CoachA magma or rock is silica-oversaturated if it has more silica than its other oxides can combine with in silicate minerals, so that quartz (or another silica mineral) appears; silica-undersaturated if it has too little to turn all its alkalis into feldspar, so that a feldspathoid (nepheline, leucite) appears instead, never together with quartz; and silica-saturated between the two.
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Earth and Space Sciences · 术语表