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首页> 外文期刊>Mineralogy and Petrology >Coexisting calderite and spessartine garnets in eclogite-facies metacherts of the Western Alps
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Coexisting calderite and spessartine garnets in eclogite-facies metacherts of the Western Alps

机译：西阿尔卑斯山的榴辉岩相微cher石中共存的钙铝榴石和十八碳三烯石榴石

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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 cooccurrence takes a variety of textural aspects, from coexisting euhedral garnets (10-100 urn 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 um in size) overgrowing colourless spessartine and showing evidence of oscillatory zoning, resoiption stages and resumed growth. Sector zoning and anisotropy are common, although not consistent features. Compositions can be expressed to 95 percent in the quadrilateral system (Ca, Mn~(2+))_3 (Al, Fe~(3+))_2 Si_3O_(12), with less than 1.0 wt percent MgO and 0.8 wt percent TiO_2 in colourless spessartine, and less than 0.2 wt percent MgO and 1.6 wt percent TiO_2 in yellow garnet. Calcium partitions into the ferric garnet. Coexisting pairs define two compositional gaps, bounded by values of the Fe~(3+)/(Al + Fe~(3+)) ratio of 10 and 15 percent for the first one, of 40 and 65 percent 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.
机译：在意大利Val d'Aoste的Saint-Marcel附近的Praborna矿山以及法国的Maurienne谷地的中生代蛇纹岩策马特-萨斯装置的榴辉岩相锰浓度中，观察到了无色和黄色石榴石的共存。 。它们既发生在与赤铁矿和褐铁矿（+少量的Mn-吡咯烷酮和钙钛矿，稀有的钛铁钙石;在不同的层中具有钙矾石或钙蒙脱石）的氧化后的亚cher石中，也包括在还原性更强的富含碳酸盐的布丁中。共生涉及多种结构方面的内容，从共存的正反面石榴石（大小为10-100毫米的钙铝石到毫米大小的斯潘塞汀），到黄色的钙铝石榴石在无色的斯潘斯汀上急剧生长，再到黄色的菜花状块状物（少量几百微米大小）无色的司巴沙丁生长过度，并显示出振荡分区，重新分配阶段和恢复生长的迹象。扇区分区和各向异性是常见的，尽管特征不一致。可以在四边形体系（Ca，Mn〜（2 +））_ 3（Al，Fe〜（3 +））_ 2 Si_3O_（12）中将成分表示为95％，其中MgO小于1.0 wt％，TiO_2小于0.8 wt％含量为无色的司贝他汀，黄色石榴石中的MgO和TiO_2含量低于0.2％（重量）。钙分配到铁石榴石中。共存的对定义了两个组成间隙，第一个间隙的Fe〜（3 +）/（Al + Fe〜（3+））的比例为10％和15％，另一个为40％和65％。光学上明显的不连续性（颜色变化和Becke线）对应于无色Spessartine和黄色Spessartine之间的较窄间隙，而宽的组成间隙出现在黄色石榴石内，黄色Spessartine和黄色钙铝石之间，并且仅由反向散射电子揭示图片。只有后者可以作为混溶性候选者。差距，如果有的话。

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