Stoppaniite, a new member of the beryl group: crystal structure and crystal-chemical implications (original) (raw)
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Crystallography Reports, 2009
O . Vernadsky named this beryl variety vorobyevite in honor of the Russian mineralogist V.I. Vorobyev. Vernadsky wrote "In the chemical composition of this mineral, two circumstances are worthy of notice: first, the presence of a considerable amount of cesium in vorobyevite, which makes it one of the most cesiumrich compounds, and, second, the presence of H, Li, and Cs in this mineral, which causes us to change our notions regarding the chemical formula of beryls." It should be noted that, with time, the chemical sense of the term "vorobyevite" unfortunately has become somewhat more general and currently is often applied Abstract -The crystal structures of high-alkali beryl, i.e., vorobyevite Cs 0.08 Na 0.42 (H 2 O) 0.18 + y × [Al 2 (Be 2.35 Li 0.65 )Si 6 O 18 ] , ( a = 9.2102(14) Å, c = 9.2179(14) Å, space group P 6/ mcc , Z = 2, ρ calcd = 2.74 g/cm 3 ) and pezzottaite Cs 0.75 Na 0.23 (H 2 O) 0.24 [Al 2 Be 2 Li(Si 6 O 18 )] ( a = 15.955(3) Å, c = 27.810(8) Å, space group
Mineralogical Magazine, 2012
The crystal structure of the rare skarn mineral trabzonite, Ca4[Si3O9(OH)]OH, from the type locality near Ikizdere, Turkey and from the Upper Chegem caldera, Northern Caucasus, Kabardino-Balkaria, Russia has been solved and refined using single-crystal X-ray data. This shows that the chemical formula should be modified from Ca4(Si3O10)˙2H2O, reported in the original trabzonite description, to an OH-bearing composition. The crystal structure, which contains Si3O10 trimers embedded in a framework of CaO6–8 polyhedra, has orthorhombic symmetry, space group Ama2, a = 20.6, b = 9.1, c = 10.3 Å. The orthorhombic A-centred cell is easily obtained by transformation from the original monoclinic cell of P21/m symmetry. The revised formula means that trabzonite and foshagite are polymorphs; foshagite has wollastonite-like silicate chains and in contrast to trabzonite it does not contain silanol groups. The structure and composition of killalaite from both localities was also studied. Single-cr...
The single-crystal, polarized-light, FTIR spectrum of stoppaniite, the Fe analogue of beryl
Physics and Chemistry of Minerals, 2007
We report here a single-crystal polarized-light study of stoppaniite, ideally (Fe,Al,Mg) 4 (Be 6 Si 12 O 36 ) (H 2 O) 2 (Na,ᮀ), from Capranica (Viterbo). Polarized-light FTIR spectra were collected on an oriented (hk0) section, doubly polished to 15 m. The spectrum shows two main bands at 3,660 and 3,595 cm ¡1 ; the former is strongly polarized for E rc, while the latter is polarized for E //c. A sharp and very intense band at 1,620 cm ¡1 , plus minor features at 4,000 and 3,228 cm ¡1 are also polarized for E //c. On the basis of literature data and considering the pleochroic behavior of the absorptions, the 3,660 cm ¡1 band is assigned to the 3 stretching mode and the 1,620 cm ¡1 (associated with an overtone 2* 2 at 3,230 cm ¡1 ) band to the 2 bending mode of "type II" water molecules within the structural channels of the studied beryl. The sharp band at 3,595 cm ¡1 is not associated with a corresponding 2 bending mode; thus it is assigned to the stretching vibration of O-H groups in the sample. The minor 4,000 cm ¡1 feature can be assigned to the combination of the O-H bond parallel to c with a low-frequency metal-oxygen mode such as the Na-O stretching mode. The present results suggest that the interpretation of the FTIR spectrum of Na-rich beryl needs to be carefully reconsidered.
Asbecasite: Crystal Structure Refinement and Crystal Chemistry
Mineralogical Magazine, 1993
The crystal structure of antimonian asbecasite in an ejectum of hypabyssal origin occurring at Tre Croci near Vetralla, Vico volcanic complex, Roman potassic province, Latium, Italy, has been refined to R = 0.042, and is compared to the original crystal structure determination carried out on the Sb-free asbecasite of hydrothermal metamorphic origin from the type-locality, Cherbandung in Binna valley, Monte Leone nappe, Switzerland. New electron microprobe analyses of samples from both localities demonstrate crystal-chemical features that permit distinction between asbecasites from the two occurrences, so far the only known localities for this mineral.
