More definitive in distinguishing plagioclase from orthoclase is the presence of fine parallel lines called “striations” on plagioclase – these striations do not occur with orthoclase.
Is plagioclase an alkali feldspar?
The most common members of the feldspar group are the plagioclase (sodium-calcium) feldspars and the alkali (potassium-sodium) feldspars. … Feldspars crystallize from magma as both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock.
How do we identify potassium feldspar?
POTASSIUM FELDSPAR
- Chemical Composition: KAlSi3O8.
- Hardness: 6.
- Color: Flesh-colored, pink, white, green or gray.
- Specific Gravity: 2.6.
- Cleavage Planes: Two perfect, at right angles.
- Crystal Structure: Prism-shaped crystals.
- Luster: Nonmetallic.
- Other Properties: Common mineral (orthoclase is a common variety).
What is the most common potassium feldspar?
In some igneous rocks and in most metamorphic rocks, microcline is the common potassium feldspar. Microcline can accommodate only a little sodium and, like orthoclase, may occur with albite.
Is albite a feldspar?
Albite, common feldspar mineral, a sodium aluminosilicate (NaAlSi3O8) that occurs most widely in pegmatites and felsic igneous rocks such as granites. It may also be found in low-grade metamorphic rocks and as authigenic albite in certain sedimentary varieties.
Is Anorthoclase a feldspar?
Anorthoclase, any member of a continuous series of feldspar minerals related to sanidine (q.v.).
Who uses feldspar?
Feldspars are used widely in the glass and ceramics industries. Alkali feldspars are more commonly used commercially than plagioclase feldspars. Albite, or soda spar as it is known commercially, is used in ceramics.
Why is K feldspar pink?
K–feldspar albite intergrowths known as pertite and albite intergrowths K–feldspar as antipertite. The hardness is from 6 to 6.5 and the relative density of 2.55–2.63. The color is usually white, and sometimes changes from pale pink to reddish due to admixtures of iron (especially microcline).
How do you know you are albite?
Albite crystallizes with triclinic pinacoidal forms. Its specific gravity is about 2.62 and it has a Mohs hardness of 6–6.5. Albite almost always exhibits crystal twinning often as minute parallel striations on the crystal face.
What rock is anorthite found in?
Anorthite is a rare compositional variety of plagioclase. It occurs in mafic igneous rock. It also occurs in metamorphic rocks of granulite facies, in metamorphosed carbonate rocks, and corundum deposits. Its type localities are Monte Somma and Valle di Fassa, Italy.
What is albite twinning?
The Albite twin law {010} indicates that the twins make a form, the faces are parallel to the mirror plane (010), i.e. perpendicular to the b-axis. Albite twinning is so common in plagioclase, that it’s presence is a diagnostic property for identification of plagioclase when seen with crossed polarizers.
What are the two types of feldspar?
These may be divided into two types of feldspar: plagioclase and alkali.
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Feldspars which are in this grouping include:
- Microcline.
- Sanidine.
- Orthoclase.
What Colour is feldspar?
Feldspars have vitreous lusters and occur in opaque shades of white to gray to pink to very dark gray. Other rarer colors are also possible! The feldspars are divided into two main groups: Potassium feldspar (“K-spar”) and plagioclase (“plag”).
Does K feldspar have twinning?
K-feldspar is similar to quartz, but quartz lacks cleavage, lacks twinning, does not alter, exhibits undulatory extinction, and is uniaxial.
What rock is feldspar found in?
“Feldspar” is the name of a large group of rock-forming silicate minerals that make up over 50% of Earth’s crust. They are found in igneous, metamorphic, and sedimentary rocks in all parts of the world.
Does feldspar have a streak?
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.
Where is feldspar used?
Feldspars play an important role as fluxing agents in ceramics and glass applications, and also are used as functional fillers in the paint, plastic, rubber and adhesive industries.
How is orthoclase feldspar formed?
Most orthoclase forms during the crystallization of a magma into intrusive igneous rocks such as granite, granodiorite, diorite, and syenite. Significant amounts of orthoclase are also found in extrusive igneous rocks such as rhyolite, dacite, and andesite.
What is microcline feldspar?
Microcline is one of the most common feldspar minerals. … The mineral occurs in feldspar-rich rocks, such as granite, syenite, and granodiorite. It is found in granite pegmatites and in metamorphic rocks, such as gneisses and schists.
Where is Bytownite found?
Bytown, Canada, gave its name to bytownite. Bytownite is found in basic plutonic rocks, some metamorphic rocks, and meteorites. Localities include Montana; South Dakota; Oklahoma; Minnesota; Wisconsin; Scotland; England; Sweden; Japan; and South Africa.
What is another name for potassium feldspar?
Synonyms for potassium feldspar include: Potash Feldspar. Alkali Feldspar. K-spar.
When was albite discovered?
Named in 1815 by Johan Gottlieb Gahn and Jöns Jacob Berzelius from Latin “albus”, white, alluding to its usual color.
What is albite chemical formula?
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8.