Gismondine-Sr, Sr4(Al8Si8O32)·9H2O, a new strontium dominant, orthorhombic zeolite of the gismondine series from the Hatrurim Complex, Israel

A new mineral, gismondine-Sr with ordered gismondine framework type (B2212 no.20, Z= 1; a = 14.0256(2) Å, b = 10.45900(10) Å, c = 13.79360(10) Å), V= 2023.44(4)Å3) and the following ideal chemical formula Sr4(Si8Al8O32)·9H2O was discovered in amygdaloidal voids of partly melted gehlenite hornfels at Halamish locality, Hatrurim Basin of the Hatrurim Complex, Negev Desert, Israel. Gehlenite hornfels is mainly composed of gehlenite, wollastonite, and garnet of the grossular-andradite-schorlomite series. In a low-temperature association occur minerals such as thomsonite-Ca, flörkeite, analcime and minerals of the tobermorite supergroup. Gismondine-Sr forms spherulitic aggregates up to 180 μm and, rarely, pseudotetragonal bipyramidal crystals up to 50 μm. Empirical crystal chemical formula of gismondine-Sr is as follows (Sr2.02Ca1.09Ba0.02K0.72Na0.62)∑4.47Al7.91Si8.09O31.85·9H2O. It is the strontium analog of gismondine-Ca and the second orthorhombic zeolite with the GIS structure topology. Crystals are transparent to translucent and feature vitreous lustre. The mineral exhibits a white color, imperfect cleavage in [101] direction, a brittle tenacity, and uneven fracture. The Mohs hardness was estimated at approximately 4. Gismondine-Sr is biaxial negative, α = 1.488(3), β = 1.492(3), γ = 1.495(3), 2Vobs = 70-80°. The Raman spectrum is characterized by a band on 465 cm-1, which is also the main band in gismondine-Ca. The structure refinement using SC-XRD (R1 = 0.0353) reveals the ordered distribution of framework cations and the disordered arrangement of extraframework cations. The aluminosilicate framework is built by crankshaft chains with 8-member apertures channels parallel to [101] and [10-1]. In gismondine-Sr, the 8-member rings are elliptically deformed and the T-O-T angle of the upward and downward tetrahedra in the double crankshaft chains is smaller compared to gismondine-Ca. Consequently, a slight rotation of the double crankshaft chains has been noticed. Similar observations have been made in partially dehydrated and the pressure-modified gismondine-Ca. The present study suggested that, in addition to high-pressure and dehydration, the elliptical deformation of the channels in GIS also arises as a consequence of the extraframework cations and H2O content. Thus, the extraframework content influences the aluminosilicate framework leading to the orthorhombic symmetry

Dynamic Disorder of Fe3+ Ions in the Crystal Structure of Natural Barioferrite

A natural barioferrite, BaFe3+12O19, from a larnite–schorlomite–gehlenite vein of paralava within gehlenite hornfels of the Hatrurim Complex at Har Parsa, Negev Desert, Israel, was investigated by Raman spectroscopy, electron probe microanalysis, and single-crystal X-ray analyses acquired over the temperature range of 100–400 K. The crystals are up to 0.3 mm × 0.1 mm in size and form intergrowths with hematite, magnesioferrite, khesinite, and harmunite. The empirical formula of the barioferrite investigated is as follows: (Ba0.85Ca0.12Sr0.03)∑1(Fe3+10.72Al0.46Ti4+0.41Mg0.15Cu2+0.09Ca0.08Zn0.04Mn2+0.03Si0.01)∑11.99O19. The strongest bands in the Raman spectrum are as follows: 712, 682, 617, 515, 406, and 328 cm−1. The structure of natural barioferrite (P63/mmc, a = 5.8901(2) Å, c = 23.1235(6) Å, V = 694.75(4) Å3, Z = 2) is identical with the structure of synthetic barium ferrite and can be described as an interstratification of two fundamental blocks: spinel-like S-modules with a cubic stacking sequence and R-modules that have hexagonal stacking. The displacement ellipsoids of the trigonal bipyramidal site show elongation along the [001] direction during heating. As a function of temperature, the mean apical Fe–O bond lengths increase, whereas the equatorial bond lengths decrease, which indicates dynamic disorder at the Fe2 site

2003b): Morphology, composition and structure of low-temperature P4/nnc high-fl uorine vesuvianite whiskers from Polar

