Texture and Geochemistry of Scheelites in the Tongshankou Deposit in Daye, Hubei, China: Implication for REE Substitution Mechanism and Multistage W Mineralization Processes

The Tongshankou skarn deposit in the Edong ore district is a typical metasomatic deposit associated with adjacent granodiorite porphyry and carbonate rocks. Using comprehensive microscopic observations, mineralogical and geochemical analysis, scheelite grains in the skarns can be classified into three major types, showing multi-stage mineralization characteristics. In the redox fluid environment, scheelites that occur with garnets usually have affinity to garnets, while in later skarn phases others exist with oxides and sulfides. They can be subdivided by trace elements, such as Nb and Eu, to discuss the detailed ore-forming process. Scheelites have three typical substitution mechanisms including: 2Ca2 + ⇌ REE3 + +Na+ (1); Ca2 + + W6 + ⇌ REE3 + +Nb5+(2); and 3Ca2 + ⇌ 2REE3++ □Ca (□Ca = Ca site vacancy) (3). Plagioclase or various hydrothermal stages can cause Eu anomalies to fluctuate from positive to negative, and these processes can cause particular zonation in W and Mo contents in scheelites. This study highlights the use of texture and geochemistry of scheelites in skarn deposits, depicting the W mineralization processes.

Controlling Multiple Orderings in Metal Thiocyanate Molecular Perovskites Ax{Ni[Bi(SCN)6]}

<p>We report four new A-site vacancy ordered thiocyanate double double perovskites,</p><p>A1􀀀xfNi[Bi(SCN)6]1􀀀x3 }, A = K⁺, NH4⁺, CH3(NH3)⁺ (MeNH₃⁺) and C(NH₂)₃⁺ (Gua⁺), including the first examples of thiocyanate perovskites containing organic A-site cations. We show, using a combination of X-ray and neutron diffraction, that the structure of these frameworks depends on the A-site cation, and that these frameworks possess complex vacancy-ordering patterns and cooperative octahedral tilts distinctly different from atomic perovskites. Density functional theory calculations uncover the energetic origin of these complex orders and allow us to propose a simple rule to predict favoured A-site cation orderings for a given tilt sequence. We use these insights, in combination with symmetry mode analyses, to show that these complex orders offer a new route to non-centrosymmetric perovskites which render them as excellent candidates for</p><p>piezo- and ferroelectric applications.</p>

Controlling Multiple Orderings in Metal Thiocyanate Molecular Perovskites Ax{Ni[Bi(SCN)6]}

<p>We report four new A-site vacancy ordered thiocyanate double double perovskites,</p><p>A1􀀀xfNi[Bi(SCN)6]1􀀀x3 }, A = K⁺, NH4⁺, CH3(NH3)⁺ (MeNH₃⁺) and C(NH₂)₃⁺ (Gua⁺), including the first examples of thiocyanate perovskites containing organic A-site cations. We show, using a combination of X-ray and neutron diffraction, that the structure of these frameworks depends on the A-site cation, and that these frameworks possess complex vacancy-ordering patterns and cooperative octahedral tilts distinctly different from atomic perovskites. Density functional theory calculations uncover the energetic origin of these complex orders and allow us to propose a simple rule to predict favoured A-site cation orderings for a given tilt sequence. We use these insights, in combination with symmetry mode analyses, to show that these complex orders offer a new route to non-centrosymmetric perovskites which render them as excellent candidates for</p><p>piezo- and ferroelectric applications.</p>

Tourmaline Composition of the Kışladağ Porphyry Au Deposit, Western Turkey: Implication of Epithermal Overprint

