How is plagioclase feldspar different from orthoclase feldspar?

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.

What is the difference between plagioclase and orthoclase feldspar?

The key difference between orthoclase and plagioclase is that orthoclase appears in green-yellow colour, whereas plagioclase appears in white colour. Orthoclase and plagioclase are important minerals. Both these forms are members of the feldspar group.

What is the difference between plagioclase and 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.

Are feldspar and orthoclase the same?

Orthoclase, or orthoclase feldspar (endmember formula KAlSi₃O₈), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for “straight fracture,” because its two cleavage planes are at right angles to each other. It is a type of potassium feldspar, also known as K-feldspar.

What is the most effective way to differentiate potassium feldspar from plagioclase feldspar?

What is the most effective way to differentiate potassium feldspar from plagioclase feldspar? Some cleavage planes of plagioclase feldspars have striations, whereas no potassium feldspars have striations.

How can cleavage distinguish between orthoclase and plagioclase?

Unlike plagioclase, orthoclase does not have striations across its cleavage plane. Cleavage faces will appear smooth. Orthoclase also typically has a colour range between white and pink, which is different than that of plagioclase.

Is potassium feldspar the same as orthoclase?

Orthoclase is a polymorph of other minerals that share the same chemistry, but have different crystal structures. If positive identification between these minerals can not be made by field methods, then the specimen may simply be referred to as a potassium feldspar or K-spar.

Is plagioclase feldspar a silicate?

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 is plagioclase feldspar used for?

Plagioclase is used in the manufacture of glass and ceramics; the iridescent varieties peristerite and labradorite are valued as gemstones or ornamental material. The primary importance of plagioclase, however, derives from its role in rock formation.

What color is plagioclase feldspar?

Plagioclase feldspar is identified by its non-metallic, vitreous to pearly luster; white to dark gray color; and two good cleavages at nearly 90o. Plagioclase feldspar has striations on some cleavage surfaces.

What makes up plagioclase feldspar?

The feldspars are divided into 2 broad categories: plagioclase, which contains calcium and sodium; and orthoclase, which contains potassium.

What are the characteristics of plagioclase feldspar?

Plagioclase Feldspar Physical Properties

What type of rock is plagioclase feldspar?

igneous rocks

Members of the plagioclase group are the most common rock-forming minerals. They are important to dominant minerals in most igneous rocks of the Earth’s crust. They are major constituents in a wide range of intrusive and extrusive igneous rocks including granite, diorite, gabbro, rhyolite, andesite, and basalt.

What minerals are plagioclase feldspar?

Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group.

How will you identify mineral plagioclase under the microscope?

In addition to simple twins, plagioclase feldspar often shows repeated or multiple twinning (Figures 57c and 59a) usually on a microscopic scale, which gives crystals a striped appearance (Figure 59c). This is often referred to as lamellar twinning and is a characteristic feature of plagioclase feldspars.

Does plagioclase feldspar react to acid?

For example, calcite (CaCO₃) is soluble in acidic solutions. This reaction shows calcium plagioclase feldspar, but similar reactions could also be written for sodium or potassium feldspars.

Is plagioclase anisotropic or isotropic?

plagioclase crystals. Colourless and nearly isotropic distinctive.

What kind of twinning does plagioclase have?

Plagioclase commonly shows this type of twinning, called the Albite Twin Law, with {010} as the twin plane. Such twinning is one of the most diagnostic features of plagioclase.

What causes twinning in plagioclase?

Plagioclase, quartz, gypsum, and spinel often exhibit contact twinning. Merohedral twinning occurs when the lattices of the contact twins superimpose in three dimensions, such as by relative rotation of one twin from the other.

How can you identify plagioclase feldspar in a thin section?

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.