New Mexico Geological Society Annual Spring Meeting — Abstracts

Authigenic zeolite mineralization in volcaniclastic sedimentary rocks is widespread where rhyolitic ash was deposited in basins in the western united States. The zeolites are diagenetic alteration products of the reaction between saline, and usually alkaline, fluids with vitric ash or clay minerals. The most common zeolites identified in sedimentary rocks in New Mexico are clinoptilolite, chabazite, erionite, mordenite, and analcime. Clinoptilolite is by far the most abundant zeolite and is present in all known zeolite occurrences in the state. The others are comparatively restricted in distribution.

Authigenic zeolites are very microcrystalline which makes their examination by ordinary light microscopy difficult. The scanning electron microscope is an ideal tool for the study of these minerals; sample preparation is easy and the instrument provides great depth of field. The form, habit, degree of crystallinity, size, and spatial and paragenetic relationships of the constituent minerals can be studied without disrupting the sample.

Scanning electron microscopy reveals that clinoptilolite occurs as subhedral to euhedral, coffin-shaped, monoclinic laths and plates, up to several micrometers in length, either isolated or in clusters. Chabazite crystals are subhedral to euhedral "cubes" or "rhombs" up to a few micrometers on a side. They are typically intergrown, forming clusters or radial stringers a few micrometers long. Erionite and mordenite, often difficult to differentiate based on morphology along, occur as thin fibers or needles tens of micrometers in lenth. Analcime is very distinctive, occurring as cubo-octehedral and trapezohedral crystals commonly 100 micrometers in diameter.