what is olivine ;the mineral?
Olivine is a group of nesosilicate minerals. 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 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. NOTE to TerryO: I have been to Kilbourne's Hole many times, and olivine in the xenoliths are the light green minerals. The dark green minerals are clinopyroxenes and the black minerals are orthopyroxenes.
See the link below, the second like gives a lot of photos. I have found olivine crystals which were very dark green, almost like emerald, but not as dark as chrome diopside. the crystals I have are very small, but have good clarity. I found them on Vandenberg Air Force Base. I also gathered some at the Kilbourne and Potrillo Mar in New Mexico.
