Plagioclase

PlagioclaseFeldsparUSGOV

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Anorthit Miyake,Japan

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Feldspar - Bytownite Sodium calcium aluminum silicate Crystal Bay Minnesota 2689

Labradorite

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01722 Andesine

Plagioclase is a series of tectosilicate minerals within the feldspar group. Rather than referring to a single mineral, plagioclase is a continuous solid solution series in which sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. The series ranges from albite (NaAlSi3O8) to anorthite (CaAl2Si2O8), with varying chemical compositions between these two endmembers.

Composition and Structure

Plagioclase minerals are composed of a framework of silicon and oxygen atoms, with sodium and calcium ions occupying specific sites in the structure. This framework forms a crystal lattice that allows for the smooth substitution between sodium and calcium, giving rise to the series' compositional range. The presence of these elements in varying proportions is what differentiates the members of the plagioclase series from one another.

Physical Properties

Plagioclase minerals are characterized by their white to off-white color, although impurities can cause variations in hue. They have a hardness of 6-6.5 on the Mohs scale of mineral hardness and display two directions of perfect cleavage at nearly right angles to each other. The luster of plagioclase minerals can range from vitreous to dull, depending on the specific composition and presence of impurities.

Occurrence

Plagioclase is a common and abundant mineral in the Earth's crust. It is a major component of igneous rocks, such as granite, diorite, and basalt, and can also be found in some metamorphic rocks and sedimentary rocks. The mineral forms through the cooling and solidification of magma or lava, and its specific composition is influenced by the chemical makeup of the parent rock material.

Economic Importance

Plagioclase minerals are not typically mined for their own sake but are important constituents of rocks that are quarried for various uses. Granite, which often contains significant amounts of plagioclase, is widely used as a dimension stone in construction. Basalt and diorite, also rich in plagioclase, are used in the production of aggregates, road base, and as a source of minerals in the manufacture of some chemicals.

Varieties

The plagioclase series is divided into several members based on their chemical composition. These include:

• Albite (NaAlSi3O8) - The sodium-rich endmember.
• Oligoclase - A sodium-calcium feldspar.
• Andesine - A balanced sodium-calcium feldspar.
• Labradorite - A calcium-sodium feldspar with significant calcium.
• Bytownite - A calcium-rich feldspar.
• Anorthite (CaAl2Si2O8) - The calcium-rich endmember.

Each of these varieties has its own specific properties and occurrences, contributing to the diversity of the plagioclase group.

Scientific and Practical Applications

In the field of geology, the study of plagioclase minerals is crucial for understanding the petrogenesis of igneous rocks. The composition of plagioclase can provide valuable information about the conditions under which the rock formed. Additionally, the presence and proportion of plagioclase in soil can influence its fertility and physical properties.