In order to discuss the silicates and their structures it is first necessary to remember that the way atoms are packed together or coordinated by larger anions, like oxygen depends on the radius ratio of the cation to the anion, Rx/Rz.Since oxygen is the most abundant element in the crust, oxygen will be the major anion that coordinates the other other cations.
Staurolite is a common mineral in medium grade metamorphic rocks, usually metamorphosed shales.In hand specimen and in thin section it characteristically is seen to show staurolite twinning, either the right-angle cross, twinned on or the oblique cross, twinned on It is monoclinic, but its optical properties are those of an orthorhombic mineral.It is very resistant to weathering and may also survives during metamorphism, allowing for dates to be obtained on the original rock prior to metamorphism (often called the protolith).In hand specimen Zircon usually occurs as tiny reddish colored crystals.As we discussed in a previous lecture, the relative abundance of elements in the Earth's crust determines what minerals will form and what minerals will be common.
Because Oxygen and Silicon are the most abundant elements, the silicate minerals are the most common.They are also nesosilicates, and therefore based on the Si O polymorphs provide rather general estimates of the pressure and temperature of metamorphism, with Kyanite indicating relatively high pressure, andalusite indicating low temperature and pressure, and sillimanite indicating high temperature.Better estimates of pressure and temperature are provided if two of the minerals are present in the same rock.In thin section, it shows extremely high relief, with = 1.968 to 2.015. Zircon has high birefringence, with interference colors in the higher orders (lots of reds, pinks and light greens).It is commonly colorless to pale brown or pinkish brown in polarized light without the analyzer.But one substitution in particular tends to mess things up a bit. If such a substitution takes place, it creates a charge imbalance that must be made up elsewhere in the silicate structure.