Cristobalite Formation in Synthetic Glasses through the Role of Mineralizing Agents in Alteration Experiments

Cristobalite, a silica polymorph, is observed outside of its stability field in volcanic domes, and is

hypothesized to be an alteration product that changes the permeability of volcanic rocks.

Impurities or mineralizing agents (Al, Na, and K) influence cristobalite formation by being

incorporated in its crystal structure, particularly at temperatures and pressures lower than its

thermodynamically stable state.

To understand the formation of cristobalite as an alteration product in volcanic domes and how

it alters the hydraulic properties of the rocks, the goals are:

1) To quantify the impurities (mineralizing agents) in the composition of the glasses and the

fluids and correlate it with the presence of cristobalite or other silica phases.

2) Reproduce the chemical processes of devitrification and vapor deposition to study their

relative textural changes in the rock microstructure at various levels of silica and mineralizing

agents at relevant pressure and temperature conditions. Compare experimental products to

altered volcanic rhyolites to confirm if laboratory results are applicable in volcanic environments.

Devitrification of volcanic glass has been seen to result in a feathery groundmass and diktytaxitic

microporous textures (Kushnir et al., 2016). While vapor deposition results in fish-scale cracking

texture of the cristobalite (Damby 2012).

3) Constrain the timescales over which dissolution and precipitation of silica are active in these

systems by performing closed and open system experiments where fluid(vapor)-solid(glass)

interact over time; changes of ionic concentration in the fluid indicate dissolution (if it increases)

or precipitation (if concentrations decrease).