Answer:It always contains certain elements,It always contains elements in definite proportions.,It cannot contain atoms from more than one element.
Step-by-step explanation:
The mineral need not occur always in a pure form.On purification we get the exact chemical composition.
The mineral always has a definite chemical composition which means the number of elements and types of elements in the mineral does not change.
Since the mineral has definite composition,the elements in the mineral has definite proportions.
The mineral can be a compound.So,there can be atoms from more than one element.