Magmatic Anhydrite

Page under construction 3/31/96
The 1982 eruption of El Chichon volcano (Mexico) yielded produced to very important facts: that eruptions of such volcanoes could vent vast quantities of SO2 into the atmosphere (with climatic effects) and that the eruptive products (tephra) could contain anhydrite.

Following that, experiment studies were carried out by Mike Carroll and 'Mac' Rutherford at Brown University (Carroll and Rutherford, 1987) and by Jim Luhr at USGS-Reston (Luhr, 1990), demonstrating the magmatic stability conditions of anhydrite in oxidized, hydrous melts.

In 1988-89, I worked with Bill Melson at the Smithsonian, on anhydrite- bearing pumices from the Colombian volcano Nevado del Ruiz. The original anhydrite that Bill discovered, in eventually determined, was hydrothermally deposited. But in the process, I discovered anhydrite as inclusions within plagioclase, pyroxene and hornblende, which I believe were probably magmatic in origin.

Here are some images of one anhydrite inclusion:

Orthopyroxene megacryst with anhydrite inclusion Which is very difficult to discern, until you have figured out exactly what you are looking for. There IS one in this transmitted light image -- can you pick it out?

Here the anhydrite is labelled ANin this BSE (backscattered electron image) where the higher atomic number CaSO4 stands out (the round pit is the electron microprobe beam damage).

An enlarged photo-micrographwhich shows the anhydrite as it appears in the standard transmitted light view. The anhydrite has such high order colors under polarized light that it is difficult to distinct from plagioclase. I usually narrow down possible areas (especially inclusions) and then use reflected light (the surface of the polished anhydrite has a characteristic roughness) and BSE imaging to locate the anhydrite. .

Later, speaking with someone at a Penrose Conference, I discovered that some of what I had believed to have been hydrothermal anhydrite, was more likely alunite--with its distinctive zoning, something that anhydrite shouldn't have (see Figure 1 A-C of my 1990 JVGR paper). I hope to follow up on this sometime.

I will post some images of anhydrite in Mt. Pinatubo pumices sometime soon...

I would be very interested in hearing from others who have found, or are looking for, anhydrite in a variety of volcanic rocks and tephras. Email me at

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