The most common and most likely answer to this crossword clue is the 8 letter word CHROMITE
I've seen this crossword clue in Financial Times
C
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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 = chromite.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 CHROMITE mean?
CHROMITE noun- A dark brown mineral species with the formula FeCr2O4.
- Any member of the chromite-magnesiochromite series that is a mixed oxide of iron, magnesium and chromium with the formula (Fe,Mg)Cr2O4. It is a commercial source of chromium.