* Pyroxene, olivine, amphibole, and plagioclase feldspar are all common minerals found in mafic rocks.

* Mafic rocks are generally dark-colored and rich in magnesium and iron.

Let's break down the minerals:

* Pyroxene and olivine are the most common minerals in mafic rocks, specifically basalt and gabbro.

* Amphibole is less common in mafic rocks but can be present in some types.

* Plagioclase feldspar is present in mafic rocks, but it will be a calcium-rich variety (like labradorite) rather than the sodium-rich varieties found in felsic rocks.

Other possibilities

While mafic rocks are the most likely source, it's technically possible that the sand could come from a slightly different type of rock:

* Ultramafic rocks like peridotite are even richer in pyroxene and olivine, but they rarely form beaches.

* Metamorphic rocks derived from mafic igneous rocks could also contain these minerals, but the sand might show signs of alteration.

In conclusion, based on the mineral composition, a mafic igneous rock is the most likely source of the sand.