A bishop about to leave RC church? This matter is insoluble (6)

I believe the answer is:

silica

'this matter is insoluble' is the definition.
(silica is insoluble)

I don't understand the remainder of the clue.

(Other definitions for silica that I've seen before include "Hard colourless compound from rocks" , "Compound in quartz" , "Common base of rock" , "Quartz used in the manufacture of glass" , "Main constituent of sandstone" .)

I've seen this clue in The Guardian.

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