Gelu Costin

Platinum-bearing chromite layers are caused by pressure reduction during magma ascent

Platinum-bearing chromitites in mafic-ultramafic intrusions such as the Bushveld Complex are key repositories of strategically important metals for human society. Basaltic melts saturated in chromite alone are crucial to their generation, but the origin of such melts is controversial. One concept holds that they are produced by processes operating within the magma chamber, whereas another argues that melts entering the chamber were already saturated in chromite. Here we address the problem by examining the pressure-related changes in the topology of a Mg2SiO4–CaAl2Si2O8–SiO2–MgCr2O4 quaternary system and by thermodynamic modelling of crystallisation sequences of basaltic melts at 1–10 kbar pressures. We show that basaltic melts located adjacent to a so-called chromite topological trough in deep-seated reservoirs become saturated in chromite alone upon their ascent towards the Earth’s surface and subsequent cooling in shallow-level chambers. Large volumes of these chromite-only-saturated melts replenishing these chambers are responsible for monomineralic layers of massive chromitites with associated platinum-group elements.Some basaltic melts become first superheated upon their ascent towards the Earth’s surface and then saturated in chromite alone after cooling in shallow chambers. Here the authors show that large volumes of these chromite-only-saturated melts are responsible for monomineralic layers of massive chromitites in layered intrusions.

Iranite, CuPb10(CrO4)6(SiO4)2(OH)2, isomorphous with hemihedrite.

This study presents the first structural report of iranite, ideally CuPb10(CrO4)6(SiO4)2(OH)2 [copper decalead hexachromate bis(orthosilicate) dihydroxide], based on single-crystal X-ray diffraction data. Iranite is isomorphous with hemihedrite, with substitution of Cu for Zn and OH for F. The Cu atom is situated at the special position with site symmetry 1. The CrO4 and SiO4 tetrahedra and CuO4(OH)2 octahedra form layers that are parallel to (120) and are linked together by five symmetrically independent Pb2+ cations displaying a rather wide range of bond distances. The CuO4(OH)2 octahedra are corner-linked to two CrO4 and two SiO4 groups, while two additional CrO4 groups are isolated. The mean Cr-O distances for the three nonequivalent CrO4 tetrahedra are all slightly shorter than the corresponding distances in hemihedrite, whereas the CuO4(OH)2 octahedron is more distorted than the ZnO4F2 octahedron in hemihedrite in terms of octahedral quadratic elongation.

Multi-stage formation of REE minerals in the Palabora Carbonatite Complex, South Africa

Abstract The 2060 Ma old Palabora Carbonatite Complex (PCC), South Africa, comprises diverse REE mineral assemblages formed during different stages and reflects an outstanding instance to understand the evolution of a carbonatite-related REE mineralization from orthomagmatic to late-magmatic stages and their secondary post-magmatic overprint. The 10 rare earth element minerals monazite, REE-F-carbonates (bastnäsite, parisite, synchysite), ancylite, britholite, cordylite, fergusonite, REE-Ti-betafite, and anzaite are texturally described and related to the evolutionary stages of the PCC. The identification of the latter five REE minerals during this study represents their first described occurrences in the PCC as well as in a carbonatite complex in South Africa. The variable REE mineral assemblages reflect a multi-stage origin: (1) fergusonite and REE-Ti-betafite occur as inclusions in primary magnetite. Bastnäsite is enclosed in primary calcite and dolomite. These three REE minerals are interpreted as orthomagmatic crystallization products. (2) The most common REE minerals are monazite replacing primary apatite, and britholite texturally related to the serpentinization of forsterite or the replacement of forsterite by chondrodite. Textural relationships suggest that these two REE-minerals precipitated from internally derived late-magmatic to hydrothermal fluids. Their presence seems to be locally controlled by favorable chemical conditions (e.g., presence of precursor minerals that contributed the necessary anions and/or cations for their formation). (3) Late-stage (post-magmatic) REE minerals include ancylite and cordylite replacing primary magmatic REE-Sr-carbonates, anzaite associated with the dissolution of ilmenite, and secondary REE-F-carbonates. The formation of these post-magmatic REE minerals depends on the local availability of a fluid, whose composition is at least partly controlled by the dissolution of primary minerals (e.g., REE-fluorocarbonates). This multi-stage REE mineralization reflects the interplay of magmatic differentiation, destabilization of early magmatic minerals during subsequent evolutionary stages of the carbonatitic system, and late-stage fluid-induced remobilization and re-/precipitation of precursor REE minerals. Based on our findings, the Palabora Carbonatite Complex experienced at least two successive stages of intense fluid–rock interaction.

