Which of the following pairs form a solid solution ? Only one answer is correct; the rest is nonsense.

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Plagioclase (Pl) and Olivine (Ol). Both can occur in basalt; this must be the right answer.

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Rhyolite and Granite; what else? These high Si solutions will be solid after slow and fast cooling, respectively.

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Gabbro (Gb) and Granite (Gr). The classical pluton pair.

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Anorthite (An) and Albite (Ab), the plagioclase endmembers. Look at that ternary diagram and you know!

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Solid solutions? This is crazy! How can a solution be solid ???

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