Synthesis and Stability Relations of Mg—Al Pumpellyite, Ca4Al5MgSi6O21(OH)7
Synthesis and Stability Relations of Mg—Al Pumpellyite, Ca4Al5MgSi6O21(OH)7
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DOI：
10.1093/petrology/21.3.441
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发表时间：
1980-08
影响因子：
3.9
通讯作者：
P. Schiffman;J. Liou
中科院分区：
文献类型：
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作者：
P. Schiffman;J. Liou
在250-600℃和2-8kbP流体温度范围内，利用高压冷封装置研究了镁铝镁钛矿的水热合成及其稳定性关系。由化学计量组成的部分结晶凝胶混合物在275-410℃、6-9kbP流体、7-90天的条件下合成了镁铝镁钛矿Ca4Al5MgSi6O21(OH)7。纯单矿物合成镁铝镁镁闪石的折射率nβ=1.624(2)，晶胞尺寸α=8.825(8)α，b=5.875(5)α，c=19.10(1)α，β=97.39(7)°。等量块体成分的高温组合由斜长石、水辉石/粗粒铝石、铝分绿泥石/绿泥石、石英和水组成。在400~500℃温度范围内，在8kb的石英存在下合成了水滑石，在400℃和8kbP流体中，水滑石+石英对水滑石+H2O是不稳定的。在8kbP流体中，铝质七氯铝石在500℃以下形成，而铝质14α绿泥石在500~600℃结晶。用合成镁铝钙镁石及其高温组合的亚等量混合物测定了镁铝钙镁石的平衡关系。反应温度为390℃(8kb)，368℃(5kb)，325℃(2kbP流体)。合成镁铝榴石的标准生成熵近似为3141J/−°K。在CaO-−-Al_2O_3-−(SiO_2-−-H_2O)伪三元系中，建立了镁铝榴石、斜长石、透闪石、粗晶石和紫辉石在过量石英和流体存在下的施莱因梅克关系式。结合对4镁铝镁铝榴石+2石英=8钠铝石+铝过氯石+2 H2O反应的勘察实验，确定了不变点[tR]约为5.7kbP流体，温度为375℃。本研究的结果与前人的实验数据[GR]不一致。用P-T取向的相关系解释了低α二氧化碳条件下重结晶低品位变质玄武岩中形成的一些天然产物。镁铝榴石的高温稳定性关系有助于指示玄武岩成分岩石中绿片岩相变质作用的开始。
Hydrothermal synthesis and investigations of stability relations of Mg—Al pumpellyite were conducted using high-pressure cold-seal apparatus over the temperature range 250–600 °C and 2–8 kbPfluid. Mg—Al pumpellyite Ca4Al5MgSi6O21(OH)7was synthesized from partially crystalline gel mixtures of stoichiometric composition at 275–410 °C, 6–9 kbPfluid, and runs of 7–90 days. Pure monomineralic synthetic Mg—Al pumpellyite has refractive index nβ= 1.624 (2) and cell dimensions α = 8.825 (8) Á,b= 5.875 (5) Á,c= 19.10 (1) Á, and β = 97.39 (7)°.The high temperature assemblage of the equivalent bulk composition consists of clinozoisite, hydrogrossular/grossular, aluminous septechlorite/chlorite, quartz, and H2O. Hydrogrossular was synthesized in the presence of quartz at 8 kb from 400–500 °C, and hydrogrossular + quartz are unstable with respect to grossular + H2O at 400 °C and 8 kbPfluid. At 8 kbPfluid, aluminous septechlorite forms at temperatures below 500 °C whereas aluminous 14 Á chlorite crystallizes at 500–600 °C.The equilibrium relations of Mg—Al pumpellyite were determined using subequal mixtures of synthetic Mg—Al pumpellyite and its high temperature assemblage. The reaction 9 Mg—Al pumpellyite = 9 clinozoisite + 6 grossular + 2 chlorite + 4 quartz + 19 H2O occurs at temperatures of 390 °C at 8 kb, 368 °C at 5 kb, and near 325 °C at 2 kbPfluid. The reversal data yield an approximate value of −3141 joules/mole °K for the standard entropy of formation for the synthetic Mg—Al pumpellyite.The Schreinemakers' relations for pumpellyite, prehnite, clinozoisite, tremolite, grossular, and amesite in the presence of excess quartz and fluid were constructed in the pseudo-ternary system CaO−Al2O3−MgO(SiO2−H2O). The results, together with reconnaissance experiments on the reaction 4 Mg—Al pumpellyite + 2 quartz = 8 prehnite + aluminous septechlorite + 2 H2O, locate the invariant point [TR] at approximately 5.7 kbPfluidand 375 °C. The results of the present study are not compatible with previous experimental data on the invariant point [GR].TheP–Toriented phase relations are used to interpret some natural parageneses developed in low-grade metabasaltic rocks recrystallized under conditions of low αco2. The high-temperature stability relations of Mg—Al pumpellyite are useful to denote the onset of greenschist facies metamorphism in rocks of basaltic composition.