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Gemstones 250x250

The plagioclase Mineral Series

Albite

The Plagioclase series is a group of related feldspar minerals that essentially have the same formula but vary in their percentage of sodium and calcium. Albite and Anorthite are the end members of the series, with the intermediary minerals Oligoclase, Andesine, Labradorite, and Bytownite.

The acclaimed Dana's System of Mineralogy lists the intermediary members as individual minerals, whereas the IMA does not recognize them as individual mineral species. Thus, some consider Oligoclase and Andesine to be a variety of Albite, and Labradorite and Bytownite to be a variety of Anorthite. For convenience to the reader, this guide will dedicate a page for each intermediary member.

The Plagioclase Feldspars are  important rock-forming minerals and occur in numerous mineral environments. In many instances, it is very hard to tell apart one from another. When a particular Plagioclase Feldspar cannot be identified, it is simply called "Plagioclase" or "Plagioclase Feldspar". The percentages of sodium and calcium for each member listed above are an approximate value; the actual percentage can vary as much as 20 percent.

Chemical Formula

(Na,Ca)Al1-2Si3-2O8

Variations in the amount of sodium and calcium; and aluminum and silicon, form different minerals in this series:
  Amount of sodium and calcium Percentage of Albite (Ab)and Anorthite (An)
Albite (Na100% , Ca 0% ) Al Si3O8 90-100% Ab ; 0-10% An
Oligoclase (Na90% , Ca10% ) Al1-2Si3-2O8 70-90% Ab ; 10-30% An
Andesine (Na70% , Ca30% ) Al1-2Si3-2O8 50-70% Ab ; 30-50% An
Labradorite (Ca70% , Na30% ) Al1-2Si3-2O8 30-50% Ab ; 70-50% An
Bytownite (Ca90% , Na10% ) Al1-2Si3-2O8 10-30% Ab ; 70-90% An
Anorthite (Ca100% , Na0% ) Al2Si2O8 0-10% Ab ; 90-100% An

Actual formula of end members are:

Albite Na Al Si 3 O 8
Anorthite Ca Al 2 Si 2 O 8

In Anorthite, an aluminum molecule takes the place of a silicon molecule. The intermediary members of this series contain an intermediary amount of sodium and calcium, as well as aluminum and silicon.

Color

White, colorless, cream, gray, yellow, orange, pink, green, blue, red, brown, black

Crystal System

Triclinic

Properties

Streak
White
Hardness
6 - 6.5
Transparency
Transparent to translucent
Specific Gravity
2.6 - 2.8
Luster
Vitreous. Pearly on cleavage surfaces.
Cleavage
2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal. The cleavage angle is about 90º.
Fracture
Conchoidal to uneven
Tenacity
Brittle

Crystal Habits

Crystals are rare for the Plagioclase Feldspars, except for Albite and Oligoclase. Crystals are usually flat and bladed, and commonly in compact groupings. Also occurs as tall prismatic and short, stubby, tabular crystals. These crystals are frequently in groupings, but may also occur singly on a matrix. Crystal twins are somewhat common. Other forms include grainy, massive, columnar, rosette, and rounded. Crystals are often striated.

Click here for more detailed information on the crystal structure of the Feldspars.

Additional Information

Composition
Sodium calcium aluminum silicate, with a varying amount of sodium and calcium.
In Group
Silicates; Tectosilicates; Feldspar Group
Striking Features
Crystal habits, cleavage, and hardness.
Rock Type
Igneous, Sedimentary, Metamorphic

Uses

The Plagioclase Feldspars are used in ceramics. Three varieties have gemstone uses: Labradorite, Moonstone, and Sunstone.

Noteworthy Localities

See the individual group members for localities.

Common Mineral Associations

Quartz, Muscovite, Biotite, Potassium Feldspar Group, Hornblende

Distingushing Similar Minerals

Potassium Feldspar Group - Don't exhibit striations on twinned crystal surfaces, whereas the Plagioclase Feldspars may. Otherwise difficult to distinguish.
Spodumene - Has a splintery fracture.
Calcite - Much lower hardness.

plagioclase Photos

Below is an image for each one of the members of the Plagioclase group. Additional images of each member mineral is indexed on the individual member pages.


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