The Dehydroxylation of Chlorite and the Formation of Topotactic Product Phases

Abstract

Abstract--Single-crystal, X-ray examination of Mg,Fe-rich chlorites that were heated at 650"C for 24 hours in air and have undergone dehydroxylation of the interlayer shows that two product phases result with a topotactic relationship, with the c axis of both phases parallel One phase ("modified chlorite" or "14-A phase") has relatively sharp reflections with a 14-A c-axis repeat, indicating that it is well crystallized and maintains the 2:1 layer from the parent. Cell parameters are a = 5.368(1)A, b = 9.297(2)A, c = 14.215(6)A, a = 89.86(3) ~ 3 = 97.15(3) ~ 3' = 89.98(2)*, and it crystallizes in Ci symmetry. A structure refinement, details of which will be reported later, indicates that the interlayer consists of two planes, each containing (M + O), where M is the interlayer cation species. These planes show about +0.SA positional disorder along the [001] direction. There is no evidence for scattering material at z = 0.5 between the 2:1 layers. The second phase in the topotactic relationship is based on a 27-A unit c axis ("27-A phase"). The diffraction data are limited with about 15 diffuse reflections observed, indicating that it is poorly crystallized. The 27-A spacing suggests that both octahedral sheets in the parent chlorite contribute to the formation of this phase.Heating Mg,Fe-rich chlorite powder in a closed system to 550"C, under either reducing or oxidizing conditions, prevents the formation of the 27-A phase. Because the 27-A phase forms in an open system, we infer that water fugacity is an important factor in its formation. Heating experiments involving samples with different polytypes and octahedral "type" (dioctahedral vs trioctahedral) of the 2:1 layer suggests that these two variables are important. However, the results are equivocal, and an ill-defined B-rich chlorite from Madagascar breaks the observed trends for both variations in stacking sequence and octahedral type. However, B-content may be a factor in the transformation also for those chlorites that contain B.

Dehydroxylation Behavior of Heat-Treated and Steam-Treated Homoionic cis-Vacant Montmorillonites

“…No evidence was found in IR spectra for the development of a porous network in the clay as proposed by Villieras et al (1994) for dehydroxylated chlorites. This supports the single-crystal study of Zhan and Guggenheim (1995). We believe that the observed decrease of the intensity of the IR band at 1635 cm -1 with preheating and the change of its intensity with relative humidity are consistent with the work of Mosser et al (1997) and Trillo et al (1992).…”

Influence of hydrothermal alteration on the elastic behaviour and failure of heat-treated andesite from Guadeloupe

“…Since chlorite is commonly found as replacement of biotite in andesistic minerals (Rimsaite 1975), its presence in the intact sample is possible. The collapse of all the characteristic peaks associated to chlorite after heat-treatment is consistent with the fact that dehydroxylation of chlorite occurs at temperatures lower than 930 • C (Zhan & Guggenheim 1995). Therefore, even if chlorite is present in the intact sample, it has 2044 A. Nicolas et al BioƟte?…”

Electrical Conductivity of Clinochlore Dehydration and Implications for High‐Conductivity Anomalies and the Melting in the Mantle Wedge

“…The oxygen in the expelled water comes entirely from other regions of the crystal that are wholly or partly converted into pores, and migration of numerous cations occurs (Taylor, 1962; Wang et al., 2017). The dehydroxylation of clinochlore and the formation of topotactic product phases has been proven using X‐ray diffraction (Zhan & Guggenheim, 1995), indicating the dehydration model of clinochlore follows the inhomogeneous model involving H + +OH − →H 2 O.…”