caa a22 4500 467890099 CHVBK 20180406152746.0 cr unu---uuuuu 170328e20060901xx s 000 0 eng 10.1007/s00710-006-0146-4 doi (NATIONALLICENCE)springer-10.1007/s00710-006-0146-4 Coexisting calderite and spessartine garnets in eclogite-facies metacherts of the Western Alps [Elektronische Daten] [B. Cenki-Tok, C. Chopin] Summary: The coexistence of a colourless and a yellow garnet was observed in eclogite-facies manganese concentrations of the Mesozoic ophiolitic Zermatt-Saas Unit, at the Praborna mine near Saint-Marcel, Val d'Aoste, Italy, and in the upper Maurienne Valley, France. They occur both in oxidised metachert with hematite and braunite (+ minor Mn-pyroxenoid and tirodite, rare tiragalloite; with ardennite or piemontite in distinct layers), and in more reduced, carbonate-rich boudins included in it. The co-occurrence takes a variety of textural aspects, from coexisting euhedral garnets (10-100 µm in size for the calderite to mm-size for spessartine) to sharp overgrowths of yellow calderitic garnet on colourless spessartine, to yellow cauliflower-like masses (a few hundreds of µm in size) overgrowing colourless spessartine and showing evidence of oscillatory zoning, resorption stages and resumed growth. Sector zoning and anisotropy are common, although not consistent features. Compositions can be expressed to 95% in the quadrilateral system (Ca, Mn2+)3 (Al, Fe3+)2 Si3O12, with less than 1.0 wt% MgO and 0.8 wt% TiO2 in colourless spessartine, and less than 0.2 wt% MgO and 1.6 wt% TiO2 in yellow garnet. Calcium partitions into the ferric garnet. Coexisting pairs define two compositional gaps, bounded by values of the Fe3+/(Al + Fe3+) ratio of 10 and 15% for the first one, of 40 and 65% for the other. The optically obvious discontinuity (colour change and Becke's line) corresponds to the narrower gap, between colourless spessartine and yellow spessartine, whereas the broad compositional gap occurs within yellow garnet, between yellow spessartine and yellow calderite, and is only revealed by back-scattered electron images. Only the latter can be a candidate for a miscibility gap, if any. Springer-Verlag, 2006 Cenki-Tok B. Laboratoire de Géologie, UMR 8538 du CNRS, Ecole normale supérieure, Paris, France aut Chopin C. Laboratoire de Géologie, UMR 8538 du CNRS, Ecole normale supérieure, Paris, France aut Mineralogy and Petrology Springer-Verlag 88/1-2(2006-09-01), 47-68 0930-0708 88:1-2<47 2006 88 710 https://doi.org/10.1007/s00710-006-0146-4 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/s00710-006-0146-4 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Cenki-Tok B. Laboratoire de Géologie, UMR 8538 du CNRS, Ecole normale supérieure, Paris, France aut NATIONALLICENCE 700 1- Chopin C. Laboratoire de Géologie, UMR 8538 du CNRS, Ecole normale supérieure, Paris, France aut NATIONALLICENCE 773 0- Mineralogy and Petrology Springer-Verlag 88/1-2(2006-09-01), 47-68 0930-0708 88:1-2<47 2006 88 710 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer