A note on the occurrence and formation of magnetite in the carbonatites of Sevvattur, North Arcot district, Tamil Nadu, Southern India

Magnetite and Green Rust: Synthesis, Properties, and Environmental Applications of Mixed-Valent Iron Minerals

Abstract: Mixed-valent iron [Fe(II)-Fe(III)] minerals such as magnetite and green rust have received a significant amount of attention over recent decades, especially in the environmental sciences. These mineral phases are intrinsic and essential parts of biogeochemical cycling of metals and organic carbon and play an important role regarding the mobility, toxicity, and redox transformation of organic and inorganic pollutants. The formation pathways, mineral properties, and applications of magnetite and green rust are currently active areas of research in geochemistry, environmental mineralogy, geomicrobiology, material sciences, environmental engineering, and environmental remediation. These aspects ultimately dictate the reactivity of magnetite and green rust in the environment, which has important consequences for the application of these mineral phases, for example in remediation strategies. In this review we discuss the properties, occurrence, formation by biotic as well as abiotic pathways, characterization t...

Signature of coseismic decarbonation in dolomitic fault rocks of the Naukluft Thrust, Namibia

Abstract: Unequivocal geological signatures of seismic slip are rare, exceptionally so in carbonate-hosted faults where carbonate minerals dissociate at temperatures lower than those required for producing a friction melt. This thermal dissociation leads to significant fault weakening by increased fluid pressure and/or nanoparticle lubrication, preventing further heating of the fault surface. Pseudotachylyte is therefore unlikely to form in carbonate-hosted faults, and other evidence for seismic slip must be identified. We studied the lower Cambrian Naukluft Thrust which crops out in central Namibia. It contains a cataclastic dolomite fault rock, referred to as ‘‘gritty dolomite’’, which we interpret as a signature of coseismic carbonate dissociation and subsequent fluid–rock interactions. The fault was active at ambient temperatures below 2001C. ‘‘Gritty dolomite’’ contains: rounded, low aspect ratio dolomite clasts with a uniform Fe-rich dolomite coating, euhedral to subhedral magnetite, quartz, and K-feldspar in a fine-grained, massive to laminated carbonate matrix of particulate dolomite and crystalline calcite cement. The fault rock texture, combined with evidence of injectites of gritty dolomite into the wallrock, indicates the cataclasite deformed as a fluidized granular flow. At seismic slip velocities, frictional heating caused dissociation of dolomite to CO 2 and Ca-, Fe- and Mg-oxides. This release of CO2 decreased the pH of the pore fluid in the fault, causing dissolution and rounding of dolomite clasts within an inertial grain flow, and precipitation of carbonate coatings and euhedral silicates and oxides during subsequent cooling and CO2 escape. Examples of similar rocks having some, if not all of these characteristics have been described from other carbonate-hosted faults. The geological setting of the Naukluft Thrust is unique in spatial extent and quality of exposure, allowing us to eliminate alternative hypotheses for sources of CO2 to drive fluidization.

Petrogenesis of the Eppawala carbonatites, Sri Lanka: A cathodoluminescence and electron microprobe study

Abstract: Field and petrographic investigations, cathodoluminescence (CL) studies as well as microprobe analyses of major rock-forming minerals were conducted to establish the crystallization processes in the Eppawala carbonatites, Sri Lanka. The well preserved magmatic textures and crystal morphologies combined with the chemistry of apatite, calcite and dolomite indicate two major stages of crystal growth, which were accompanied by dynamic crystallization conditions. Initially, nucleation of apatite, ilmenite and possibly olivine was associated with rapid crystal growth during slow cooling of the carbonatite melt at depth. The heat loss through the roof and crystallization processes induced the development of turbulent convective currents, which in turn prevented further nucleation and growth of crystals and led to the dispersion of these earlier formed crystals within the magma chamber. Then, rapid upward movement of magma along structural weaknesses led to (i) the transport of mineral clusters, (ii) deformation of ilmenite, (iii) fracturing of apatite and (iv) the emplacement of the carbonatite melt as dykes. Here, the conditions were favourable for the simultaneous crystallization of magnetite, calcite and dolomite in a non-turbulent environment. Subsequent subsolidus alteration caused the hydrothermal overprint of the documented mineral assemblages, particularly along grain boundaries. The study demonstrates that detailed textural examinations of carbonatites combined with mineral chemical analyses and CL investigations can reveal the crystallization processes within carbonatite melts.

