The most common and most likely answer to this crossword clue is the 10 letter word OLIGOCLASE
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The straight part
We know that "mineral" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mineral = oligoclase.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does OLIGOCLASE mean?
OLIGOCLASE noun- A plagioclase feldspar, the second member of the Albite-Anorthite solid solution series. Primarily found as small crystals in impure marble. Oligoclase contains a small amount of calcium substituting for some of the sodium in its formula. Oligoclase with reddish-golden inclusions found in Norway and Canada is called sunstone.