FMF - Friends of Minerals Forum, discussion and message board

[Peter Megaw]

Carlos M. noted a peculiar geode from Chihuahua, Mexico in an image I posted a while back and rather than simply sending him a better picture of the piece, I thought it might be of interest to others...and that a broader theme of pseudomorphs and epimorphs and other features that trace the evolution of geode fillings over time might be fun to explore. For most of the Chihuahua geode, and related agate localities, the geodes form in vesicles and devitrification spherules in Tertiary rhyolite to andesite composition volcanics rocks. Much of the filling is likely formed through vapor phase deposition and/or crystallization from a silica gel at high temperatures as the volcanic piles cooled and degassed. Many show interesting transitions from one form of silica deposition to another with products ranging from hyalite to chalcedony to crystalline quartz. Very unusual geodes include fluorite crystals and fluorite pseudomorphs after scalenohedral calcite.

First let me say I do not know the precise location for the first pictured geode despite the fact that they have come out in abundance. I've been given several conflicting stories so to date have only labeled them as Sierra Madre Occidental, Chihuahua. They are distinctly different from anything from the classic Las Choyas locality or Las Trancas which produces the nice chalcedony geodes.

This geode is a devitrification cavity roughly 20 cm in diameter and crowded with elongate spikes up to 6 cm long covered with gel-like druplets of hyalite (?) and tiny quartz crystals. Other specimens show that the spikes are hollow with smooth internal walls, making these epimorphs. Cross-sections and overall morphology indicate orthorhombic symmetry suggesting anhydrite or aragonite precursor. Flat terminations (pinacoids?) suggest anhydrite. The same geodes also contain silica epimorphs after scalenohedral calcite and tabular orthorhombic precursors (Baryte or anhydrite?) (photos to come when I get home from China)

[Pierre Joubert]

Absolutely fantastic Peter!