How do you identify alkali feldspar?
Alkali feldspars can often be distinguished from plagioclase feldspars because most grains of the latter exhibit albite twinning (see above Crystal structure), which is manifested by parallel lines on certain cleavage surfaces, whereas grains of alkali feldspars do not.
What is alkali feldspar minerals?
alkali feldspar, any of several common silicate minerals that often occur as variously coloured, glassy crystals. They are used in the manufacture of glass and ceramics; transparent, highly coloured, or iridescent varieties are sometimes used as gemstones.
What elements are in alkali feldspar?
alkali feldspar A group of silicate minerals that contain the alkali metal elements potassium and sodium.
How do you identify alkali feldspar in a thin section?
In general, keys to identifying K-feldspar are its (lack of) color, its low birefringence, and its twinning. In thin section, microcline, orthoclase and sanidine are distinguished by their twinning, optical sign, and 2Vo.
Does alkali feldspar have cleavage?
Feldspars have two cleavage planes that intersect at 90°. Fragments of pure feldspar crystals thus tend to form rectangular blocks with irregular ends. Feldspars have vitreous lusters and occur in opaque shades of white to gray to pink to very dark gray.
What are the two end members in the alkali feldspar series?
Alkali-Feldspar Series Complete solid solution exists between the potassium (K) and sodium (Na) end members of the series.
What is the luster of feldspar?
Feldspars have a hardness of 6, have a smooth, glassy or pearly luster, and show good cleavages along two planes at nearly right angles to each other. Specific gravity is about 2.6. The streak is white, but the color of the mineral is highly variable.
What is the classification of feldspar?
In the classification of igneous rocks of the International Union of Geological Sciences (IUGS), the feldspars are treated as two groups: the alkali feldspars and the plagioclase feldspars. The alkali feldspars include orthoclase, microcline, sanidine, anorthoclase, and the two-phase intermixtures called perthite.
What is the physical properties of feldspar?
Physical Properties of Feldspar Minerals
| Chemical Classification | Silicate |
|---|---|
| Mohs Hardness | 6 to 6.5 |
| Specific Gravity | 2.5 to 2.8 |
| Diagnostic Properties | Perfect cleavage, with cleavage faces usually intersecting at or close to 90 degrees. Consistent hardness, specific gravity and pearly luster on cleavage faces. |
What color is alkali feldspar?
Alkali feldspar (also called potassium feldspar or K-feldspar) has a color range from white to brick-red, and it’s typically opaque. Many rocks have both feldspars, like granite.
Is alkali feldspar K-feldspar?
Solid solutions between K-feldspar and albite are called alkali feldspar. Solid solutions between albite and anorthite are called plagioclase, or, more properly, plagioclase feldspar.
What are the two types of feldspar?
These may be divided into two types of feldspar: plagioclase and alkali.
- Plagioclase feldspar:
- Orthoclase feldspar:
- Uses:
- Abundance:
- References:
Is alkali feldspar granite?
Alkali feldspar granite, some varieties of which are called ‘red granite’, is a felsic igneous rock and a type of granite rich in the mineral potassium feldspar (K-spar). It is a dense rock with a phaneritic texture….Alkali feldspar granite.
| Composition | |
|---|---|
| Secondary | Plagioclase; dark minerals |
Does alkali feldspar have twinning?
Twinning is very common in alkali feldspars and the type of twinning represents the main tool to distinguish them under the microscope: sanidine and orthoclase show only simple twinning between two crystals (most commonly Carlsbad twinning), which forms during their growth.
What is the hardness of feldspar?
6
Feldspars have a hardness of 6, have a smooth, glassy or pearly luster, and show good cleavages along two planes at nearly right angles to each other. Specific gravity is about 2.6.
Is alkali feldspar granite intrusive or extrusive?
What is the difference between granite and feldspar?
The principal constituent of granite is feldspar. Both plagioclase feldspar and alkali feldspar are usually abundant in it, and their relative abundance has provided the basis for granite classifications. In most granite, the ratio of the dominant to the subdominant feldspar is less than two.
How can you tell the difference between alkali and K-feldspar?
Alkali feldspar (also called potassium feldspar or K-feldspar) has a color range from white to brick-red, and it’s typically opaque. Many rocks have both feldspars, like granite. Cases like that are helpful for learning to tell the feldspars apart.
What is an alkali feldspar?
Alkali Feldspar 1 Basic Mineralogy. The alkali feldspars include monoclinic feldspars (orthoclase and sanidine) and triclinic feldspars (microcline and anorthoclase). 2 Volume 1. 3 Plutonic Geology. 4 Igneous rocks
What is the difference between alkali feldspar and plagioclase?
The alkali feldspars include orthoclase, microcline, sanidine, anorthoclase, and the two-phase intermixtures called perthite. The plagioclase feldspars include members of the albite-anorthite solid-solution series.
What is the difference between albite and feldspar?
Strictly speaking, however, albite is an alkali feldspar as well as a plagioclase feldspar.