YELLOWISH GREEN DIOPSIDE AND TREMOLITE FROM MERELANI, TANZANIA

GEMS & GEMOLOGY SUMMER 2007 t the 2006 Tucson gem shows, Steve Ulatowski showed one of the authors (BML) some yellowish green crystals that he purchased as diopside while on buying trips to Tanzania in August and November 2005. The material was reportedly produced during this time period from Block D at Merelani, in the same area that yielded some large tsavorite gem rough (see Laurs, 2006). Mr. Ulatowski obtained 1,200 grams of the green crystals, mostly as broken pieces ranging from 0.1 to 50 g (typically 1–5 g). More recently, in May 2007, he obtained some additional pieces of gem-quality material weighing 0.1–2 g. The “mint” green color is quite attractive, but most of the rough is not cuttable due to the presence of cleavage planes and, in some cases, the flat morphology of the crystal fragments. In 2006, Mr. Ulatowski was informed by a few of his customers that the flatter crystals might be tremolite, rather than diopside. This was consistent with the diamond-shaped cross-section of these crystals (typical of an amphibole), which was distinct from the blocky cross-section (typical of diopside, which is a pyroxene) shown by other crystals in the parcels. Mr. Ulatowski loaned one example of both types of crystals to GIA for examination (figure 1), and we also

The Mafic–Ultramafic Dykes in the Yanbian Terrane (Sichuan Province, SW China): Record of Magma Differentiation and Emplacement in the Emeishan Large Igneous Province

Numerous olivine-rich dykes intruded the late Proterozoic Yanbian terrane, Sichuan, SW China. The dykes are less than 10 m thick and show thin aphanitic chilled margins (c. 10–20 cm wide), whereas most of the dyke volume comprises coarse porphyric rocks made up of olivine grains up to 2 cm in size set in a groundmass consisting of clinopyroxene, plagioclase, Fe–Ti oxides, a second generation of olivine, hornblende and biotite. Chrome-spinel occurs mostly as inclusions in the olivine phenocrysts, but also in the groundmass. The geochemical investigation of 25 dykes suggested a meimechite-type composition of the coarse-grained rocks and a basaltic composition of the chill margins. The sum of REE contents is 46–67 ppm in the coarse rock and 104–137 ppm in the chill margins. Chondrite-normalized REE and primitive mantle-normalized values show identical trends in all dykes, suggesting that they are comagmatic. The initial Sr/Sr and eNd(t) (t1⁄4260 Ma) of the dykes display values in the ranges 0 7041–0 7060 and 0 66–5 25, respectively. Olivine composition varies greatly, with Mg# values of 72 0–93 9. The Cr number of the spinel is 0 62–0 76. Clinopyroxene is Carich (0 76–0 89 Ca atoms per formula unit). Although Ar/Ar investigations did not yield definitive results, the mineralogical and geochemical similarities between the studied dykes and the ultramafic lavas in the Emeishan large igneous province support their emplacement during Emeishan magmatism. Almost all elements unrelated to olivine (Ti, Al, Ca, P, Cu, Zr, Sr, Ba, Y, REE, U, Th, etc.) show strong positive inter-element correlation both in the chill margins and in the coarse-grained rocks, suggesting their concentration in the melt. The chilled margins (Mg#1⁄4 55–63) would have been in equilibrium with moderately Mg-rich olivine (Mg# up to 84). Therefore, the Mg-rich olivine crystallized from a more primitive magma and was transported in the dykes by a more evolved melt with a composition similar to that of the chill margins. Flow differentiation could explain the concentration of the olivine phenocrysts in the inner parts of the dykes while some melt was expelled towards the dyke walls. Based on the composition of the most Mg-rich olivine, the Mg# of the primary magma VC The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com 513 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 3, 513–538 doi: 10.1093/petrology/egx025 Advance Access Publication Date: 1 June 2017

Silicification and organic matter preservation in the Anisian Muschelkalk: Implications for the basin dynamics of the central European Muschelkalk Sea

Anisian Muschelkalk carbonates of the southern Germanic Basin containing silicified ooidal grainstone are interpreted as evidence of changing pH conditions triggered by increased bioproductivity (marine phytoplankton) and terrestrial input of plant debris during maximum flooding. Three distinct stages of calcite ooid replacement by silica were detected. Stage 1 reflects authigenic quartz development during the growth of the ooids, suggesting a change in the pH–temperature regime of the depositional environment. Stages 2 and 3 are found in silica-rich domains. The composition of silica-rich ooids shows significant Al2O3 and SrO but no FeO and MnO, indicating that late diagenetic alteration was minor. Silicified interparticle pore space is characterized by excellent preservation of marine prasinophytes; palynological slides show high abundance of terrestrial phytoclasts. The implications of our findings for basin dynamics reach from paleogeography to cyclostratigraphy and sequence stratigraphy, since change...

Tyrrellite, Cu(Co0.68Ni0.32)2Se4, isostructural with spinel.

Tyrrellite, a naturally occurring Co-Ni-Cu selenide, has been studied by single-crystal X-ray diffraction. It possesses the normal spinel-type structure, with Cu occupying the tetrahedral site and (Co+Ni) the octahedral site. The average Cu-Se distance of 2.3688 (2) A is close to that of 2.3703 (8) A in CuCr(2)Se(4), whereas the average (Co+Ni)-Se distance of 2.3840 (1) A appears to be slightly shorter than most octahedral Co-Se or Ni-Se distances ( approximately 2.40-2.50 A) in other selenides. The refined structure provides a basis for a redefinition of the ideal chemical formula of tyrrellite, which should be Cu(Co,Ni)(2)Se(4), rather than the previously suggested (Cu,Co,Ni)(3)Se(4).

