Lithium-containing Na-Fe-amphibole from cryolite rocks of the Katugin rare-metal deposit (Transbaikalia,Russia): chemical features and crystal structure

Detailed chemical and structural studies were carried out for Li-Na-Fe-amphibole from cryolite rocks of the Katugin deposit, Transbaikalia. The rocks contain 30-70 vol.% cryolite, mafic minerals as Fe-silicates (Li-Na-Fe-amphibole, Li-containing fluorannite, and bafertisite), oxides (magnetite, ilmenite, pyrochlore, cassiterite, and others), and sulfides (sphalerite, pyrite, and chalcopyrite). Quartz, K-feldspar, polylithionite, REE-fluorides, and albite occur as minor or accessory phases. The chemical composition of amphibole (wt.%) varies as follows: SiO₂, 48.5-48.9; TiO2, 0.4-0.8; M2O3, 1.6-2.2; Fe2O3, 15.9-17.1; FeO, 17.6-18.4; MnO, 0.8-0.9; ZnO, 0.3-1.1; MgO, 0.2-0.3; CaO, < 0.1; Na2O, 8.4-8.7; K₂O, 1.4-1.5; Li₂O, 0.6-0.8; H₂O, 0.7-0.8; and F, 2.2-2.5. The amphibole has a specific composition intermediate among the F-Fe members of the Na-amphibole subgroup: 40-45 mol.% ferro-ferri-fluoro-nyb0ite, 40-45 mol.% ferro-ferri-fluoro-leakeite, and 10-20 mol.% fluoro-riebeckite ± fluoro-arfvedsonite. The mineral is monoclinic, space group C2/m, a = 9.7978(2), b = 17.9993(3), c = 5.33232(13) A, P = 103.748(2)°, V = 913.43(3) A³, and Z = 2. The structural formula of Li-Na-Fe-amphibole is (Nao.₄6Ko.₂₄do.₃o)Na₂.oo(Fea95Mgo.o5)₂- (Fe0⁺ 95Ti0.025Mg0.025)2(Li0.37Fea48Mn0.10Zn0.05)[(Si0.91Al <).09)4Si4O22](F0.58(OH)0.42)2. R^amaⁿ and Mossbauer spectroscopy data are given for this amphibole.     

汞银黝铜矿Ag6 (Cu4Hg2) Sb4S13——黝铜矿族新矿物

作为黝铜矿族矿物的新成员,汞银黝铜矿(Argentotetrahedrite-(Hg),IMA 2020-079),Ag6(Cu4 Hg2) Sb4S13,发现于湘黔汞矿带北段之保靖东坪Hg-Ag矿床中,是该矿床的主要矿石矿物和回收对象.汞银黝铜矿单晶晶体尺寸约为5～20 μm,呈粒径20～300 μm的粒状、片状集合...     

Ab initio quantum chemical investigation of arsenic sulfide molecular diversity from As4S6 and As4

The structural diversity of arsenic sulfide molecules in compositions between As₄S₆ and As₄ was investigated using ab initio quantum chemical calculations. The As₄S₆ molecule consists of four trigonal pyramid coordinations of As atoms bonding to three S atoms. In the As₄S₅ composition, only one type of molecular configuration corresponds to an uzonite-type molecule. In the As₄S₄ composition, two molecular configurations exist with realgar-type and pararealgar-type molecules. Three molecular configurations are in the As₄S₃ composition. The first configuration comprises trigonal pyramidal As atom coordinations of two types: bonding to two S atoms and one As atom, and bonding to one S atom and two As atoms. The second is the molecular configuration of dimorphite. The third comprises trigonal pyramidal As atom coordinations of two types: bonding to three As atoms, and bonding to one As atom and two S atoms. The As₄S₂ composition allows molecular configurations of two types. One is comprised of trigonal pyramidal As atom configurations of one type bonding to two As atoms and one S atom. The other comprises trigonal pyramidal As atom coordinations of three types: bonding to two S atoms and one As atoms, bonding to one S atom and two As atoms, and bonding to three As atoms. The As₄S molecule has trigonal pyramidal As atom coordinations of two types: bonding to one S atom and two As atoms, and bonding to three As atoms. The As₄S composition permits only one molecular configuration, which suggests that the mineral duranusite comprises the As₄S molecular geometry. In all, ten molecular configurations are predicted in the molecular hierarchy of the arsenic sulfide binary system. The simulated Raman spectral profiles are helpful in searching for undiscovered arsenic sulfide minerals.     

