|The Olivine Group
The Olivine Group is a term that is sometimes incorrectly applied to just two minerals that are often lumped together and simply called olivine. The two minerals are fayalite and forsterite and are perhaps best referred to as the Olivine Series. Although olivine is not an official mineral name in itself, it is a term that is used to denote intermediate specimens between fayalite and forsterite. The true Olivine Group is more inclusive and is a group of similarly structured orthorhombic nesosilicates. The structure of the Olivine Group is composed of a layered closest-packed oxygen framework with the silicon ions occupying tetrahedral sites and the metal ions occupying octahedral sites called M1 and M2 The M1 site is a slightly distorted octahedron. The silicon tetrahedral sites or silicate tetrahedrons, SiO4, are not adjacent to each other and therefore independent to each other making them true nesosilicates.
The general formula for this group is X2SiO4. The X ion can be either calcium, magnesium, iron, manganese and/or nickel. In olivine minerals with calcium, the formula is better represented as CaXSiO4. The X ion can be either magnesium, manganese and/or iron. The calcic olivines are referred to as the Monticellite - Glaucochroite Series and differ in the fact that the calcium ions occupy a specific site, the M2 octahedrons, within the structure whereas the other olivines of mixed chemistry have a random non-specific distribution of their X ion.