The Stability of Andradite, Hedenbergite, and Related Minerals in the System Ca—Fe—Si—O—H
The Stability of Andradite, Hedenbergite, and Related Minerals in the System Ca—Fe—Si—O—H
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DOI：
10.1093/petrology/15.3.455
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发表时间：
1974
影响因子：
3.9
通讯作者：
W. Gustafson
中科院分区：
文献类型：
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作者：
W. Gustafson
采用固相氧缓冲剂控制氧含量，采用常规水热技术测定了3CaO·2FeOx·3SiO 2+过量H2O和CaO·FeOx·2SiO 2+过量H2O的物相关系。在550 °C以上合成的钙铁榴石Ca 3Fe 3 + 2Si 3 O 12的平均晶胞边缘ao为12.055±0.001 nm，折射率n为1.887±0.003。低于该温度，ao增加而n减少，表明形成了钙铁榴石-水钙铁榴石固溶体的一员。在2000巴下，Pfluidandradite在800 °C下在1015巴以上和400 °C下在10- 32巴以上的anfO 2是稳定的。在较低的温度下，钙铁榴石+流体逐渐让位于磁铁矿+硅灰石、铁橄榄石的凝聚组合（CaFe 2 + SiO 4）+硅灰石和硅镁石+硬硅钙石（Ca 6Si 6 O 17（OH）2）。合成的铁镁矾CaFe 2 + Si 2 O 6具有平均晶胞尺寸fao = 9.857± 0.004 μ m，bo= 9.033±0.002 μ m，折射率n α = 1.731±0.003，n γ = 1.755±0.005。在2000巴的压力下，在800 °C下10- 13巴和400 °C下10- 28巴的压力下，Hedenbertenis稳定。当fO 2值高于此值时，硅灰石+O2分解为钙铁榴石+磁铁矿+石英，钙铁榴石+钙铁榴石的矿物对限制了它们在平衡条件下形成联合体的fO 2范围，硅灰石水化为硬硅钙石的温度比以前的实验结果低得多。该反应的暂定高温极限设定为185°±15 °C和5000±25巴以及210°±15 °C和2000±20巴。由于从硅灰石+H_2O中生长硬硅钙石从未完成，所以这种高温极限并不代表平衡单变曲线。在研究钙铁榴石和钙铁榴石时遇到了9个相。它们是钙铁榴石、钙铁榴石、磁铁矿、硅灰石、铁硅镁石、硬硅钙石、石英、硅铝钙石和蒸气（流体）。使用Schreinemakers方法的不变点分析显示了所涉及的反应的拓扑关系。由此产生的网格可用于解释自然现象。
Phase relations for the bulk compositions 3CaO·2FeOx·3SiO2+excess H2O and CaO·FeOx·2SiO2+excess H2O were determined using conventional hydrothermal techniques with solid phase oxygen buffers to controlfO2.Andradite, Ca3Fe3+2Si3O12, synthesized above 550 °C has an average unit cell edge,ao, of 12.055±0.001 Å, and an index of refraction,n, of 1.887±0.003. Below this temperature,aoincreases whereasndecreases, indicating the formation of a member of the andradite-hydroandradite solid solution. At 2000 bars Pfluidandradite is stable above anfO2of 1015bar at 800 °C and 10-32bar at 400 °C. At lowerfO2andradite+fluid gives way at successively lower temperatures to the condensed assemblages magnetite+wollastonite, kirschsteinite (CaFe2+SiO4)+ wollastonite and kirschsteinite+xonotlite (Ca6Si6O17(OH)2).Synthetic hedenbergite, CaFe2+Si2O6, has average unit cell dimensions ofao= 9.857± 0.004 Å,bo= 9.033±0.002 Å,co= 5.254±0.002 Å and β = 104.82°±0.03°, and refractive indices ofnα = 1.731±0.003 andnγ = 1.755±0.005. At 2000 bars Pfiuid, hedenbergite is stable below anfO2of 10-13bar at 800 °C and 10-28bar at 400 °C. Above thesefO2values, hedenbergite+O2breaks down to andradite+magnetite+quartz.The mineral pair andradite +hedenbergite thus limit thefO2range possible for their joint formation under equilibrium conditions.The hydration of wollastonite to xonotlite occurs at much lower temperatures than previous experimental work indicated. A tentative high temperature limit for this reaction is set at 185°±15 °C and 5000±25 bars and 210°±15 °C and 2000±20 bars. Inasmuch as the growth of xonotlite from wollastonite + H2O was never accomplished, this high temperature limit does not represent an equilibrium univariant curve.Nine phases were encountered in the study of andradite and hedenbergite. They are andradite, hedenbergite, magnetite, wollastonite, kirschsteinite, xonotlite, quartz, ilvaite, and vapor (fluid). An invariant point analysis using the method of Schreinemakers shows the topologic relations of the reactions involved. The resulting grid can be used to interpret natural occurrences.