Iron-bearing K-dioctahedral 1M and 1Md micas are abundant in diverse geological environments and vary in composition from illite to celadonite through Fe-illite, Al-glauconite, and glauconite. The chemistry and structural features of these micas are complex and heterogeneous,
reliable diagnostic criteria are lacking, and the conventional mineralogical nomenclature is ambiguous, which complicate the identification of these mica varieties. The objectives of the present study were to reveal the structural and crystal-chemical variability in Fe-bearing, K-dioctahedral
1M micas and to define composition ranges and identification criteria for the mica varieties in the series. A collection of samples of various compositions was studied using X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy. Analysis of the relationships between
unit-cell parameters and cation composition showed that the series included four groups, namely, Fe-bearing illites, Al-glauconites, glauconites, and celadonites and each group was characterized by a specific combination of unit-cell parameters and variation ranges. The illite group contained
two distinct subgroups; Fe-bearing, Mg-rich illites and Fe-illites; which differ in the range of cation compositions and in FTIR characteristics. The boundary between Fe-illites and Al-glauconites occurs at a unit cell b value of ~9.05 Å and at ratios of octahedral Al to total
trivalent octahedral cations that range between 0.60 and 0.65. The partially overlapping cation composition and cell parameter ranges may complicate the distinction between Al-glauconites and glauconites, which can still be unambiguously differentiated using FTIR data. The dramatically different
XRD and FTIR characteristics confirmed that glauconite and celadonite should be treated as separate mineral species. The distinctive features of celadonite are relatively low csinβ values and reduced |ccosβ/a| values combined with b parameters lower than
glauconites, but similar to Fe-illites. Celadonites also have distinct and sharp FTIR absorption bands at specific positions in the Si-O and OH stretching regions.
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Document Type: Research Article
August 1, 2017
This article was made available online on July 11, 2017 as a Fast Track article with title: "CRYSTAL-CHEMICAL REGULARITIES AND IDENTIFICATION CRITERIA IN Fe-BEARING K-DIOCTAHEDRAL 1 M MICAS FROM X-RAY DIFFRACTION AND INFRARED SPECTROSCOPY DATA.".
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