The microtexture of analcime phenocrysts in igneous rocks

Analcime which has formed by ion-exchange processes from leucite has a characteristic microporous texture when observed by scanning electron microscopy (SEM) and is composed of an aggregate of sub-micrometre crystals, whereas analcime crystallised from a fluid phase in hydrothermal veins has a well crystalline structure. SEM observations of analcime phenocrysts on broken surfaces of volcanic igneous rocks from Sardinia show a microtexture very similar to that in analcime known to be ion-exchanged. Analcime phenocrysts from a lamproite from Jumilla, Spain also show a similar porous microtexture as well as direct evidence of leucite to analcime transformation. This suggests strongly that the analcime phenocrysts in the Sardinian igneous rocks are secondary