Refinement of the crystal structure of beryllonite,NaBePO4

Summary The refined lattice parameters of beryllonite are:a=8.178 (3) Å,b=7.818 (2) Å,c=14.114 (6) Å, =90.00° (2); space groupP2₁/n,Z=12. Integrated Weissenberg photographs were taken by using CuK radiation and multiple film packs. The anisotropic refinement of the crystal structure by means of least-square methods gave a finalR value of 0.063 for the 1388 observed reflections. In the crystal structure PO₄ and BeO₄ tetrahedra, linked by shared oxygen atoms in a three-dimensional network, form pseudo-ditrigonal rings perpendicular to theb axis. The independent Na atoms lying in the channels formed by the rings are coordinated as an irregular nine-cornered polyhedron and as distorted octahedra.

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Verfeinerung der Kristallstruktur des Beryllonits,NaBePO ₄

Zusammenfassung Verfeinerte Gitterkonstanten des Beryllonits lauten:a=8,178 (3) Å,b=7,818 (2) Å,c=14,114 (6) Å, =90,00 (2)°; Raumgruppe:P2₁/n,Z=12. Integrierte Weissenbergaufnahmen wurden mit CuK-Strahlung und multiplen Filmpaketen aufgenommen. Die anisotrope Verfeinerung der Kristallstruktur nach der Methode der kleinsten Quadrate ergab für 1388 beob. Reflexe einen abschließendenR-Wert von 0,063. In der Kristallstruktur bilden PO₄-und BeO₄-Tetraeder, die über gemeinsame Sauerstoffe zu einem dreidimensionalen Gerüst verknüpft sind, pseudo-ditrigonale Ringe senkrecht zurb-Achse. Die kristallographisch unabhängigen Arten von Na-Atomen, welche in den aus Ringen gebildeten Kanälen liegen, sind in der Form eines unregelmäßigen neuneckigen Polyeders bzw. in der Form verzerrter Oktaeder koordiniert.

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With 2 Figures     

Refinement of the crystal structure of papagoite,CaCuAlSi2O6(OH)3

Summary The crystal structure of papagoite, CaCuAlSi₂O₆(OH)₃, monoclinic,a = 2.926 (3),b = 11.496 (3),c = 4.696 (1) Å, = 100.81 (2)°, V = 685.4 (3) ų, space groupC 2/m,Z = 4, has been refined to anR index of 3.4% for 913 observed reflections measured with MoK X-radiation. The single unique Cu cation is surrounded by six anions in a pseudo-octahedral arrangement showing strong Jahn-Teller distortion. Edge-sharing rutile-like chains of pseudo-octahedrally coordinated Cu and Al extend in the Y direction, and are cross-linked into an octahedral sheet by Ca₂Ø₁₀ (O = unspecified ligand, O or OH) dimers. These sheets are linked by (Si₄O₁₂) rings to form a mixed tetrahedral-octahedral framework.

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Verfeinerung der Kristallstruktur von Papagoit, CaCuAlSi₂O₆(OH)₃

Zusammenfassung Die Kristallstruktur von Papagoit, CaCuAlSi₂O₆(OH)₃, monoklina = 12,926 (3),b = 11.496 (3),c = 4.696 (1) Å, = 100.81 (2)°,V = 685.4 (3) ų, RaumgruppeC2/m,Z = 4, wurde für 913 beobachtete, MoK-Röntgenstrahlung gemessene Reflexe aufR = 3,4% verfeinert. Das eine symmetrieunabhängige Kupfer-Kation ist pseudooktaedrisch von sechs Anionen mit einer starken Jahn-Teller-Verzerrung umgeben. Die pseudo-oktaedrisch koordinierten Cu- und Al-Atome werden über Kanten zu parallel zur y-Achse verlaufenden rutil-ähnlichen Ketten verbunden, die zusammen mit den Ca₂Ø₁₀-Gruppen (Ø = nicht spezifsierter Ligand, O bzw. OH) Oktaederschichten bilden. Diese Schichten werden durch (Si₄O₁₂)-Ringe zu einem aus Tetraedern und Oktaedern bestehenden Netzwerk verknüpft.

