Original Paper

Contributions to Mineralogy and Petrology

, Volume 157, Issue 2, pp 163-172

First online:

Diffusion-controlled spherulite growth in obsidian inferred from H2O concentration profiles

  • Jim WatkinsAffiliated withDepartment of Earth and Planetary Science, University of California Email author 
  • , Michael MangaAffiliated withDepartment of Earth and Planetary Science, University of California
  • , Christian HuberAffiliated withDepartment of Earth and Planetary Science, University of California
  • , Michael MartinAffiliated withAdvanced Light Source, Lawrence Berkeley National Laboratory

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Abstract

Spherulites are spherical clusters of radiating crystals that occur naturally in rhyolitic obsidian. The growth of spherulites requires diffusion and uptake of crystal forming components from the host rhyolite melt or glass, and rejection of non-crystal forming components from the crystallizing region. Water concentration profiles measured by synchrotron-source Fourier transform spectroscopy reveal that water is expelled into the surrounding matrix during spherulite growth, and that it diffuses outward ahead of the advancing crystalline front. We compare these profiles to models of water diffusion in rhyolite to estimate timescales for spherulite growth. Using a diffusion-controlled growth law, we find that spherulites can grow on the order of days to months at temperatures above the glass transition. The diffusion-controlled growth law also accounts for spherulite size distribution, spherulite growth below the glass transition, and why spherulitic glasses are not completely devitrified.

Keywords

Spherulites Obsidian FTIR Advection–diffusion