Biopyribole evolution during tremolite synthesis from dolomite and quartz in CO2-H2O fluid

Abstract

the present study deals with the formation of tremolite from dolomite + quartz in a CO 2 -H 2 O fl uid; this has greater relevance to nature. EXPERIMENTAL METHODS Starting materials and sample treatmentNatural white dolomite (northwest Adirondacks, New York) and quartz (locality unknown) were used in this study. Microprobe analysis (procedure described below) of the dolomite indicated that it was unzoned and had a uniform composition of Ca 1.024(7) Mg 0.975(7) Fe 0.001(1) (CO 3 ) 2 , where the number in parenthesis is the uncertainty in the last digit. Dolomite and quartz were ground (but unsized) and mixed in the weight proportions of dolomite to quartz of 65.7 to 34.3 wt%, respectively, ABSTRACT A series of experiments with and without sample retreatment was performed on a starting mixture of dolomite plus quartz at 0.5 GPa and 600 °C in a CO 2 -H 2 O fl uid with a mole fraction of CO 2 of 0.2 for durations up to 582 h. The initial reaction of dolomite and quartz led to rapid formation of talc and calcite instead of tremolite and calcite by the intended reaction: 5 dolomite + 8 quartz + H 2 O = tremolite + 3 calcite + 7 CO 2 . With continued treatment, the talc + calcite + quartz assemblage gradually reacts to form increasing amounts of tremolite and diopside with the eventual loss of quartz and nearly complete loss of talc.The detailed structure of the amphibole and the nature of the Mg enrichment were revealed using AEM analysis of individual amphibole crystals. The most abundant defects are triple-chain chain multiplicity faults (CMFs), which appear as isolated lamellae with single periodicity in short-duration experiments and increasingly as groups of lamellae with variable multiplicity and periodicity in longer duration experiments and especially in a long-duration experiment without retreatment. In this latter experiment, a calcian clinojimthompsonite domain extending 15 unit cells along the b axis was observed. The sample after 582 h and fi ve retreatments shows, on average, 3.5% true solid solution with Mg expressed as the Mg-cummingtonite (MC) component after correction for the presence of CMFs. This sample is thought to most closely approach the equilibrium composition for the amphibole. The sample after 250 h with three retreatments has about 10%, whereas that after 582 h without retreatment has 0.6% MC component, the latter having a relatively high density of CMFs. This study affi rms the importance that precursor non-amphibole biopyriboles play in the formation of tremolitic amphibole.

Micro- and nano-scale study of deformation induced mineral transformations in Mg-phyllosilicate-rich fault gouges from the Galera Fault Zone (Betic Cordillera, SE Spain)

“…Considering that a lattice fringe in a high-resolution image would include at least unit cells in depth, when assuming an average width of 20 nm per lath, the variability in the measured spacings could be the result of different proportions of the two types of chains, representing a weighted average of the spacings of the two polysomes. Nevertheless, we have not found individual lattice fringes corresponding to the palygorskite spacing in a matrix of sepiolite, contrary to the case described for pyriboles by Bozhilov et al (2007). Hence, our results cannot be considered conclusive in relation to the mechanism of transformation between sepiolite and palygorskite, however chemical analysis confirms that the Al content appears often towards the edges of the crystals (Fig.…”

“…Polysomatic reactions are seen in the case of smectite to illite through illite/smectite mixed-layers (Hower et al, 1976), or in the case of the transformation from pyroxenes to amphiboles through pyriboles (Veblen and Buseck, 1981). In both of these cases, high-resolution images display the new polysome as individual unit cells within the former phase showing their respective characteristic spacings, which allow the clear identification of the areas in which the new polysome is present (Bozhilov et al, 2007;Vázquez et al, 2014). In other cases, the new phase nucleates as discrete crystals, sometimes assisted by topotactic or epitactic mechanisms (Sánchez-Navas, 1999), but without the existence of intermediate stages (e.g.…”

Synthesis of Ni-Doped Tremolite Fibers to Help Clarify the Aetiology of the Cytotoxic Outcome of Asbestos

“…Although synthesized tremolite (TN8) shows a slightly lower Ca/Mg value (0.36) of the ideal products of the starting mixtures (0.5), this Mg excess in synthetic tremolite is considered as Mg-cummingtonite Mg 7 Si 8 O 22 (OH) 2 component depending on the thermodynamic growth conditions [36,51]. Indeed, it is worth mentioning that, on the basis of the numerous experimental researches carried out on synthesized tremolite, synthetic tremolite products tend to be cummingtonite-tremolite solid solutions with variable (7-3%) Mg amount [17,37,41,44,45,52].…”

The effect of solid solution on the stability of talc and 10-Å phase

“…The fact that our results are broadly consistent with calculations using Perple_X suggests that our experiments have reached equilibrium. Achieving equilibrium is an issue of concern in synthesis experiments involving talc, which can nucleate outside its stability field (e.g., Bozhilov et al 2007).…”