White micas with mixed interlayer occupancy a possible cause of pitfalls in applying illite Kübler index (“crystallinity”) for the determination of metamorphic grade
White micas with mixed interlayer occupancy a possible cause of pitfalls in applying illite Kübler index (“crystallinity”) for the determination of metamorphic grade
复制标题
DOI：
10.1127/0935-1221/2004/0016-0469
复制
发表时间：
2004-05
影响因子：
2.1
通讯作者：
P. Árkai;K. Livi;M. Frey;A. Brukner-Wein;C. Sajgó
中科院分区：
文献类型：
--
作者：
P. Árkai;K. Livi;M. Frey;A. Brukner-Wein;C. Sajgó
综合显微结构观察，X射线粉末衍射（XRPD）模态组成，伊利石Kubler指数和结晶度的测定和镜质组反射率的测量进行了泥灰岩板岩从选定的配置文件的中央阿尔卑斯山，瑞士的Helvetic区。研究的概况是：上侏罗统来自布里恩茨的维尔德霍恩推覆体，上侏罗统来自格拉鲁斯阿尔卑斯山的阿尔赫西夫准原地，始新世来自格拉鲁斯阿尔卑斯山的格里斯斯托克推覆体，上侏罗统来自莱茵河河谷的阿克森推覆体。在一些地方的研究，伊利石Kubler指数（“结晶度”）值非常高，只产生成岩条件，而伊利石“结晶度”和镜质组反射率显示anchi-and epizonal变质条件。对Ca饱和和乙醇酸化的样品进行的详细XRPD观察表明，白色云母中存在少量的溶胀（蒙脱石）夹层。除了主要的K-白云母之外，可以从XRPD（00，10）基底反射中检测到以规则夹层或具有混合（K = Na> NH 4）夹层组成的云母形式存在的离散钠云母和钠云母相以及吐贝利石杂质的痕量。根据镜质体反射率推测的有机成熟度、有机相和无机相之间N和H的可能分配以及对<2 μm的脱碳酸盐和氧化样品进行的元素（C、H、N、S）分析结果，可以估算无机相中固定的NH 4+的量。FTIR光谱中1400 - 1440 cm−1之间的吸收带虽然小，但却系统地出现，这明确地证明了所研究的岩石中存在铵。通过电子能量损失谱（EELS）检测到云母薄片内的少量N，证实NH 4+确实固定在层间位点位置。使用扫描透射电子显微镜（STEM）的能量色散谱（EDS）揭示了层间阳离子占位的不均匀性。虽然K总是占主导地位，但可以看到Na的不规则，局部，域状富集。描述所研究的早期变质白色云母的不平衡状态的最准确模型是具有不规则变化的夹层占位率和次要数量的溶胀混合层的二八面体云母结构。目前的工作表明，如果这些白色云母是用于变质成岩的目的，应特别注意其详细的表征，特别是在有机质丰富的岩性往往具有高Al/Si的散装化学比。表面上看，具有混合层间结构的白色云母可能比迄今普遍预期的更普遍。
Integrated microstructural observations, X-ray powder diffractometric (XRPD) modal composition, illite Kubler index and chlorite “crystallinity” determinations and vitrinite reflectance measurements were carried out on marly slates from selected profiles of the Helvetic zone of the Central Alps, Switzerland. The studied profiles were: Upper Jurassic from the Wildhorn nappe, Brienz, Upper Jurassic from the Parautochthonous of the Aar massif, Glarus Alps, Eocene from the Griesstock nappe of the Glarus Alps and Upper Jurassic from the Axen nappe, Rhine Valley. In some of the localities studied, illite Kubler index (“crystallinity”) values were anomalously high, yielding only diagenetic conditions, while chlorite “crystallinity” and vitrinite reflectance showed anchi- and epizonal metamorphic conditions. Detailed XRPD observations carried out on Ca-saturated and glycolated mounts indicated subordinate amounts of swelling (smectitic) interstratifications in white mica. In addition to the dominant K-white mica, traces of discrete paragonite and paragonitic phases and tobelitic impurities in the form of either regular interstratifications or micas with mixed (K≫Na>NH4) interlayer compositions could be detected from the XRPD (00,10) basal reflections. On the basis of the organic maturity assumed from vitrinite reflectance, probable partitioning of N and H between organic and inorganic phases, and the results of elemental (C, H, N, S) analyses carried out on the <2 μm fraction decarbonated and oxidized samples, the amount of NH4+ fixed in inorganic phases could be estimated. Small, but systematically appearing, absorption bands between 1400 and 1440 cm−1 in the FTIR spectra unequivocally proved the presence of ammonium in the rocks studied. Small amounts of N within the mica flakes were detected by electron energy loss spectrometry (EELS), confirming that NH4+ is indeed fixed in the interlayer site position. Energy dispersive spectroscopy (EDS) using scanning transmission electron microscopy (STEM) revealed heterogeneities in the interlayer cation occupancies. Although K is always dominant, irregular, local, domain-like enrichments in Na could be seen. The most accurate model to describe the disequilibrium state of the incipient metamorphic white micas studied is that of a dioctahedral mica structure with irregularly varying interlayer occupancies combined with subordinate amounts of swelling mixed-layers. The present work shows that, if these white micas are to be used for metamorphic petrogenetic purposes, special attention should be paid to their detailed characterization, especially in organic matter rich lithologies often characterized by high Al/Si bulk chemical ratios. Seemingly, white micas with mixed interlayer occupancies may be more widespread than has generally been anticipated so far.