done with both distinct heating rates (3 K/min and 10 K/min) and particle sizes (crystals < 25 mu m, and between 25 and 50 mu m). In addition, time-dependent isothermal runs at 381, 490, and 726 K were performed for fine and coarse grained samples to track dehydration accompanied by structural evolution. The non-equilibrium dynamic dehydration is more influenced by the heating rate than by the crystal size. In general, dehydration steps involve less pronounced volume modifications and are shifted to higher temperature than found for "equilibrated" single-crystals. Isothermal release of 1H(2)O at 381 K is characterized by a first rapid increase of cell volume related to thermal motion of extra framework H2O, then the internal pressure is released by H2O liberation and the cell volume continuously decreases. The coarse fraction reacts time-delayed compared to the fine one. At 490 K 2H(2)O are expelled. This process converges to a low volume plateau, which is reached for the fine sample in a broad time span between 770 and 1880 s. Surprisingly, the coarse sample reacts faster reaching the slightly lowered volume plateau after ca. 300 s. The faster reaction of the coarse sample is assigned to lower partial H2O pressure due to differences of coarse and fine particle packing in the capillary. At 726 K we expected release of 3H(2)O. However, the coarse sample rapidly decreased to a volume plateau characteristic of the 2H(2)O modification and remained crystalline until the end of the experiment (2200 s). The fine sample dehydrated faster, continuously lost crystallinity, and became amorphous after already 490 s.

Khesinite, Ca 4 Mg 2 Fe 3+ 10 O 4 (Fe 3+ 10 Si 2 )O 36 , is a new member of the rhonite group of the sapphirine supergroup. Khesinite was discovered in thin veins of paralavas within fine-grained gehlenite rocks (hornfels) of the Hatrurim Complex in the Negev Desert, Israel. Paralavas are composed of rankinite, pseudowollastonite (rarely wollastonite), flamite, kalsilite, cuspidine and members of the solid-solution series: schorlomite–andradite, gehlenite–ackermanite–“Fe 3+ -gehlenite”, magnesioferrite–spinel and fluorapatite–fluorellestadite. Accessory and rare minerals are represented by baryte, walstromite, fresnoite, vorlanite, barioferrite, hematite, perovskite, gurimite, zadovite, aradite and hexacelsian. Electron-microprobe analysis of the holotype khesinite gives the following empirical formula for 40 oxygens and 28 cations: Ca 4 (Fe 3+ 8.528 Mg 1.635 Ca 0.898 Ti 4+ 0.336 Ni 2+ 0.217 Mn 2+ 0.155 Cr 3+ 0.132 Fe 2+ 0.098 ) Σ12 [(Fe 3+ 6.827 Al 2.506 Si 2.667 ) Σ12 O 40 ]. Khesinite is black to dark brown. It has semi-metallic lustre and does not show fluorescence. Cleavage and parting are not observed, fracture is irregular. Khesinite has a Mohs9 hardness of 6; microhardness VHN 50 is 943 kg mm −2 . The calculated density is 4.097 g cm −3 . In reflected light khesinite is grey with weak internal brown reflections. Reflectance data for the COM (Commission of Ore Mineralogy, IMA) wavelengths vary from ~13.4% (470 nm) to ~11.8% (700 nm). The crystal structure of khesinite [ P 1 ¯ a = 10 . 5363 ( 1 ) , b = 10.9242(2), c = 9.0612(1) A, α = 106.340(1)°, β = 95.765(1)°, γ = 124.373(1)°, V = 780.54(2) A 3 ] was refined from X-ray single-crystal data to R 1 = 0.046. The khesinite structure is close to that of the synthetic compounds SFCA and SFCAM. Khesinite crystallized in paralava from melt, sometimes forming isolated crystals, but more commonly reaction rims on magnesioferrite in association with pseudowollastonite and flamite at temperature not lower than 1200 °C.

