4.6: The Silicate Minerals - Nine of "The Big Ten"

Olivine

Chemically, olivine is mostly silica, iron, and magnesium and typically green in color. Olivine is the primary mineral component in mantle rock (called peridotite) and ocean floor rock (called basalt). It is characteristically green when not weathered. The chemical formula is \(\ce{(Fe,Mg)2SiO4}\). The comma between iron (Fe) and magnesium (Mg) indicates these two elements occur in a solid solution. Not to be confused with a liquid solution, a solid solution occurs when two or more elements have similar properties and can freely substitute for each other in the same location in the crystal structure. The crystal structure of olivine is built from independent silica tetrahedra.

Figure \(\PageIndex{1}\): The iron- and magnesium-rich silicate mineral olivine (green). (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

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 polymerized 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: \(\ce{(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6}\).

Figure \(\PageIndex{2}\): The iron- (Fe) and magnesium- (Mg) rich silicate mineral augite is a member of the pyroxene family. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

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 \(\ce{(Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH)2}\).

Figure \(\PageIndex{3}\): The iron- (Fe) and magnesium- (Mg) rich silicate mineral hornblende is a member of the pyroxene family. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

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.

Figure \(\PageIndex{4}\): The sheet silicate mineral biotite. The color indicates that Fe and/or Mg exists in the mineral’s chemistry. Note that one thin sheet of dark colored biotite can be virtually colorless. The ability to separate into thin sheets is described as the mineral’s cleavage. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Figure \(\PageIndex{5}\): The sheet silicate mineral muscovite. This mineral belongs to the same family as biotite (mica minerals) however it does not contain any Fe/Mg therefore it is clear in color. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

The Framework Silicates

Quartz and feldspar are the two most abundant minerals in the continental crust. In fact, feldspar itself is the single most abundant mineral in the Earth’s crust (see pie chart). 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 where albite is most sodium-rich and anorthite is most calcium-rich. Potassium feldspar and sodium-rich plagioclase feldspar are abundant in the rock of the continental crust while calcium-rich plagioclase feldspar is abundant in the rock of oceanic crust. Together with quartz, 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.

Figure \(\PageIndex{6}\): Calcium-rich plagioclase feldspar, a framework silicate mineral. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Figure \(\PageIndex{7}\): Sodium-rich plagioclase feldspar, a framework silicate mineral. Use your trackpad as a zoom. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Figure \(\PageIndex{8}\): Potassium-rich feldspar, a framework silicate mineral. Use your trackpad as a zoom. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

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, rose quartz, and citrine.

Figure \(\PageIndex{9}\): Quartz, a framework silicate mineral composed of almost entirely \(\ce{SiO2}\). This is typically how we find quartz in common rocks, both clear and smoky grey in color. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)