The most common and most likely answer to this crossword clue is the 9 letter word MARCASITE
M
A
R
C
A
S
I
T
E
Buddy explains!
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 = marcasite.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 MARCASITE mean?
MARCASITE noun- The orthorhombic form of iron disulfide, FeS2, occurring as yellow crystals.
- (jewellery) Pyrite.
- Any of various metal sulfides, usually iron sulfide minerals.