Discussion
The reaction Opx + Plagioclase = Garnet + Cpx + Quartz is :
Answer: B
It is a solid-solid net-transfer reaction.
In a net-transfer reaction, phases are produced and consumed, so the modal amounts of minerals change and material is transferred from one set of phases to another. Here orthopyroxene and plagioclase are used up to make garnet, clinopyroxene and quartz, all solids, with no fluid involved. Its strong pressure dependence makes it a widely used geobarometer in granulites.
An exchange reaction only swaps elements, such as Fe and Mg, between existing minerals without changing their amounts; such reactions are used as geothermometers. Exsolution is the unmixing of one mineral from another during cooling, as in perthite. 'Ion exchange' is a surface process in clays and resins.
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