The microtexture of analcime phenocrysts in igneous rocks

Analcite which has been produced from leucite by ion-exchange processes has a characteristic microporous texture when observed by SEM, and is seen to be composed of an aggregate of sub-mu m crystals, whereas analcite crystallized from a fluid phase in hydrothermal veins has a well developed crystalline structure. SEM observations of analcite phenocrysts on broken surfaces of volcanic rocks from Sardinia show a microtexture very similar to that in analcite known to be ion-exchanged. Analcite phenocrysts from a lamproite from Jumilla, Spain, show a similar porous microtexture as well as direct evidence of leucite ->> analcite transformation. This suggests strongly that the analcite phenocrysts in the Sardinian volcanic rocks are secondary.