Oddly bromidic sand tainted volcanic glass (8)

I believe the answer is:

obsidian

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'volcanic glass' is the definition.
(obsidian is a kind of volcanic glass)

I don't understand the rest of the clue.

(Other definitions for obsidian that I've seen before include "Dark glass-like rock" , "Dark volcanic glass" , "Dark glassy rock formed from lava" , "A hard black volcanic glass" , "Dark volcanic rock" .)

I've seen this clue in the Sydney Morning Herald.

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