4.5: The Silicate Minerals - Nine of "The Big Ten"
- Page ID
- 33300
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Starting at the top of Bowen's Reaction Series's discontinuous branch, you find the most simple minerals. The first one is olivine which is mostly silica, iron, and magnesium and typically green in color when not weathered. Olivine is the primary mineral component in mantle rock (called peridotite) and ocean floor rock (called basalt). The chemical formula is \(\ce{(Fe,Mg)2SiO4}\). The comma between iron (Fe) and magnesium (Mg) indicates these two elements can substitute for one another. That "spot" can host either of those elements.
The Pyroxene Family
Augite is the most common mineral of the pyroxene family and one of our Big Ten Minerals. Augite is iron and/or magnesium-rich forming a complex structure of elements bonded to single chains of silica tetrahedra. Augite is typically black or dark green in color. The chemical formula for augite is complex, indicating that different elements may substitute in the structure depending on what is available in the cooling magma: \(\ce{(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6}\).
The Amphibole Family
As we move down Bowen’s Reaction Series, the internal crystal structure of each mineral becomes increasingly more complex. Amphibole minerals are built from polymerized double silica chains. The most common amphibole, hornblende, is usually black. The chemical formula is very complex with several solid solution opportunities and generally written as \(\ce{(Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH)2}\).
The Sheet Silicates
Biotite and muscovite are varieties of mica. The silica tetrahedra in the micas are arranged in continuous sheets. Bonding of elements between sheets is relatively weak which allows these minerals to be split easily along the sheets (this refers to the mineral’s characteristic pattern of breaking, or cleavage).
The difference between the two micas is that biotite will contain iron and/or magnesium while muscovite mica, the location of the iron/magnesium is replaced by potassium. Therefore, biotite is dark while muscovite is light in color.
The Framework Silicates
Quartz and feldspar are the two most abundant minerals in the continental crust. The main feldspar minerals are potassium feldspar, (a.k.a. K-feldspar or K-spar) and the continuum of sodium- to calcium-rich plagioclase feldspars found on the right side of Bowen's Reaction Series. These minerals are classified as framework silicates because they are built with a three-dimensional framework of silica tetrahedra. Within the framework of the feldspar minerals are holes and spaces into which potassium, sodium, and calcium can fit giving rise to a variety of compositions.
Quartz is composed of pure silica, \(\ce{SiO2}\) with the tetrahedra arranged in a three dimensional framework. Quartz is the final mineral on Bowen’s Reaction Series. It is the last mineral to crystallize from a silica rich magma. In quartz, the silica tetrahedra are bonded in a “perfect” three-dimensional framework. Pure quartz is composed entirely of \(\ce{SiO2}\) however, impurities consisting of atoms within this framework give rise to many varieties of quartz among which are gemstones like amethyst (purple), rose quartz (pink), and citrine (yellow).
- augite - a common single-chain silicate mineral rich in iron and magnesium though other elements often substitute in making its formula complex; the most common mineral of the pyroxene family
- biotite - a sheet silicate rich in iron and magnesium making it light in color
- hornblende - a common double-chain silicate mineral; it has a complex formula due to its many possible substitututions
- feldspar - a framework silicate mineral with a wide variety of compositions as potassium, sodium, and calcium fit into the structure
- muscovite - a sheet silicate rich in potassium making it light in color
- olivine - a common silicate mineral rich in iron and magnesium making up much of the ocean floor and the mantle
- quartz - a framework silicate mineral composed entirely of silicon and oxygen though small amounts of other elements can cause color changes