The SHAND quaternary system for evaluating the supersilicic or subsilicic crystal-chemistry of eclogite minerals, and potential new UHPM pyroxene and garnet end-members |
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Authors: | D C Smith |
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Institution: | (1) Laboratoire de Tectonique, CNRS/UMR7072, Université Paris VI, Paris, France |
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Abstract: | Summary The stoichiometry of pyroxenes {viiiX2+
viY2+
ivZ4+
2 O6} and garnets {xiiX2+
3
viY3+
2
ivZ4+
3 O12} is re-evaluated by a theoretical crystal-chemical approach that takes into account natural phenomena that do not fit with
conventional anhydrous stoichiometric mineralogy: the existence in eclogites of microinclusions of other minerals that may
have been exsolved from previous supersilicic or subsilicic UHPM pyroxene or garnet. Different definitions of supersilicic
and subsilicic are discussed and the one based on the ability to exsolve SiO2 and leave behind a stoichiometric pyroxene or garnet is recommended for general adoption. The SHAND system (S = Si et al.; H = H; A = Al et al.; N = Na et al.; D = divalents)
for projecting multivariate chemical space involving 23 cations and 104 selected natural or potential mineral species on to
two essential diagrams (SAND and SHND) is described in full for the first time. Numerous possible chemical exchanges are considered
and justified with respect to known mineral phenomena such as cation vacancies, octahedral silicon or protonation. Several
new potential end-members are presented, in particular “supersilipyx”, “supersiligar” and “subsiligar”. It is suggested that
small quantities of these end-members can be incorporated into UHPM solid-solutions and lead to various exsolution phenomena
during eclogite exhumation. |
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Keywords: | |
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