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With 4 Figures     

The crystal structure of arsentsumebite, Pb2Cu[(As, S)O4]2(OH)

Summary The crystal structure of arsentsumebite, ideally, Pb₂Cu[(As, S)O₄]₂(OH), monoclinic, space group P2₁/m, a = 7.804(8), b = 5.890(6), c = 8.964(8) ?, β = 112.29(6)°, V = 381.2 ?³, Z = 2, d_calc. = 6.481 has been refined to R = 0.053 for 898 unique reflections with I> 2σ(I). Arsentsumebite belongs to the brackebuschite group of lead minerals with the general formula Pb₂ Me(XO₄)₂(Z) where Me = Cu²⁺, Mn²⁺, Zn²⁺, Fe²⁺, Fe³⁺; X = S, Cr, V, As, P; Z = OH, H₂O. Members of this group include tsumebite, Pb₂Cu(SO₄)(PO₄)(OH), vauquelinite, Pb₂Cu(CrO₄)(PO₄)(OH), brackebuschite, Pb₂ (Mn, Fe)(VO₄)₂(OH), arsenbracke buschite, Pb₂(Fe, Zn)(AsO₄)₂(OH, H₂O), fornacite, Pb₂Cu(AsO₄)(CrO₄)(OH), and feinglosite, Pb₂(Zn, Fe)[(As, S)O₄]₂(H₂O). Arsentsumebite and all other group members contain M = M–T chains where M = M means edge-sharing between MO₆ octahedra and M–T represents corner sharing between octahedra and XO₄ tetrahedra. A structural relationship exists to tsumcorite, Pb(Zn, Fe)₂(AsO₄)₂ (OH, H₂O)₂ and tsumcorite-group minerals Me(1)Me(2)₂(XO₄)₂(OH, H₂O)₂. Received June 24, 2000; revised version accepted February 8, 2001     

Geology and mineralogy of the Alakha spodumene granite porphyry deposit,Gorny Altai,Russia

The Alakha lithium–tantalum deposit in the southern Altai, Russia, is represented by a stock of spodumene-bearing granite porphyry localized in the Kalba–Narym–Koktogai lithium–tantalum rare-metal granitic belt, unique in extent (more than 1000 km). This belt is a part of the Altai accretionary–collisional system. Judging from forecasting, the Alakha deposit can be regarded as an uneroded proxy of a pegmatite body both in dimensions and mean Li₂O and Ta₂O₅ contents (0.98 wt % and 114 ppm, respectively); however, the oregenerating potential of this deposit remains insufficiently studied and had not yet been claimed. In this paper, we attempt to fill this gap with a detailed mineralogical study, which allows us to provide insights into the crystallization of Li-bearing high-silicic magma and redistribution of components during magmatic and postmagmatic processes. Accessory mineral assemblages in muscovite–spodumene–K-feldspar granite porphyry and muscovite albitite—the main petrographic rock varieties of the Alakha stock—turned out to be almost identical. A significant similarity in the chemistry of major rock-forming minerals is established for spodumene granite porphyry of the Alakha stock and spodumene pegmatites from large deposits, which makes it possible to suggest that they are close in the petrogenetic mechanism of their formation. The mineral assemblages of muscovite albitite in the apical portion of the Alakha stock are connected by gradual transition with those of spodumene granite porphyry. Such a transition is caused by postmagmatic metasomatic alteration of the latter.     

