Trabzonite, Ca4[Si3O9(OH)]OH: crystal structure, revised formula, new occurrence and relation to killalaite

The crystal structure of the rare skarn mineral trabzonite, Ca 4[Si 3O 9(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 Ca 4(Si 3O 10)̇2H 2O, reported in the original trabzonite description, to an OH-bearing composition. The crystal structure, which contains Si 3O 10 trimers embedded in a framework of CaO 6-8 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 P2 1/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-crystal X-ray structure refinement of killalaite from the Northern Caucasus confirmed it to be non-stoichiometric with a composition between Ca 6[Si 2O 6(OH)] 2(OH) 2 and Ca 7[Si 2O 7] 2(OH) 2(Z = 2). Trabzonite, killalaite and dellaite form a series of modular structures which differ mainly in the degree of condensation of the SiO 4 units

Vapnikite Ca<SUB>3</SUB>UO<SUB>6</SUB> – a new double-perovskite mineral from pyrometamorphic larnite rocks of the Jabel Harmun, Palestinian Autonomy, Israel

The new mineral species vapnikite, Ca3UO6, was found in larnite pyrometamorphic rocks of the Hatrurim Formation at Jabel Harmun in the Judean desert, Palestinian Autonomy, Israel. Vapnikite is an analogue of the synthetic ordered double-perovskite β-Ca3UO6 and is isostructural with the natural fluorperovskite – cryolite Na3AlF6. Vapnikite Ca3UO6 (P21/n,Z = 2, a = 5.739(1), b = 5.951(1), c = 8.312(1) Å, β = 90.4(1)°, V = 283.9(1) Å3) forms yellow-brown xenomorphic grains with a strong vitreous lustre. Small grains up to 20 – 30 m m in size are wedged between larnite, brownmillerite and ye'elimite. Vapnikite has irregular fracture, cleavage and parting were not observed. The calculated density is 5.322 g cm–3, the microhardness is VHN25 = 534 kg mm–2 (mean of seven measurements) corresponding to the hardness of ~5 on the Mohs scale. The crystal structure of vapnikite Ca3UO6 differs from that of its synthetic analogue β-Ca3UO6 by having a larger degree of Ca, U disorder. Vapnikite formed at the high-temperature retrograde stage of pyrometamorphism when larnite rocks were altered by fluids/melts of high alkalinity

The modular structure of dovyrenite, Ca 6 Zr[Si 2 O 7 ] 2 (OH) 4 : Alternate stacking of tobermorite and rosenbuschite-like units

absTracT The average structure, space group Pnnm [subcell: A = 5.666(16), B = 18.844(5), C = 3.728(11) Å, V = 398.0(2) Å 3 , Z = 1], of the new mineral dovyrenite Ca 6 Zr[Si 2 O 7 ] 2 (OH) 4 has been refined from single-crystal X-ray data to R = 7.97%. The modular structure of dovyrenite is build by alternate stacking of Ca-polyhedral layers characteristic of the tobermorite structure and octahedral layers with attached disilicate groups known from the rosenbuschite group of minerals. No indications of ordered polytypes were detected for the potential OD-structure. Either the small crystal size producing only weak diffraction intensities did not allow detecting diffuse diffraction features (or &quot;super-structure&quot; reflections) or the structure is build by disordered stacks of OD layers. Nevertheless, the resolved average structure allowed unraveling the possible order patterns within the rosenbuschite-like octahedral layers. The key for understanding the polytypic character of this structure is the short periodicity of the tobermorite-like Ca polyhedral layer of only 3.73 Å along c, whereas the periodicity of the attached rosenbuschite-like octahedral layer is doubled. In dovyrenite Ca occurs in sixfold-, sevenfold-, and eightfold-coordination. The octahedral Ca site is only half occupied and may reveal additional vacancies, which must be charge balanced by disordered OH-groups replacing O. A corresponding modular structure with the same subunits but different composition and without octahedral vacancies exists for rinkite (Ti,Nb,Al,Zr)(Na,Ca) 3 (Ca,Ce) 4 [Si 2 O 7 ] 2 (O,F) 4 , which has hitherto been considered as heterophyllosilicate