Mineralogical Magazine, 2009
is triclinic, space group C1, a = 10.6965(7) Å , b = 13.7861(9) Å , c = 21.478(2) Å , a = 99.345(1)º, b = 92.315(2)º, g = 89.993(2)º, V = 3122.6(4) Å 3 , Z = 4, D calc. = 4.018 g cm À3 , from the Verkhnee Espe alkaline deposit, Akjailyautas Mountains, Kazakhstan, has been solved and refined to R 1 5.87% on the basis of 6682 unique reflections (F o >4sF). The crystal structure of cámaraite can be described as a combination of a TS block and an intermediate (I) block. The TS (titanium silicate) block consists of HOH sheets (H-heteropolyhedral, O-octahedral), and is characterized by a minimal cell based on translation vectors t 1 and t 2 , with t 1~5 .5 and t 2~7 Å and t 1^t2 close to 90º. We describe the crystal structure of cámaraite using a double minimal cell, with 2t 1 and 2t 2 translations. In the O sheet, there are eight [6]-coordinated M O sites occupied mainly by Fe 2+ and Mn, with minor Fe 3+ , Mg, Zr, Ca and Zn with <M O Àj> = 2.185 Å. Eight M O sites give, ideally Fe 2+ 8 p.f.u. In the H sheet, there are four [6]-coordinated M H sites occupied almost solely by Ti (Ti = 4 a.p.f.u.), with <M H Àj> = 1.963 Å , and eight [4]-coordinated Si sites occupied solely by Si, with <SiÀO> = 1.621 Å. The topology of the TS block is as in Group II of the Ti-disilicates (Ti = 2 a.p.f.u. per minimal cell) in the structure hierarchy of Sokolova (2006). There are six peripheral (P) sites, four [8À12]-coordinated Ba-dominant A P sites, giving ideally 3 Ba p.f.u., and two [10]-coordinated Na-dominant B P sites, giving ideally 1 Na p.f.u. There are two I blocks: the I 1 block is a layer of Ba atoms (two A P sites); the I 2 block is a layer of Ba (two A P sites) and Na atoms (two B P sites). Along c, there are two types of linkage of TS blocks: (1) TS blocks link via A P cations which constitute the I 1 block, and (2) TS blocks link via common vertices of M H octahedra (as in astrophyllite-group minerals) and A P and B P cations which constitute the I 2 block. Cámaraite is the only mineral of Group II with two types of linkage of TS blocks and two types of I blocks in its structure. The relation of cámaraite to the Group-II minerals is discussed.
Mineralogical Magazine, 2011
Rinkite, ideally Na 2 Ca 4 REETi(Si 2 O 7 ) 2 OF 3 , is a common mineral in alkaline and peralkaline rocks. The crystal structures of five rinkite crystals from three alkaline massifs: Ilimaussaq, Greenland; Khibiny, Kola Peninsula, Russia and Mont Saint-Hilaire, Canada, have been refined as two components related by the TWIN matrix (À1 0 0, 0 À1 0, 1 0 1) (Mo-Ka radiation). The crystals, a = 7.4132À7.4414, b = 5.6595À5.6816, c = 18.8181À18.9431 Å , b = 101.353À101.424(2)º, V = 776.1À786.7 Å 3 , space group P2 1 /c, Z = 2, D calc = 3.376À3.502 g cm À3 , were analysed using an electron microprobe subsequent to collection of the X-ray intensity data. Transmission electron microscopy confirmed the presence of pseudomerohedral twinning in rinkite crystals. The crystal structure of rinkite is a framework of TS (titanium silicate) blocks. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). The TS block in rinkite exhibits linkage and stereochemistry typical for Group I (Ti = 1 a.p.f.u.) of Ti disilicate minerals: two H sheets connect to the O sheet such that two (Si 2 O 7 ) groups link to the trans edges of a Na polyhedron of the O sheet. The crystal chemistry of rinkite and nacareniobsite-(Ce) is discussed.
The crystal chemistry of roméite
Contributions to Mineralogy and Petrology, 1997
Roméite (Ca, Fe, Mn, Na) 2 (Sb 5+ , Ti 4+) 2 (O, OH, F) 7 is a rare mineral found in metamorphic ironmanganese deposits and in hydrothermal Sb-bearing veins. It is isostructural with the pyrochlore-group minerals of the general formula A 2-m B 2 X 6-w Y 1-n • pH 2 O. The pyrochlore-group minerals are important Nb and Ta ores, and are also used as an actinide host phase in radioactive waste. The crystal chemistry of roméite from the type locality Praborna (Italy), from Massiac (France), and from four newly discovered localities in the Swiss Alps, and of ''lewisite'', a questionable species related to roméite from Tripuhy (Brazil), is compared to that of pyrochlore. A wide range of substitutions has been observed including (1) independent substitutions on the A-and B-sites, and (2) coupled substitutions between the A-and Band between the A-and Y-sites. Only the roméite from Massiac, derived from weathering of stibnite, contains significant H 2 O (up to 14 wt %). The A-site vacancies in roméite appear to be controlled by the primary conditions of crystallization, and not by post-crystallization alteration. The Y-site chemistry of roméite varies from locality to locality; it can be dominated by F, OH, or be fully vacant. The ''lewisite'' octahedral crystals studied are a sub-microscopic mixture of roméite with a mineral structurally related to pyrochlore, which grows at the expense of roméite.
The Canadian Mineralogist, 2012
Crystals of beryl from miarolitic pockets, hosted in pegmatitic dikes on the Island of Elba, in Italy, were analyzed for the major and minor elements, including the light elements, by EMPA and SIMS techniques. Back-scattered electron (BSE) images and X-ray maps were used to define the complex textures of these crystals and to highlight the pattern of element distribution in each sample. The majority of the analyzed crystals exhibit a complex pattern of zoning caused by changes in the chemical composition, which follow the main mechanisms of substitution defined for the crystal chemistry of beryl. The following main features are illustrated: (1) the occurrence of crystals with octahedral-site or tetrahedral-site substitutions in different domains of the same crystal, (2) the occurrence of crystals with exclusively tetrahedral or exclusively octahedral substitutions, and (3) the occurrence of crystals or crystal domains having a limited extent of octahedral-site and tetrahedral-site substitutions. However, the analytical data confirm the presence of a gap between the two series, having octahedral-site or tetrahedral-site substitutions. The observed enrichment in Cs and Li is strictly related to the geochemical and petrological features of the LCT host pegmatites. The presence of Ca in some beryl crystals in pegmatites emplaced in carbonate rocks suggests an interaction of the late hydrothermal fluids with country rocks. An enrichment in Fe could also be ascribed to the circulation of fluids having such a contaminant element attributed to the contribution of country rocks or the destabilization of early-crystallized Fe-rich minerals.