Clinoeulite (magnesian clinoferrosilite) in a eulysite of a metamorphosed iron formation in the vredefort structure, South Africa
Clinoeulite (magnesian clinoferrosilite) in a eulysite of a metamorphosed iron formation in the vredefort structure, South Africa
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南非 vredefort 构造中变质铁岩层中的斜钙石（镁斜铁硅石）
DOI：
10.1007/bf00372283
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发表时间：
1978
影响因子：
3.5
通讯作者：
W. Müller
中科院分区：
文献类型：
--
作者：
W. Schreyer;D. Stepto;K. Abraham;W. Müller
摘要：一种独特的斜辉石（En19Fs78Wo3），斜锂石，空间群P21/c，
$${\text{(Fe}}_{{\text{1}}{\text{.48}}} {\text{Mg}}_{{\text{0}}{\text{.37}}} {\text{Mn}}_{{\text{0}}{\text{.08}}}^{{\text{2 + }}} {\text{Ca}}_{{\text{0}}{\text{.05}}} {\text{Al}}_{{\text{0}}{\text{.01}}} {\text{)}}_{{\text{1}}{\text{.99}}} {\text{ [Si}}_{{\text{2}}{\text{.01}}} {\text{O6],}}$$
含有锋利的铁辉石 (En17Fs43Wo40) 出溶片层，从中可以计算出 En17Fs70Wo13 附近以前存在的铁红辉石。这种双辉石共生体是一种藻溶岩的主要成分，还含有磁铁矿、橄榄石 (Fo9Fa86Te5)、石英、寡长石-钾长石共生体和逆行闪辉石，其中约 76% 的钙镁橄榄石端元。这种最不寻常的岩石类型在 Vredefort 构造中心的出现归因于一段高温变质作用时期（至少 800 年）。 °–850 °C），随后在 Vredefort 事件期间岩石发生热变形，因此可能通过冷却时的退溶和反转阻止了斜方辉石的常见形成。经过这次构造变形，岩石在斜硅石的低温稳定范围内再结晶，产生细小的退火织构。温度远低于 500 °C 时的后期平衡包括以前高温斜长石的完全分离、斜锂石和橄榄石中 Mg/Fe 的重新分布，以及随着水的引入，通过斜锂石、橄榄石和石英的反应部分形成角闪石。在风化过程中，橄榄石转变为几乎不透明的无水硅酸铁，除了 Fe2+ 变为 Fe3+ 以及氧气的引入外，它在很大程度上保留了其原始化学性质。
AbstractA unique clinopyroxene (En19Fs78Wo3), clinoeulite, space group P21/c,
$${\text{(Fe}}_{{\text{1}}{\text{.48}}} {\text{Mg}}_{{\text{0}}{\text{.37}}} {\text{Mn}}_{{\text{0}}{\text{.08}}}^{{\text{2 + }}} {\text{Ca}}_{{\text{0}}{\text{.05}}} {\text{Al}}_{{\text{0}}{\text{.01}}} {\text{)}}_{{\text{1}}{\text{.99}}} {\text{ [Si}}_{{\text{2}}{\text{.01}}} {\text{O6],}}$$
contains sharp exsolution lamellae of ferroaugite (En17Fs43Wo40) from which the former presence of a ferropigeonite near En17Fs70Wo13 can be calculated. This two-pyroxene intergrowth is the main component of a eulysite containing also magnetite, olivine (Fo9Fa86Te5), quartz, oligoclase-K feldspar inter-growth, and retrograde cummingtonite with about 76 % grunerite end member.The occurrence of this most unusual rock type in the center of the Vredefort structure is attributed to a period of high-temperature metamorphism (at least 800 °–850 °C) which was followed by hot deformation of the rock during the Vredefort event thus probably preventing the common formation of orthopyroxene through pigeonite exsolution and inversion upon cooling. After this tectonic deformation, the rock recrystallized within the low-temperature stability range of clinoeulite to yield fine annealing textures. Late-stage equilibria at temperatures well below 500 °C include the complete unmixing of a former high-temperature anorthoclase, a Mg/Fe redistribution in the clinoeulite and olivine and, with the introduction of water, the partial formation of cummingtonite through reaction of clinoeulite, olivine, and quartz. During weathering the olivine was transformed to a nearly opaque, anhydrous ferrisilicate which, except for the change of Fe2+ to Fe3+ and the oxygen introduction, largely retained its original chemistry.