Extreme chromite alteration in Antanimbary chromitites from the Maevatanana belt, Tsaratanana sheet (Northern Madagascar)

The Maevatanana Belt, located in the north-central Madagascar, is the westernmost of three belts (Andriamena, Beforona and Maevatanana) belonging to the Tsaratanana Sheet. This tectonic unit, individuated by Collins (2006), is composed of mafic gneisses, tonalites, chromite-bearing ultramafic rocks and meta-pelites, some of which were metamorphosed to ultra-high temperatures at 2.5 Ga (Goncalves et al., 2004). Chromitites were found close to the village of Antanimbary in the northern part of Maevatanana Belt, but unlike nearby chromitites, hosted within ultramafic bodies of Andriamena belt, they have never been studied. Seven separate chromitite lenses, forming a 3 km long, NE to SW trending, alignment, outcrop close to the Ikopa river bank. They are included within a metamorphic unit, known in literature as orthoamphibolite, composed of sodic plagioclase, horneblenditic amphibole and minor biotite and quartz. All chromitites are massive with more than 70 and up to 90 modal % chromite and have a cumulus texture where the cumulus phase is always chromite with intercumulus silicates. Chromite grains, completely altered in ferritchromite, ranging from 0.5 to 0.1 mm in size, are euhedral, with fractured cores and porous rims, and enclosed in an anhedral silicate gangue. The most common silicate mineral is chlorite, whose composition falls in the fields of sheridanite and clinochlore and shows detectable Cr2O3 contents (1-2 wt%), which are anyway low if compared to kammererite usually associated to ferritchromite. Tremolitic to actinolitic amphibole and orthopyroxene also occur, often in intergrowth. Serpentine and titanite are present as accessory silicate phases. Ilmenite, as tiny inclusions in chromite grains, is the most common non silicate gangue mineral, followed by rutile, monazite, magnetite and rarely pyrrhotite. Chromite crystals (isolated or in aggregates) preserve the original shape even if they are completely altered in ferritchromite. A slight core to rim zonation occurs with broken and porous ferritchromite cores surrounded by a more porous corona showing a stronger alteration. Grains never preserve composition of primary chromite. Generally ferritchromites are very low in Cr2O3, never exceeding 43 Cr2O3 wt%. FeO is high, ranging between 29.23 and 32.81 wt%, calculated Fe2O3 is never below 6.60 wt% and reaches very high values, up to 28 wt%, in more altered ferritchromite grains. MgO is extremely low, systematically below 2.14 wt%. Al2O3 content is strongly variable, with the lowest and highest limits at 3.33 and 21.96 wt%. Ferritchromite composition, plotted in XFe vs XCr and XFe vs XFe3+ \u201cfried egg diagrams\u201d, shows a best fit with layered mafic-ultramafic intrusions (Barnes and Roeder, 2001), confirmed by composition of chromite alteration rims from Bird River Sill (Ohnenstetter et al., 1986). Quite constant high values of XFe, ranging between 0.89 and 0.94, together with variable XCr evidence that alteration of primary chromite is complete and occurred firstly at reducing conditions with a primary substitution of Fe2+ on Mg. Only after almost complete loss of Mg iron is oxidized to Fe3+ and substitutes for Al and Cr

The origin of Madagascar chromitites

Precambrian rocks of Madagascar host numerous chromitite occurrences, ranging from centimeter-thick lenses and seams to orebodies containing millions of tons. Production of chromite concentrates and lumpy, coming from Bemanevika mine that was estimated to have a remaining life of 15 years (Rahaga, 2009), establishes Madagascar as the world 15th chromite producer. The five most important chromitite localities, investigated for this work, are all characterized by outcropping chromitite bodies hosted within mafic/ultramafic intrusions of poorly understood age. They may range from Archean to Cambrian in age although they probably date back to Neoproterozoic to Cambrian. Metamorphism and alteration have variously affected all of the chromitites, but never completely obliterated their primary characteristics. Chromitite host rocks are peridotite, orthopyroxenite or orthoamphibolite, and primary gangue phases are orthopyroxene, olivine, rare plagioclase, ilmenite, rutile, pyrrhotite and pentlandite. Secondary assemblages comprise serpentine, talc, Cr-chlorite, tremolitic to actinolitic amphibole and magnetite. Geologic, textural, mineralogical and mineral chemistry data best fit a layered intrusion origin for North Toamasina, North Belobaka, Antanimbary and Andriamena chromitites, while Befandriana chromitites, even in a general layered intrusion scenario, show some contrasting features more similar to ophiolite chromitites. Differences between the studied chromitites can be ascribed to the position of the chromitite bodies within the stratigraphic sequence of a layered intrusion. The most striking chromitites are those from Antanimbary that show features assimilating them to the Cr-bearing Ti-magnetite layers of the Upper Zone of Bushveld complex. Chromitite alteration mostly affected gangue silicates whose primary assemblage was partially to almost totally obliterated, while chromites underwent at North Belobaka and North Toamasina partial and at Antanimbary complete ferritchromitization

Pyranocoumarin and Triterpene from Millettia richardiana.

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Effect Of Nanoparticle Size on The Photoelectrochemical Property of Anatase Nanocrystals in Photocatalysis

Using light energy to make electrochemical reaction, photocatalysis is among the most promising technology for water treatment. In heterogeneous catalysis, electrochemical reactions occur at the liquid/solid of gas/solid interface. The development of materials with a high surface area (nanomaterials) has then been considered as the best route achieving an efficient system. In this paper, we studied the effect of the crystal size on the photoelectrochemical properties of anatase. Two synthesis routes were used to get nanoparticles with different size and the comparison of their efficiency for the degradation of rhodamine B under ultraviolet (UV) light excitation showed that crystallite size is most important than surface area consideration. Comparing results obtained under UV lamp and under sunlight excitation, we also demonstrated that photocatalysis is more efficient under sunlight radiation