Khesinite,Ca4Mg2Fe103+O4(Fe103+Si2) O-36, is a new member of the rhonite group of the sapphirine supergroup. Khesinite was discovered in thin veins of paralavas within fine-grained gehlenite rocks (hornfels) of the Hatrurim Complex in the Negev Desert, Israel. Paralavas are composed of rankinite, pseudowollastonite (rarelywollastonite), flamite, kalsilite, cuspidine and members of the solid-solution series: schorlomite-andradite, gehlenite-ackermanite-"Fe3+-gehlenite", magnesioferrite-spinel and fluorapatite-fluorellestadite. Accessory and rare minerals are represented by baryte, walstromite, fresnoite, vorlanite, barioferrite, hematite, perovskite, gurimite, zadovite, aradite and hexacelsian. Electron-microprobe analysis of the holotype khesinite gives the following empirical formula for 40 oxygens and 28 cations: Ca-4(Fe8.5283+Mg1.635Ca0.898Ti0.3364+Ni0.2172+Mn0.1552+Cr0.1323+Fe0.0982+) (Sigma 12)[(Fe6.8273+Al2.506Si2.667)(Sigma 12)O-40]. Khesinite is black to dark brown. It has semi-metallic lustre and does not show fluorescence. Cleavage and parting are not observed, fracture is irregular. Khesinite has a Mohs' hardness of 6; microhardness VHN50 is 943 kg mm(-2). The calculated density is 4.097 g cm (-3). In reflected light khesinite is grey with weak internal brown reflections. Reflectance data for the COM (Commission of Ore Mineralogy, IMA) wavelengths vary from similar to 13.4% (470 nm) to similar to 11.8% (700 nm). The crystal structure of khesinite [P(1) over bar a = 10.5363(1), b = 10.9242(2), c = 9.0612(1) angstrom, alpha = 106.340(1)degrees, beta =95.765(1)degrees, gamma=124.373(1)degrees, V= 780.54(2) angstrom(3)] was refined from X-ray single-crystal data to R-1 = 0.046. The khesinite structure is close to that of the synthetic compounds SFCA and SFCAM. Khesinite crystallized in paralava from melt, sometimes forming isolated crystals, but more commonly reaction rims on magnesioferrite in association with pseudowollastonite and flamite at temperature not lower than 1200 degrees C.

The thermal behavior of Na-exchanged stellerite and stilbite was investigated by in-situ single crystal X-ray diffraction. For comparison with the exchanged forms new data were collected on natural stellerite and stilbite under the same experimental conditions. With the increase of temperature, strong disorder at T and O sites of the tetrahedra of the four-membered ring developed in natural forms. Such disorder was associated with the rupture of T-O-T connections and transition from the A to the B phase. Differently from previous studies, stellerite B at T > 300 degrees C was found to be monoclinic (space group A2/m). In addition, at 400 degrees C, a new T-O-T connection occurred, analogous to that in the B phase of barrerite.Na-stellerite and Na-stilbite were at RT monoclinic, space group F2/m. Upon heating, they also displayed the same structural modifications as observed in natural barrerite and Na-barrerite and adopted space group A2/m. Compared to natural stellerite and stilbite different T-O-T connections ruptured leading to a different topology of the B phase. The total volume contraction was 16% at 350 degrees C compared to -8% of pristine materials. The highly-condensed D phase, which does not form in natural stellerite and stilbite, was obtained by heating a Na-stellerite crystal ex-situ at 525 degrees C. The structure corresponded to the D phase of natural barrerite and Na-barrerite.All investigated STI members, after being exchanged with Na, have identical symmetry and demonstrate corresponding behavior upon heating and associated dehydration. Thus, a previously assumed memory effect of the symmetry of the natural parent structure, is not confirmed. (C) 2017 Elsevier Inc. All rights reserved.

Abstract The new mineral gazeevite, BaCa6(SiO4)2(SO4)2O (R3̅m, a = 7.1540(1), c = 25.1242(5) Å, V = 1113.58(3) Å3, Z = 3), was found in an altered xenolith in rhyodacites of the Shadil-Khokh volcano, Southern Ossetia and at three localities in larnite pyrometamorphic rocks of the Hatrurim Complex; Nahal Darga and Jabel Harmun, Judean Mountains, Palestinian Autonomy, and Har Parsa, Negev Desert, Israel. Larnite, fluorellestadite-fluorapatite, srebrodolskite-brownmillerite andmayenite-supergroup minerals are the main minerals commonly associated with gazeevite. Gazeevite is isostructural with zadovite and aradite; the 1:1 type AB6(TO4)2(TO4)2W, occurs together with the structurally related minerals of the nabimusaite series, 3:1 type AB12(TO4)4(TO4)2W3, where A = Ba, K, Sr…; B=Ca, Na…; T = Si, P, V5+, S6+, Al…; W=O2-, F-. Single antiperovskite layers {[WB6](TO4)2} in the structure type of gazeevite-zadovite and triple {[W3B12] (TO4)4} layers in arctite-nabimusaite are intercalated with single A(TO4) layers. These minerals with an interrupted antiperovskite structure are characterized by a modular layered structure derived from hatrurite, Ca3(SiO4)O. Gazeevite is colourless, transparent, with a white streak and vitreous lustre. Gazeevite is brittle, shows pronounced parting and imperfect cleavage on {001}; it is uniaxial (-), ω = 1.640(3), ε = 1.636(2) (λ = 589 nm) and nonpleochroic; Mohs’ hardness is ∼4.5, VHN50 = 417 kg mm-2. The calculated density is = 3.39 g cm-3. The main lines of the calculated powder X-ray diffraction pattern are as follows (d(Å)/I/hkl): 3.58/100/110, 3.07/91/021, 2.76/47/116, 1.789/73/220, 3.29/60/113, 2.78/36/024, 2.12/25/125, 2.21/21/208. Raman spectra of gazeevite are compared with spectra of other minerals. The formation of gazeevite and minerals of the nabimusaite-dargaite series is connected with high-temperature alteration of an early assemblage of clinker minerals affected by later fluids generated by volcanic activity or combustion processes.