The Petrography, Mineralogy and Origins of Calcium Sulphate within the Cold Bokkeveld CM Carbonaceous Chondrite
The Petrography, Mineralogy and Origins of Calcium Sulphate within the Cold Bokkeveld CM Carbonaceous Chondrite
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冷 Bokkeveld CM 碳质球粒陨石中硫酸钙的岩相学、矿物学和起源
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发表时间：
1993
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通讯作者：
Martin R. Lee
中科院分区：
文献类型：
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作者：
Martin R. Lee
-对Cold Bokkeveld的详细扫描和透射电镜研究表明，在钙化球粒、基质方解石颗粒和富钙、富铝包裹体（CAI）中，硫酸钙分布广泛，除了μm大小的矿脉和不规则的溶解孔外，还堵塞了~ 30 μm宽的基体切割裂缝。大多数硫酸钙晶体具有纤维习性，特别是那些生长在直边裂缝和静脉中的晶体。矿物学上，硫酸钙由半水和硬石膏的精细混合物组成，可能是样品制备过程中原生石膏脱水形成的。
— A detailed scanning and transmission electron microscopy study of Cold Bokkeveld has shown that calcium sulphate is widespread, occluding ∼30 μm wide matrix-cutting fractures in addition to μm-sized veins and irregular dissolution pores within calcitized chondrules, matrix calcite grains and Ca- and Al-rich inclusions (CAI). The majority of calcium sulphate crystals have fibrous habits, especially those which have grown within straight-sided fractures and veins. Mineralogically, the calcium sulphate is composed of a fine-scale mixture of hemihydrate and anhydrite which have probably formed by the dehydration of primary gypsum during sample preparation.
In all contexts, calcium sulphate precipitation was the last identifiable diagenetic event in the pre-terrestrial history of Cold Bokkeveld and followed pervasive aqueous alteration of the meteorite matrix to phyllosilicates. The fibrous fracture- and vein-filling calcium sulphates are morphologically similar to a terrestrial form of gypsum termed “satinspar” and so may have formed in a similar manner by precipitation from supersaturated aqueous solutions during fracture and vein dilation. The aqueous solutions were most probably generated by melting of water ice due to internal heating and/or post-accretional impact heating of the parent body. Although impact-produced fractures and veins may have provided conduits for fluid advection, the dissolution of calcite, alteration of metal sulphides and precipitation of gypsum probably took place when aqueous solutions were more-or-less static. Despite their similarity to late-stage mineralized fractures in CI meteorites, the calcium sulphate-filled fractures in Cold Bokkeveld probably do not have any significance regarding a shared CM-CI parent body.