Serpentine phase compositions

In sediments and the maflc portion of the subducted crust, all of the reactions involving hydrous phases and carbonates involve solid solutions whose compositions depend on the bulk composition, in addition to pressure and temperature. Different bulk compositions cause different phase compositions and thus cause reactions to shift in P-T space, i.e., to start shallower or deeper. In peridotites, amphibole and to some extent chlorite are controlled by continuous and discontinuous reactions however, the other volumetrically important hydrous phases (e.g., brucite, serpentine, talc, and phase A ) in altered harzburgites display a relatively restricted compositional range, at least compared to those present in mafic eclogites. As a result, breakdown reactions of hydrous phases in harzburgites are dominated (in a first approximation) by discontinuous reactions, and take place over a restricted depth range of only a few kilometers. 

Figure 1. Representation of the ideal compositions of some major phyllosilicate phases in the MR - 2R - 3R coordinates. M = muscovite, paragonite P - phlogopite Py = pyrophyllite Kaol = kaolinite S serpentine T = talc Chlor = chlorite, 14 8 or aluminous 7 8 polymorphs Ce = celadonite F = feldspar.

Stalder R. and Uhner P. (2001) Phase relations of a serpentine composition between 5 and 14 GPa significance of chnohumite and phase E as water carriers into the transition zone. Contrib. Mineral. Petrol. 140, 670—679. (also cf. errata Contrib. Mineral. Petrol. 140, 754). 

Dehydration, or more generally, devolatilization of the oceanic crust is a process that combines continuous and discontinuous reactions in a variety of heterogeneous bulk compositions. In addition, within a vertical column—the sedimentary, mafic, and serpentinized peridotite layers— each experience a significant thermal gradient. The result is a continuous, but not constant, production of a fluid or melt, with the rate of mobile phase production generally decreasing with depth. Peaks in the volatile flux result from significant discontinuous reactions. However, despite the continuous fluid flux, trace elements may not necessarily be released continuously. 

Clnysotile belongs to the serpentine group of minerals, v arieties of which are found in most of the important mountain ranges and precambrian shields (8). Only a small part of these serpentine occurrences are in the asbestiform clnysotile variety. Chrysotile fibers are found as veins in serpentines or related minerals in serpentinized ultramafic rocks and in serpentinized dolomitic marbles (9). It has been suggested that the ultrabasic rocks (forsterite, Mg-rich pyroxenes, and ampliiboles) are first attacked in an hydrothermal process and transformed in serpentines in a later hydrothermal event, the serpentines are partially redissolved and crystallized as chrysotile fibers (9). (Heady, the genesis of each chrysotile deposit must have involved specific features related to the composition of the precursor minerals, the stress and defomiations in the host matrix, the water content, the temperature cycles, etc. Nonetheless, it is generally observed that the chemical composition of the fibrous phase is closely related to that of the surrounding rock matrix (9). 

The differential PEM fuel cell reactor is motivated by considering a small element in the serpentine flow channel fuel cell as shown in Figure 3.1 [12]. Mathematical models of fuel cells use differential mass, momentum and energy balances around the differential element as the defining equations for modeling larger and more complex flow fields [12]. In the differential element the only compositional variations are transverse to the membrane. The key element of a differential fuel cell is that the compositions in the gas phases in the flow channels at the anode and cathode are uniform. 

Hydrothermal syntheses at low temperature and pressure by Yoder [1952], Roy and Roy [1955], Nelson and Roy [1954, 1958], and Cillery [1959] have demonstrated that the 7 A serpentine structure can be formed for any composition between chrysotile and amesite. At higher temperatures and pressures, above 400 to 500°C and 10,000 to 15,000 Ib/in., aluminian serpentines between penninite and amesite in composition invert slowly to the 14 A chlorite structure. It is not certain whether the lower-temperature 7 A structures are thermodynamically stable or metastable. Phase equilibrium diagrams for both possibilities have been given by Nelson and Roy [1958], shown here as Figure 26. 

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