Erebus crystal is an anorthoclase mineral, a type of feldspar found in the immediate area surrounding Mount Erebus in Antarctica near McMurdo Station. . This particular feldspar crystal is rich in sodium, potassium, and aluminum silicate. Similar crystals can also be found on Mount Kenya.

Though the formation and growth of these crystals is not well understood, it is evident that the crystals grow in the magma beneath Mount Erebus and are ejected out of the mountain encased in glassy volcanic bombs. This glass structure quickly weathers away leaving the mountainside covered in crystals.

References

1. The Antarctic Sun: Mount Erebus Throws Fit (Page 2)
2. Origins: Antarctica: Field Notes: Five Senses on Erebus