Here's why:
* Peridotite: This is an ultramafic rock, meaning it's very rich in magnesium and iron. Olivine is the dominant mineral in peridotite, often making up over 50% of the rock.
* Gabbro: This is a mafic rock, meaning it's rich in magnesium and iron, but not as much as peridotite. Gabbro contains olivine, but it's usually in smaller amounts compared to other minerals like pyroxene and plagioclase feldspar.
Therefore, olivine is the mineral that distinguishes peridotite from gabbro in terms of its abundance.