Refinement of the crystal structure of bonshtedtite, Na3Fe(PO4)(CO3)

The crystal structure of bonshtedtite, Na₃Fe(PO₄)(CO₃) (monoclinic, P2₁/m, a = 5.137(4), b = 6.644(4), c = 8.908(6) Å, β = 90.554(14)°, V = 304.0(4) ų, Z = 2) has been refined to R ₁ = 0.041 on the basis of 1314 unique reflections. The structure is similar to that of other minerals of the bradleyite group. It is based on the [Fe(PO₄)(CO₃)]^3? layers oriented parallel to (001). The layers are formed by corner-sharing PO₄ tetrahedra and FeO₄(CO₃) complexes, where FeO₆ tetrahedra and CO₃ triangles are edge-shared. The topology of the octa-tetrahedral layer in bonshtedtite is similar to that of the autunite-group minerals, but it differs from the latter in terms of local topological properties.     

Uniquely high-grade iodide mineralization in the oxidation zone of the Rubtsovskoe base-metal deposit,Northwest Altai,Russia

Unusual, high-grade iodide mineralization comprising marshite, miersite, and iodargyrite has been discovered in the oxidation zone of the Rubtsovskoe VHMS base-metal deposit, Northwest Altai, Russia. The distribution of iodides reveals distinct zoning. Iodargyrite is widespread in the upper part of the oxidized orebody at the hypsometric level of +156 to 163 masl. The iodargyrite zone extends for more than 150 m. The content of iodargyrite in gossan occasionally reaches 1–5 vol %. Marshite is localized at the lower level (+146 to +151 masl); the zone enriched in marshite is about 50 m in extent. The marshite content in the high-grade oxidized ore with native copper and in the zone of wall-rock argillic alteration is typically close to 1 vol %, occasionally reaching 7–10 vol %. Miersite occurs sporadically in association with both iodargyrite and low-Ag marshite, which are antagonistic mineral species. Iodargyrite is stoichiometric AgI (2H polytype with a = 4.574 and c = 7.519 Å). Isostructural cubic marshite CuI and miersite (Ag,Cu)I make up an isomorphic series within compositional limits Mar₁₀₀Mie₀-Mar₉Mie₉₁ and a break between Mar_82.5Mie_17.5 and Mar₅₇-Mie₄₃; the a parameter of their unit cells varies from 6.050 to 6.424 Å. The crystal morphology, properties, and mode of iodide occurrence are described in the paper. According to the suggested genetic model, the source of iodine was related to exhalations of seafloor fumaroles accompanying volcanic-hydrothermal-sedimentary ore formation. Iodine is absorbed by clay in the wide zone of wall-rock argillic alteration. During the early stages of sulfide ore oxidation, sulfuric acid oxidized iodine to (I⁵⁺O₃)^?, and it was subsequently reduced and fixed in poorly soluble Ag⁺ and Cu⁺ iodides.     

阿尔泰可可托海3号脉伟晶岩型稀有金属矿床云母和长石的矿物学研究及意义

可可托海3号脉伟晶岩型稀有金属矿床是阿尔泰造山带产出的规模最大的伟晶岩脉,其完美的同心环状结构分带举世闻名。云母和长石作为3号脉9个结构带的贯通性矿物,由外向内表现不同的结构和成分特征。其中,云母由白云母系列向锂云母系列演化,白云母呈黄-绿色中细粒→白色或绿色中粗粒-巨晶→白色或绿色书状集合体→白色或绿色中粗粒-巨晶,锂云母呈玫瑰紫中细粒鳞片状或楔状集合体,BSE图像下云母表现出成分分带及不平衡和交代结构;长石主要为钾长石和钠长石,及少量斜长石,钾长石主要呈块体产出,钠长石呈细粒→叶片状→薄片状产出。本次研究通过电子探针(EMPA)和激光剥蚀等离子质谱(LA-ICP-MS)获得3号脉各结构带云母和长石的主微量成分。3号脉云母具有高Li(249×10^-6~35932×10^-6)、Rb(1240×10^-6~22825×10^-6)、Cs(35.9×10^-6~13980×10^-6)、Ta(13.3×10^-6~447×10^-6)含量、低K/Rb值(4.23~59.4)和K/Cs值(6.53~2368),钾长石具有低K/Rb值(35.4~1865),且由外向内,随K/Rb值降低,云母的Li、Rb、Cs、F、Ta含量升高,表明3号脉是一个由外向内结晶的分异演化程度较高的伟晶岩脉。另外,连续相邻结构带中云母和长石的主微量成分呈振荡变化,该现象主要受熔体不混溶过程的控制,也受矿物结晶不平衡影响,而熔体不混溶过程也是控制3号脉结构分带的机制之一。外部带(I-IV带)和内部带(V-VIII带)的云母和碱性长石在成分(FeO、Li、Rb、Cs、F、Ta含量和K/Rb值及K/Cs值)和结构(不平衡和交代结构)上具有明显差异,内部带演化程度明显加大,流体组分比例升高,表明体系由以熔体为主的阶段(外部带)进入以熔流体为主相对不稳定的阶段(外部带)。结合野外观察的证据,促使体系在IV带和V带间发生突然转变而进入熔流体阶段的是一个泄压事件。     

