Quartz and potassium feldspar (Alaskite) (see also Quartz)
Some granites have such a low ferromagnesian mineral content that they appear to consist only of quartz and potassium feldspar. Such granites are called Alaskites. Alaskite is a leucogranite (light-colored) with pinkish K-feldspar and whitish-gray quartz, with almost no dark minerals.
In geology, leucogranites (from greek λευκος / leucos « white ») are granite rocks in which dark minerals (typically micas of the biotite and amphibole type) are poorly represented, quartz and feldspar being therefore very dominant. These granites have muscovite or white mica with sometimes other minerals such as pink garnet or tourmaline in black prisms. Muscovite granites are leucogranites (less than 5% ferro-magnesian minerals) with two micas (biotite and muscovite). They generally form small massifs of a few kilometers at most; they are relatively low temperature puddles, formed by fusion in the presence of water from sedimentary rocks. They are distinguished from the cordierite granites which are richer in ferro-magnesians, and which can easily be granodiorites.
What is the difference between quartz and feldspar ?
A feldspar is a mineral from the family of tectosilicates, the composition of which is that of an aluminosilicate of sodium, potassium or calcium. There are many feldspars, the main ones being orthosis or microcline (potassium), albite (sodium) and anorthite (calcium).
A distinction is made between alkaline feldspar, often translucent, whitish or pink and rich in alkali (Na +, K +), plagioclases which differ from the previous ones by the presence of calcium (Ca2 +) and the absence of potassium (K +). Plagioclases constitute a continuous series of minerals (a solid solution), the pure poles of which are albite and anorthite. The alkaline feldspars also constitute a continuous series (whose poles are albite and orthosis) at high temperature, but with only partial miscibility at low temperature.
Feldspar is the most volumetrically important group of minerals in the earth’s crust.
Quartz
Quartz is a hard, crystalline mineral composed of silicon and oxygen atoms. The atoms are linked in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Quartz is the second most abundant mineral in Earth’s continental crust, behind feldspar.
There are many different varieties of quartz, several of which are semi-precious gemstones.
