In air, the mechanisms of amorphous calcium carbonate (ACC) transformation into crystalline polymorphs of CaCO3 and whether the atomic ordering is attributable to a solid-state transformation or a dissolution and reprecipitation process are still under debate. While some studies observed a significant influence of relative humidity on ACC transformation, other studies suggested a dehydration process of ACC prior to crystallization. In the present study, we focus on the metastability of additive-free ACC in air and in particular on its interaction with relative humidity. Our findings indicate that the transformation of ACC into crystalline CaCO3 is triggered only after the physisorption of a critical H2O level. Consequently, ACC metastability was prolonged by retarding H2O uptake and by keeping the physisorbed H2O below the critical level, ACC remained in its metastable state. Therefore, the conceptual formation of a “thin film” of about four monolayers of physisorbed H2O is considered to govern the transformation of ∼90 nm sized ACC particles via partial dissolution and reprecipitation. Furthermore, we observed simultaneous formation of calcite, vaterite, and aragonite from ACC, where distinct proportions correspond to different H2O exposure conditions. Thus, polymorph formation from ACC depends also on physicochemical boundary conditions during transformation rather than on prestructural formation within ACC alone.

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Troctolitic gabbros from Valle Fe´rtil and La Huerta Ranges, San Juan Province, NW-Argentina exhibit multi-layer corona textures between cumulus olivine and plagioclase. The corona mineral sequence, which varies in the total thickness from 0.5 to 1 mm, comprises either an anhydrous corona type I with olivine|orthopyroxene|clinopyroxene+spinel symplectite|plagioclase or a hydrous corona type II with olivine|orthopyroxene|amphibole|amphibole+spinel symplectite|plagioclase. The anhydrous corona type I formed by metamorphic replacement of primary olivine and plagioclase, in the absence of any fluid ⁄ melt phase at <840°C. Diffusion controlled metamorphic solid-state replacement is mainly governed by the chemical potential gradients at the interface of reactant olivine and plagioclase and orthopyroxene and plagioclase. Thus, the thermodynamic incompatibility of the reactant minerals at the gabbro-granulite transition and the phase equilibria of the coronitic assemblage during subsequent cooling were modelled using quantitative lMgO-lCaO phase diagrams. Mineral reaction textures of the anhydrous corona type I indicate an inward migration of orthopyroxene on the expense of olivine, while clinopyroxene+spinel symplectite grows outward to replace plagioclase. Mineral textures of the hydrous corona type II indicate the presence of an interstitial liquid trapped between cumulus olivine and plagioclase that reacts with olivine to produce a rim of peritectic orthopyroxene around olivine. Two amphibole types are distinguished: an inclusion free, brownish amphibole I is enriched in trace elements and REEs relative to green amphibole II. Amphibole I evolves from an intercumulus liquid between peritectic orthopyroxene and plagioclase. Discrete layers of green amphibole II occur as inclusion-free rims and amphibole II+spinel symplectites. Mineral textures and geochemical patterns indicate a metamorphic origin for amphibole II, where orthopyroxene was replaced to form an inner inclusion-free amphibole II layer, while clinopyroxene and plagioclase were replaced to form an outer amphibole+spinel symplectite layer, at <770°C. Calculation of the possible net reactions by considering NCKFMASH components indicates that the layer bulk composition cannot be modelled as a ÔclosedÕ system although in all cases the gain and loss of elements within the multi-layer coronas (except H 2 O, Na 2 O) is very small and the main uncertainties may arise from slight chemical zoning of the respective minerals. Local oxidizing conditions led to the formation of orthopyroxene+magnetite symplectite enveloping and ⁄ or replacing olivine. The sequence of corona reaction textures indicates a counter clockwise P-T path at the gabbro-granulite transition at 5-6.5 kbar and temperatures below 900°C.

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We document various textural occurrences of greenockite associated with several ore minerals in varying parageneses: pyrite + sphalerite + galena + chalcopyrite ± arsenopyrite ± pyrrhotite ± bornite ± tennantite ± tetrahedrite ± covellite ± chalcocite ± aikinite ± wittichenite ± matildite ± berryite ± miharaite ± geocronite ± hematite ± magnetite from mineralized samples of the Caledonia Group mines, Marayes mining district, La Huerta Range, San Juan province, Argentina, based on reflected light and scanning electron microscope investigations. The mineralization is hosted mainly in metacarbonate outcrops in contact with the granulite basement and associated intermediate to mafic meta-intrusive rocks. We recognize three types of greenockite on the basis of textural and chemical criteria. Type-1 greenockite contains less than 1 mol.% ZnS, but its Fe content varies from 0.2 up to 4.5 mol.% FeS. Type 2 is Zn-rich, with 10-25 mol.% ZnS component and <0.1 to 3 mol.% FeS component, and type 3 has a Zn content of 5-7 mol.% ZnS component and FeS <0.1 mol.%. The presence of complex Ag-Cu-Bi-Pb sulfides point to a latestage, low-temperature overprint of the primary sulfide parageneses, accompanied by the formation of hypogene greenockite, at least partly at the expense of sphalerite. The near-end-member composition of greenockite and late-stage sphalerite also indicates a low temperature of formation.