Vinciennite and Cu-excess tennantite from the Layo (Cu,Sn, As,Au) epithermal deposit (Southern Peru)

Summary The Layo epithermal deposit, cutting Miocene-Pliocene calc-alkaline volcanites of the Tacaza group, includes a well-developed eastern zone (Vetas 7 and 8) in which brecciated and banded textures are associated with a large, intensely argillized zone containing diaspore and alunite. The vetas contain a typical Cu-As mineralogy of the acid-sulfate type (pyrite, enargite, Cu-excess tennantite, chalcopyrite, covellite) with an associated original stanniferous paragenesis including vinciennite and mawsonite. The vinciennite is close to the ideal end-member (Cu₁₀Fe₄SnAsS₁₆) and the Cu-excess tennantite (Cu₁₁FeAs₄S₁₃) shows a total absence of Zn and Ag; its very specific chemical composition suggests that all the iron is Fe³⁺, equilibrated by Cu⁺ and probably minor Cu²⁺.This particular mineralogical association implies deposition at relatively low temperature (300°C) and high aS₂ (10^–6.5 decreasing to 10^–8.5), from a Cu-S-rich and Fe-Zn-poor fluid. The acid-sulfate epithermal mineralization of the eastern vetas of Layo appears to have preceded an adularia-sericite epithermal mineralization expressed in the western vetas of Layo and also in the nearby large epithermal veins at Orcopampa and Shila. It also supports the genetic relationship that is commonly evoked between porphyry copper and epithermal deposits.

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Le minéralogie originale à vinciente et tennantite riche du gisement épithermal (Cu, Sn, As, Au) de Layo (Sud Péru)

Résumé Le gîte épithermal de Layo est encaissé dans les volcanites calco-alcalines Miocène-Pliocène du groupe de Tacaza. Sa zone orientale renferme des corps minéralisés (Vetas 7 et 8) bien développés, à textures bréchiques et rubanées associées à une large zone intensément argilisée contenant diaspore et alunite.Les vetas présentent une association minéralogique à As-Cu typique des gisements acide-sulfate (pyrite, énargite, tennantite riche en cuivre, chalcopyrite, covellite) et une paragenèse stannifère originale renfermant vinciennite et mawsonite. La vinciennite est proche du pôle théorique (Cu₁₀Fe₄SnAsS₁₆) et la tennantite riche en Cu (Cu₁₁FeAs₄S₁₃) est dépourvue de Zn et de Ag; sa composition chimique suggère que tout le fer se présente sous la forme Fe³⁺, en équilibre avec Cu⁺, avec probablement une participation mineure de Cu²⁺.Cette association minéralogique particulière implique une mise en place sous forte fugacité en soufre (aS₂ = 10^–6.5 décroissant jusqu'à 10^–8.5) à une température relativement basse de l'ordre de 300°C à partir d'un fluide riche en Cu et S et pauvre en Fe et Zn. La minéralisation de type acide-sulfate des vetas orientales de Layo précéderait celles de type adulaire-sericite des vetas occidentales de Layo et des gisements voisins de Orcopampa et Shila. Elle contribue à renforcer le lien fréquemment évoqué entre les porphyres cuprifères et les gisements épithermaux.