ABSTRACT Whiskers and needles of F-rich vesuvianite were found together with diopside in cavities of an altered magnesian skarn in the Tas-Khayakhtakh Mountains of Polar Yakutia, in Russia. The acicular crystals are strongly zoned and formed between two generations of diopside. The chemical composition of the vesuvianite whiskers is more homogeneous and resembles that of the outermost rim of the vesuvianite needles. In the last stage, fluorapophyllite, prehnite, titanite, calcite and quartz overgrew vesuvianite. Whiskers of vesuvianite crystallized at low activity of CO 2 and P-T conditions corresponding to the prehnitepumpellyite facies. Single-crystal X-ray refinements of the structure of three vesuvianite whiskers, for which electron-microprobe data also were collected, revealed P4/nnc space-group symmetry and (F, Cl) substitution at O(10) within disordered strings running parallel to the four-fold axis. In addition, there is partial substitution of F at O(11), usually occupied by OH in lowtemperature vesuvianite. The high symmetry (P4/nnc) in low-temperature (<350°C) whiskers of vesuvianite adds evidence that the degree of string order is determined not only by the temperature of crystallization, as hitherto assumed, but also by the prevailing composition of the fluid and the regime of crystal growth leading to substitutions that disturb intra-rod order and particularly long-range rod order. Long-range rod order leads to reduced symmetry (P4/n or P4nc), typical of vesuvianite crystallized at low temperature in rodingites. Vesuvianite whiskers formed in a kinetic regime where the growth rates were selectively influenced by surface-active substances poisoning the prism faces. Growth of faces in vesuvianite whiskers is explained by a tangential layer-by-layer mechanism without participation of a central screw dislocation. Keywords: low-temperature vesuvianite, whisker, fluorine, composition, crystal structure, crystal growth, infrared spectra, Yakutia, Russia. SOMMAIRE Nous avons découvert des trichites et des aiguilles de vésuvianite riche en fluor avec diopside dans des cavités d'un skarn magnésien altéré dans les montagnes Tas-Khayakhtakh, en Yakoutie polaire, en Russie. Les cristaux aciculaires sont fortement zonés et se sont formés entre deux générations de diopside. La composition chimique de la vésuvianite trichitique est plus homogène, et ressemble à celle de la bordure des aiguilles de vésuvianite. Au stade ultime de croissance, fluorapophyllite, prehnite, titanite, calcite et quartz ont englobé la vésuvianite. Les trichites de vésuvianite ont cristallisé à une faible activité de CO 2 à des conditions de P et de T correspondant au faciès prehnite-pumpellyite. Des affinements de la structure par diffraction X sur trois monocristaux trichitiques, pour lesquels nous possédons des données sur la composition obtenues par analyses à la microsonde § (Traduit par la Rédaction) Mots-clés: vésuvianite de faible température, trichite, fluor, composition, structure cristalline, croissance cristalline, spectroscopie infra-rouge, Yakoutie, Russie. tals elongate along c. However, there are no condensed chains of SiO 4 tetrahedra in the vesuvianite structure, and thus whisker formation is structurally not expected. The symmetry of vesuvianite is defined by its growth conditions and thermal history. Comparative structural investigations of vesuvianite of variable genesis lead to a distinction of high-temperature, high-symmetry tetragonal vesuvianite (>350-400°C) with long-range disorder (space group P4/nnc), and low-temperature, low-symmetry vesuvianite (<350-400°C, from rodingites) with specific long-range "string" order (space grou

Merohedral Mechanism Twining Growth of Natural Cation-Ordered Tetragonal Grossular

Garnet supergroup minerals are in the interest of different applications in geology, mineralogy, and petrology and as optical material for material science. The growth twins of natural tetragonal grossular from the Wiluy River, Yakutia, Russia, were investigated using single-crystal X-ray diffraction, optical studies, Raman spectroscopy, microprobe, and scanning electron microscopy. The studied grossular is pseudo-cubic (a = 11.9390 (4), c = 11.9469 (6) Å) and birefringent (0.01). Its structure was refined in the Ia3¯d, I41/acd, I41/a, and I4¯2d space groups. The I41/a space group was chosen as the most possible one due to the absence of violating reflections and ordering of Mg2+ and Fe3+ in two independent octahedral sites, which cause the symmetry breaking according to the group–subgroup relation Ia3¯d → I41/a. Octahedral crystals of (H4O4)4−-substituted grossular are merohedrally twinned by twofold axis along [110]. The mechanism of twining growth led to the generation of stacking faults on the (110) plane and results in the formation of crystals with a long prismatic habit