Verneite, Na₂Ca₃Al₂F₁₄, a New Aluminum Fluoride Mineral from Icelandic and Vesuvius Fumaroles

Verneite, Na2Ca3Al2F14, is a new mineral first discovered in fumarolic samples from both Hekla, Iceland and Vesuvius, Italy. Additional occurrences are so far from Eldfell and Fimmvörduhals, both on Iceland. Verneite is cubic, I213, a = 10.264(1) Å, V = 1081.4(3) Å3, Z = 4, and corresponds to the known synthetic compound. The empirical formula is Na2.01Ca2.82Al2.17F14.02 (scanning electron microscopy with energy dispersive spectrometer from an unpolished sample). It appears in crystals up to 20 μm in diameter, with {110}, {100}, and {111} as the main forms. In the crystal structure of its synthetic analogue, Na is coordinated by 7 F atoms in the form of a capped octahedron, Ca with 8 F atoms in the form of a bisdisphenoid, and Al with 6 F atoms in the form of an octahedron. The crystal structure of Na2Ca3Al2F14 contains sinuous chains of Ca coordination polyhedra interlacing with similarly sinuous chains of Na coordination polyhedra and forming together with them layers parallel to {100}. The intersecting layers parallel to three equivalent crystallographic planes form a three-dimensional mesh with Al coordinations imbedded in its holes. The characteristics of Ca coordinations in fluorides, as well as their relations to other ternary Na⁻Ca⁻Al fluorides are discussed. Verneite is named after Jules Verne

Ca- pyrophosphate dihydrate deposition disease (pseudogout) of the temporomandibular joint

Calcium pyrophosphate dihydrate (CPPD) crystals deposition disease or “pseudogout” is an arthropathy rarely affecting the temporomandibular joint (TMJ). Less than 30 clinical cases are reported in literature, whereas mineralogical, structural and chemical studies on CPPD crystals are very few and concern only data on synthetic phases. Medical and mineralogical studies were carried out on CPPD crystals deposition in TMJ area of a 63- year-old woman operated in the 2010. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) investigations were performed to identify the phosphate crystals and to understand the mechanism of their formation. Two phosphates were characterized: a monoclinic (m-CPPD) and a triclinic (t-CPPD) phase, which are the natural analogues of synthetic phases. It is likely that the crystallization of two phosphates during this human disorder was originated by a catalyst activity. One option is to rely on the activity of the bacteria or cellular enzymes during the first stage of the growth of these minerals

Crystal chemistry of silicofluorides

The family of silicofluorides includes compounds of alkaline metals or ammonium with silicium and fluorine. Their common structural characteristic is the presence of isolated [SiF6] octahedra. There is a major difference between the coordination of sodium compared with that of potassium and ammonium. As a result, the crystal structure of malladrite (Na2SiF6) [1][2] is significantly different from those of potassium or ammonium compounds. In malladrite fluorine at oms form a hexagonal eutaxy. In this arrangement half of vacant octahedrally-coordinated XF6 sites are occupied in ratio 1SiF6:2NaF6. On the contrary, the structures of potassium or ammonium silicofluorides are based on eutactic arrangements where both fluorine and potassium/ammonium partake and silicon atoms fill isolated octahedral holes surrounded exclusively by fluorine atoms. Structures with both cubic-eutaxy and hexagonal-eutaxy stackings can be found in this part of the family, as well as with a special ...ABCBABCB... stacking of layers. In the structure of the new mineral heklaite (KNaSiF6) [3][4] the eutactic arrangement of fluorine and Na/K atoms is not present, due to distortion of K coordination. It is a peculiarity of this structure that the cation with shorter bond lengths (Na) has a larger coordination number (10) than the cation with the longer bond lengths (K; CN 9) [3]. The fundamental differences between the Na and K coordinations in heklaite, plus the difference between its structure and those of Na2SiF6 and K2SiF6 polymorphs suggest absence of solid solution in the pseudo-binary phase field Na2SiF6-K2SiF6 which is confirmed by the field observations on the type locality for this mineral, Hekla volcano on Iceland [4]. (18) (PDF) Crystal chemistry of silicofluorides. Available from: https://www.researchgate.net/publication/276859488_Crystal_chemistry_of_silicofluorides [accessed Oct 05 2021]