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With 10 Figures     

The nature of the chemical bonds in sulvanite,Cu3VS4 1

Summary It is shown that a completely tetrahedral structure for Cu₃VS₄ would be unstable because of a large residual electric charge on the vanadium atom. The observed structure of sulvanite places six copper atoms, as well as four sulfur atoms, within bonding distance of each vanadium atom. The consideration of alternative valence-bond structures that are compatible with the electroneutrality principle (atomic charges restricted to the range between–1 and +1) leads to the bond numbers 0.50 for Cu–S and 0.29 for V–Cu, with 1.00 assumed for V–S. These bond numbers correspond reasonably well with the observed bond lengths and with electroneutrality (charges-0.27 for Cu, +0.06 for V, +0.19 for S). The bond angles at the sulfur atom indicate some bond-bending strain, estimated at 0.85 kcal mole^–1 per sulfur atom. This study of the sulvanite structure thus provides an explanation of the concentration of the four bonds formed by the sulfur atom into a small solid angle on one side of the atom.

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Zusammenfassung Es wird gezeigt, daß eine Struktur mit durchgehend tetraedrischer Koordination für Cu₃VS₄ wegen der großen Restladung des Vanadiumatoms instabil wäre. In der beobachteten Struktur des Sulvanites liegen sowohl vier Schwefelatome wie sechs Kupferatome im Bindungsabstand zu jedem Vanadiumatom. Die Betrachtung anderer kovalenter Strukturen, die mit dem Elektroneutralitäts-Prinzip (Atomladungen auf den Bereich zwischen –1 und +1 beschränkt) verträglich sind, führt auf Bindungszahlen 0,50 für Cu–S und 0.29 für V–Cu, wenn 1,00 für V–S angenommen wird. Diese Bindungszahlen entsprechen den beobachteten Bindungslängen und der Elektroneutralität (Ladungen –0,27 für Cu, +0,06 für V, +0,19 für S) ziemlich gut. Die Bindungswinkel am Schwefelatom deuten auf eine gewisse Spannung durch Verbiegung der Bindungen hin, die auf 0,85 kcal·mol^–1 pro Schwefelatom geschätzt wird. Diese Untersuchung der Sulvanitstruktur liefert so eine Erklärung für die Konzentration der vier von Schwefel ausgehenden Bindungen in einem kleinen räumlichen Winkelbereich an einer Seite des Atoms.

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With 1 Figure

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Dedicated to ProfessorF. Machatschki on the occasion of his 70th birthday.     

滇西茅草坪脉状铜矿床电气石的发育特征、成分及其意义

位于滇西兰坪盆地西缘的茅草坪矿床是近年来新发现的一个脉状铜矿床,与该区其他脉状铜矿不同,它发育大量热液电气石。本文对矿床电气石的发育特征和化学成分进行了研究,以此探讨电气石生长和成分的控制因素、成矿流体来源及成矿过程中氧逸度的变化。茅草坪矿床铜矿体由石英-碳酸盐-硫化物脉及伴生的热液蚀变晕组成。电气石可分为两类:较早沉淀的蚀变晕电气石(T1)和较晚沉淀的脉体内电气石(T2)。T1电气石与石英共生,出现在变碎屑岩(T1-Q)和大理岩(T1-M)中,矿物强烈定向生长,长轴平行于围岩的剪切面理,颗粒相对细小,此阶段无含铜硫化物生成;T2电气石伴有黄铜矿生成,矿物长轴多与围岩的剪切面理斜交、呈弱定向生长,或不定向地生长,矿物颗粒相对粗大。由此反映在矿床热液矿化过程中,控制热液矿物生长的构造应力由强剪切到弱剪切及剪切停止的趋势变化。电子探针分析结果表明,所有电气石均属于碱性族,主体为镁电气石。T2电气石在背散射图像下存在明暗环带,与Ti、Fe、Mg含量的差异有关,也反映该期成矿流体的物理化学条件呈波动变化。茅草坪电气石的化学组成主要受流体成分控制,但T1-Q电气石相对富Al,T1-M电气石相对富Ca,表明围岩对蚀变晕电气石成分有一定影响。矿床以镁电气石为主的特征,暗示成矿流体不可能直接来自未经历水-岩相互作用的壳源花岗岩岩浆水。T2电气石较T1电气石具有高的Fe~(3+)/(Fe~(3+)+Fe~(2+))值,表明矿床成矿流体从早期到晚期可能是一个氧逸度升高的过程。结合区域脉状铜矿流体特征、金属组合、围岩和蚀变特征,推测茅草坪矿床及滇西脉状铜矿床迁移Cu的流体可能是CO_2缓冲p H值的还原性流体,在矿化部位流体氧逸度升高很可能是含铜硫化物沉淀的重要机制之一。     