In situ major and trace element analysis of magnetite from carbonatite-related complexes: Implications for petrogenesis and ore genesis

Abstract: Magnetite (Fe3O4) is one of the most common accessory minerals in magmatic rocks, and it can accommodate a wide variety of major, minor and trace elements that can be measured by laser ablation ICP-MS. In this study, we investigate the chemical compositions of magnetite from four carbonatite complexes (Oka, Mushgai Khudag, Hongcheon and Bayan Obo). The minor elements (Mg, Ti, Al, Mn) in magnetite vary significantly both within and between different complexes. High field strength elements (Zr, Hf, Nb, Ta, U, Th) are generally depleted in magnetite from carbonatite complexes, whereas K, Rb, Cs, Ca and P are commonly below detection limits. V and Zn display significant variations from tens to thousands of ppm. Co, Ni and Ga are present in ppm or tens of ppm, whereas Cu, Sr, Y, Ba and Pb are characterized by sub-ppm levels. Mo and Ge are identified at the ppm level, whereas a consistent concentration of 2–5 ppm is observed for Ge. The determined chemical compositions of magnetite from carbonatite complexes are quite distinguishable compared to those formed in silicate and sulfide melts. This is clearly shown using multielement variation diagrams, and the distinct signatures of carbonatite-related magnetite include strong positive anomalies of Mn and Zn and negative anomalies of Cu, Co and Ga. The discriminant diagrams of Ti vs. Zr + Hf, Ti vs. Nb + Ta and Ni/Cr vs. Ti are applicable for distinguishing magmatic and hydrothermal magnetite in carbonatite-related environments. In addition, the discriminant diagram of Zn/Co vs. Cu/Mo and Cu vs. Zr + Hf can be used to distinguish carbonatite-related magnetite from magnetite that formed in other environments.

The ore minerals and their intergrowths

Abstract: (partial) Introduction to the general section Intergrowths of the ore minerals Genetic systematics of ore deposits Meteorites Magmatic sequence Sedimentary sequence Metamorphic sequence Ore textures - principles of the classification of the ore intergrowths The fabric properties considered from a purely geometric point of view Genetic fabric types The relationship of ore textures to industrial minerals and benefication problems Descriptive section Elements and intermetallic compounds Alloy-like compounds and tellurides Common sulphides and 'sulphosalts' Oxidic ore minerals Gangue mineral and non-opaque oxide ore minerals

Laboratory handbook of petrographic techniques

Abstract: Contents include: Thin-section preparation. Aids in thin-section study. Grain size, modal analysis, and photomicrography. Rotation methods for the polarizing microscope. Mineral separation, Powder methods of x-ray diffraction. Application of x-ray powder data to specific mineral groups. Specific gravity determination, Refractive index determination. X-ray fluorescence spectrometry. Atomic absorption spectrophotometry. Determination of chemical components not attainable by the foregoing methods. Display of data. Thermal analysis techniques. References. Appendix. Index. This handbook stresses the practical aspects of commonly-used petrographic techniques that are generally applicable to igneous and metamorphic rocks, Author is with the University of Malaya, Kuala Lumpur, Malaysia. -- AATA

Trace Elements and Strontium Isotopes in Carbonatites, Fluorites and Limestones from India and Pakistan

Abstract: Carbonatites discovered in south-west Asia have typical trace elements, but some have unusually high ratios of strontium-87 to strontium-86. Related hydrothermal fluorites have quite different strontium isotope ratios and no distinctive trace elements, emphasizing difficulties in identifying ultimate sources of fluorites.