The Kışladağ porphyry Au deposit occurs in a middle Miocene magmatic complex comprising three different intrusions and magmatic-hydrothermal brecciation related to the multiphase effects of the different intrusions. Tourmaline occurrences are common throughout the deposit, mostly as an outer alteration rim around the veins with lesser amounts disseminated in the intrusions, and are associated with every phase of mineralization. Tourmaline mineralization has developed as a tourmaline-rich matrix in brecciated zones and tourmaline-quartz and/or tourmaline-sulfide veinlets within the different intrusive rocks. Tourmaline was identified in the tourmaline-bearing breccia zone (TBZ) and intrusive rocks that had undergone potassic, phyllic, and advanced argillic alteration. The tourmaline is present as two morphological varieties, aggregates of fine crystals (rosettes, fan-shaped) and larger isolated crystals and their aggregates. Four tourmaline generations (tourmaline I to IV) have different compositions and substitutions. Tourmaline I in TBZ and INT#1 is distinguished by the highest Fetot and enriched in Fe3+. Tourmalines II and III occur as fine aggregates, accompanied by the formation of isolated crystals and are characterized by lower Fetot and Fe3+. Tourmaline IV is characterized by the lowest Fetot, enriched in Cl, and has the highest proportion of X-site vacancy among all the tourmalines. Tourmaline I may be attributed to the potassic stage in INT#1 and early tourmaline in TBZ. Tourmalines II and III from INT#1 and the TBZ could be referred to the phyllic stage. The low Fe content in tourmaline is caused by the simultaneous deposition of sulfide minerals. Tourmaline IV from the TBZ and tourmaline II from INT#3 are distinguished by the high X-site vacancy proportion up to the formation of X-site vacant species as well as enriched in Cl; they can be attributed to the argillic stage of the hydrothermal process. The textural and especially chemical data of the tourmaline from the Kışladağ Au deposit provide information on the physico-chemical conditions during the porphyry to epithermal transition and subsequent epithermal overprinting.

Tourmaline from the Solnechnoe tin deposit, Khabarovsk Krai, Russia

Tourmaline from the Solnechnoe hydrothermal granitoid-related tin deposit in the Khabarovsk Krai, Russian Far East has been studied with electron microprobe, infrared and Mössbauer spectroscopy. Tourmaline formed in three distinct stages with different types of chemical substitution. Tourmaline from the first unmineralised stage is classified as dravite or schorl, which could be enriched locally in Ca, the X-site vacancy and F. This tourmaline is characterised by the Fe ↔ Mg and X vacancy + Al ↔ Na + Fe substitutions. The second, molybdenum-stage tourmaline, is schorl–dravite and fluor-schorl–fluor-dravite enriched in Ca, and a few compositions belong to the calcic group. The predominant substitution is Ca + Mg ↔ Na + Al. The third, tin-stage tourmaline, is classified as schorl–dravite with some tourmalines being fluor-schorl, oxy-schorl, foitite and magnesio-foitite. The tin-stage tourmaline is characterised by the substitutions Fe2+ ↔ Mg, Altot + O2– ↔ Fe2+ + OH–, and Fe3+ ↔ Altot. An increase of the Fe3+/Fetot value from 3–9% in the molybdenum stage to 12–16% in the tin-stage tourmalines indicates an increase in oxidation potential, which possibly contributed to cassiterite deposition. Comparison of tourmalines from greisen, porphyry and intrusion-related tin deposits worldwide shows they differ in primary chemical substitutions so can be characterised by this mechanism. The Fe3+/Fetot value in tourmaline also appears to be one of the indications for the tin deposit type. The Fe3+/Fetot value increases from <10% in greisen tourmaline through 15% in tourmaline from intrusion-related deposits to 20% in tourmaline from porphyry deposits.

Ordered B-Site Vacancies in an ABX3 Formate Perovskite

<p>We report the synthesis and structural characterisation of a series of aliovalently doped metal–formate ABX₃ perovskite frameworks [C(NH₂)₃]Mn²⁺_1–<i>x</i>(Fe³⁺_2<i>x</i>/3,V_<i>x</i>/3)(HCOO)₃ (V = B-site vacancy). For sufficiently large <i>x</i>, the vacancies order, lowering the crystal symmetry from orthorhombic <i>Pnna</i> to monoclinic <i>P</i>2/<i>n</i>. This system establishes B-site vacancies as a new type of defect in formate perovskites, and one with important chemical, structural, and functional implications. Monte Carlo simulations driven by a nearest-neighbour vacancy repulsion model show checkerboard vacancy order to emerge for <i>x</i> > 0.6, in accord with experiment.</p>