Isokite, CaMg(PO4)F0.8(OH)0.2, isomorphous with titanite

This study presents the first structural report of natural isokite (calcium magnesium phosphate fluoride), with the formula CaMg(PO(4))F(0.8)(OH)(0.2) (i.e. some substitution of OH for F), based on single-crystal X-ray diffraction data. Isokite belongs to the C2/c titanite mineral group, in which Mg is on an inversion centre and the Ca, P and F/OH atoms are on twofold axes. The structure is composed of kinked chains of corner-sharing MgO(4)F(2) octahedra that are crosslinked by isolated PO(4) tetrahedra, forming a three-dimensional polyhedral network. The Ca(2+) cations occupy the interstitial sites coordinated by six O atoms and one F anion.

Exfoliation of a non-van der Waals material from iron ore hematite

ABSTRACTWith the advent of graphene, the most studied of all two-dimensional materials, many inorganic analogues have been synthesized and are being exploited for novel applications. Several approaches have been used to obtain large-grain, high-quality materials. Naturally occurring ores, for example, are the best precursors for obtaining highly ordered and large-grain atomic layers by exfoliation. Here, we demonstrate a new two-dimensional material ‘hematene’ obtained from natural iron ore hematite (α-Fe2O3), which is isolated by means of liquid exfoliation. The two-dimensional morphology of hematene is confirmed by transmission electron microscopy. Magnetic measurements together with density functional theory calculations confirm the ferromagnetic order in hematene while its parent form exhibits antiferromagnetic order. When loaded on titania nanotube arrays, hematene exhibits enhanced visible light photocatalytic activity. Our study indicates that photogenerated electrons can be transferred from hematene to titania despite a band alignment unfavourable for charge transfer.A new non-van der Waals 2D material hematene, exfoliated from natural iron ore hematite, shows ferromagnetic ordering and enhanced photocatalytic activity.

Chemistry, Morphology and Origin of Magmatic-Reaction Chromite Stringers Associated with Anorthosite in the Upper Critical Zone at Winnaarshoek, Eastern Limb of the Bushveld Complex

The occurrence of numerous chromitite layers within the Rustenburg Layered Suite of the Bushveld Complex, South Africa, has been widely cited in models to explain the origin of the igneous layering. Most hypotheses are based around the principle of episodic replenishment of the magma chamber. Chromitite layers occur in both the Lower Critical Zone (LCZ), which is wholly ultramafic, and the Upper Critical Zone (UCZ), where they are part of repetitive units that include pyroxenite (6 minor harzburgite), norite and anorthosite. The UCZ also reveals stringers of disseminated chromite, which, despite being only a few millimetres thick, are laterally very persistent. We investigate chromite stringers from the uppermost part of the UCZ at Winnaarshoek in the Eastern Limb of the intrusion, where they are preferentially located on contacts between layers of pyroxenite and anorthosite. Stringers overlain by anorthosite are of particular interest as they are located in the centre of units and are unlikely to have developed from replenishment by basal flows of magma. The uppermost of the two chromite stringers associated with the Merensky Reef is unusual as it is located wholly within a layer of pyroxenite, and does not demarcate a lithological contact. Stringers are categorized as Type III chromite to distinguish them from the thicker layers of chromitite, Type I (in the LCZ) and Type II (in the UCZ). The Cr-spinel in the stringers is characterized by relatively low Cr/Fe ratios and is associated with recrystallized, unusually calcic, plagioclase as the principal silicate phase. Accessory phases include rutile, corundum, zircon and baddeleyite, in addition to base-metal sulphides and PGM. The origin of Type III stringers is ascribed to replenishment by sheets of picritic magma injected, not as basal flows, but as sills into an earlier-formed crystalline substrate dominated by well-defined layers of norite and anorthosite. In the framework of this hypothesis, the units that characterize the UCZ are, therefore, not differentiation cycles: the ultramafic components crystallized from U-type (picritic) magmas and the norite–anorthosite from A-type (tholeiitic) magmas. The two components of the units were emplaced non-sequentially, but the ultramafic rocks (6 chromitite layers and/or chromite stringers) still occur in stratigraphic sequence relative to each other. Type III stringers developed on either the lower or upper contacts of the picritic magma sheets, dependent on whether they were emplaced above or below a layer of anorthosite. Nucleation of Cr-spinel was triggered by contamination of the picritic magma by partial melting of the anorthosite. Melting of low-temperature oikocrysts of pyroxene and interstitial plagioclase produced a thin boundary layer of melt mush. This boundary layer achieved rapid saturation in Cr-spinel, in part owing to Cr2O3 released from the pyroxene oikocrysts. Heat was insufficient to melt the main framework of plagioclase crystals in the anorthosite or the noritic wall-rocks. The different occurrences of chromite in the Bushveld should not be aggregated into a single VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 1551 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2018, Vol. 59, No. 8, 1551–1578 doi: 10.1093/petrology/egy071 Advance Access Publication Date: 10 July 2018