How is plagioclase feldspar different from orthoclase feldspar?

Asked By: Valdas Christians | Last Updated: 27th April, 2020
Category: science geology
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Note that the plagioclase differs from the orthoclase in terms of the additional metals, plagioclase having calcium or sodium and orthoclase having potassium. The feldspars have similar physical properties and typically have a luster ranging from glassy to pearly.

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Simply so, what is the main difference between orthoclase feldspar and plagioclase feldspar?

Plagioclase feldspars lack potassium, are light colored and are usually striated. The other k-spar minerals are sanidine, microcline and anorthoclase . Orthoclase is the more common of the k-spars. The differences between these minerals are minor in hand samples.

Subsequently, question is, what is the most common element found in orthoclase feldspar but not in plagioclase feldspar? It is the proportions of these 3 elements which help determine which specific feldspar is formed. The feldspars are divided into 2 broad categories: plagioclase, which contains calcium and sodium; and orthoclase, which contains potassium.

Correspondingly, is plagioclase a feldspar?

Plagioclase is a member of the feldspar group (like orthoclase) and is a framework silicate. Plagioclase consists of a solid solution between the albite and anorthite end-members, and together with quartz is the most common of the rock forming minerals.

What are the two types of feldspar?

The feldspars are divided into two main groups: Potassium feldspar ("K-spar") and plagioclase ("plag").

39 Related Question Answers Found

What are the types of feldspar?

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 type of rock is pink feldspar?

Orthoclase is a feldspar mineral with a chemical composition of KAlSi3O8. It is one of the most abundant rock-forming minerals of the continental crust. Orthoclase is most widely known as the pink feldspar found in many granites and as the mineral assigned a hardness of "6" in the Mohs hardness scale.

How do you identify feldspar?

In fact, feldspar is the standard for hardness 6 in the Mohs scale. Feldspars usually are white or nearly white, though they may be clear or light shades of orange or buff. They usually have a glassy luster. Feldspar is called a rock-forming mineral, very common, and usually makes up a large part of the rock.

What color is Feldspar?

In general, potassium feldspars commonly have pink to reddish hues, while the plagioclase feldspars tend to be white or gray, but both mineral groups may exhibit similar colors, so the presence or absence of striations is more diagnostic.

What is feldspar made of?


The mineralogical composition of most feldspars can be expressed in terms of the ternary system Orthoclase (KAlSi3O8), Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8). Chemically, the feldspars are silicates of aluminium, containing sodium, potassium, iron, calcium, or barium or combinations of these elements.

How is feldspar formed?

Feldspars crystallize from magma in both intrusive and extrusive rocks, and they can also occur as compact minerals, as veins, and are also present in many types of metamorphic rock. Rock formed entirely of plagioclase feldspar is known as anorthosite.

How do you identify a feldspar in a thin section?

Hardness (6-6.5) and cleavage are good indicators of a plagioclase as opposed to other similar silicates. In thin section or grain mount, the general lack of pleochroism and the twinning are good indicators. The presense of parallel striae on cleavage surfaces distinguishes plagioclase from K-feldspar.

What color is quartz?

Pure quartz is white or colorless, but is commonly found in almost any color imaginable. Since quartz is naturally clear, any chemical impurities will allow the color to "shine" through. Amethyst, tiger eye, adventurine, and agates are all varieties of quartz with impurities in them to cause their distinctive color.

Who found feldspar?

Johan Gottschalk Wallerius

Why is olivine green?


Olivine is named for its typically olive-green color, thought to be a result of traces of nickel, though it may alter to a reddish color from the oxidation of iron. The metamorphism of impure dolomite or other sedimentary rocks with high magnesium and low silica content also produces Mg-rich olivine, or forsterite.

Does feldspar react to acid?

Minerals - Reaction to Hydrochloric Acid. Carbonate minerals such as calcite tend to fizz or efferves when tested with Hydrochloric Acid (HCl). Non-carbonate minerals, especially silicates will not react to HCl.

Why is K feldspar pink?

Kfeldspar albite intergrowths known as pertite and albite intergrowths Kfeldspar 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).

Which feldspar has exsolution lamellae?

Perthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar (near K-feldspar, KAlSi3O8, in composition) includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar (near albite, NaAlSi3O8, in composition).

How do you identify hornblende?

Identification of Hornblende
Hornblende minerals as a group are relatively easy to identify. The diagnostic properties are their dark color (usually black) and two directions of excellent cleavage that intersect at 124 and 56 degrees.

What is pyroxene used for?


In Our Society: The Economic Importance of Pyroxene
Spodumene is mined as an important source of lithium, used in ceramics, and is also prized as a gemstone. Jadeite is one of two minerals commonly known as jade (nephrite, an amphibole mineral, is the other jade variety).

What does plagioclase feldspar look like?

Plagioclase Series of the Feldspar Group of Silicate Minerals. Typically white to gray, may also range from colorless, through shades of off-white, to yellowish, pink, reddish brown or nearly black. Common twinning in the crystal structure that creates fine parallel grooves or striations on some cleavage surfaces.

What is black mica?

Biotite is a name used for a large group of black mica minerals that are commonly found in igneous and metamorphic rocks. These micas vary in chemical composition but are all sheet silicate minerals with very similar physical properties.