Whole-rock geochemistry and U-Pb ages of Devonian bimodal-type rhyolites from the Rudny Altai,Russia: Petrogenesis and tectonic settings

The paper presents new original data on the Devonian felsic volcanism of the NW Rudny Altai (Russia) in the west of Central Asian Orogenic Belt (CAOB) – the front part of the Altai convergent margin of the Siberian continent. Two geochemical types of subvolcanic rhyolites were emplaced synchronously with the bimodal rhyolite-basalt association, which began to form in the end-Emsian, and clearly manifested on the border of the Givetian and the Frasnian. The rhyolites yield zircon U-Pb ages of ca. 390 Ma (R1-type) and 380 Ma (R2- and R3-types), reflecting two peaks of the volcanic activity. Most of these rocks have extreme petrochemical characteristics of high SiO₂ contents and have contrast Na/K ratios. Their compositions are transition between calc-alkaline and tholeiite series: (La/Yb)_n ~ 2–7, Zr/Y ~ 4 (Zr < 350 ppm) and La/Sm ~ 0.55–1. Rhyolites bear the distinctive geochemical signature of A-type felsic magma, such as enrichments in Zr, Nb, Y and Ce (>350 ppm), Zr (>250 ppm), and high Ga/Al (>2.6) values. The island-arc-like R1-rhyolite formed immediately after the beginning of rifting due to widespread crustal melting under reduced conditions. The generation of rift-like R2- and R3-rhyolites took place under non-equilibrium conditions, synchronously with the rise in the upper crust of Givetian-Frasnian basic magmas, as a result of the active lithospheric extension and high thermal input from the underlying hot mantle. We propose an extension regime in the transition area between the island-arc and back-arc basin for the origin of rhyolites. The study of the Devonian volcanism of the Rudny Altai gives important information about the processes that occurred at the initial stage of the formation of the Altai convergent margin.     

A bond-topological approach to theoretical mineralogy: crystal structure, chemical composition and chemical reactions

Here, I describe a theoretical approach to the structure and chemical composition of minerals based on their bond topology. This approach allows consideration of many aspects of minerals and mineral behaviour that cannot be addressed by current theoretical methods. It consists of combining the bond topology of the structure with aspects of graph theory and bond-valence theory (both long range and short range), and using the moments approach to the electronic energy density-of-states to interpret topological aspects of crystal structures. The structure hierarchy hypothesis states that higher bond-valence polyhedra polymerize to form the (usually anionic) structural unit, the excess charge of which is balanced by the interstitial complex (usually consisting of large low-valence cations and (H₂O) groups). This hypothesis may be justified within the framework of bond topology and bond-valence theory, and may be used to hierarchically classify oxysalt minerals. It is the weak interaction between the structural unit and the interstitial complex that controls the stability of the structural arrangement. The principle of correspondence of Lewis acidity–basicity states that stable structures will form when the Lewis-acid strength of the interstitial complex closely matches the Lewis-base strength of the structural unit, and allows us to examine the factors that control the chemical composition and aspects of the structural arrangements of minerals. It also provides a connection between a structure, the speciation of its constituents in aqueous solution and its mechanism of crystallization. The moments approach to the electronic energy density-of-states provides a link between the bond topology of a structure and its thermodynamic properties, as indicated by correlations between average anion coordination number and reduced enthalpy of formation from the oxides for ^[6]Mg _m ^[4] Si _n O_(m+2n) and MgSO₄(H₂O) _n .