Andradite mineral. Andradite is a calcium-iron silicate mineral, belonging to the garnet group (Ca3.Fe2.Si3.O12). It belongs to the cubic crystal system, occurring as well-developed dodecahedral or trapezohedral crystals of yellow-green to brownish-green colour. It is formed in metamorphic as well as in alkaline igneous rocks. This sample, from Franklin, New Jersey, USA, is 18 centimetres across.