The plagioclase feldspar mineral series

PLAGIOCLASE FELDSPAR Series
Feldspar Group
plagioclase feldspar - mineral #15AB6

Chemical Formula

(Na,Ca)Al_1-2Si_3-2O₈

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	(Na _100% , Ca_0% ) Al Si ₃ O ₈	90-100% Ab ; 0-10% An
Oligoclase	(Na _90% , Ca _10% ) Al _1-2 Si _3-2 O ₈	70-90% Ab ; 10-30% An
Andesine	(Na _70% , Ca _30% ) Al _1-2 Si _3-2 O ₈	50-70% Ab ; 30-50% An
Labradorite	(Na _30% , Ca _70% ) Al _1-2 Si _3-2 O ₈	30-50% Ab ; 70-50% An
Bytownite	(Na _10% , Ca _90% ) Al _1-2 Si _3-2 O ₈	10-30% Ab ; 70-90% An
Anorthite	(Na _0% , Ca _100% ) Al ₂ Si ₂ O ₈	0-10% Ab ; 90-100% An

Actual formula of end members are:

Albite	Na Al Si₃ O ₈
Anorthite	CaAl ₂ Si₂ O₈

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.

Composition

Sodium calcium aluminum silicate, with a varying amount of sodium and calcium. This forms a series, with Albite, the sodium-rich variety, as one end member, ranging through Oligoclase, Andesine, Labradorite, Bytownite, until the the calcium-rich end member, Anorthite.

Color

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

Streak

White

Hardness

6 - 6½

Crystal Forms
and Aggregates

(Triclinic) Crystals are rare for all Plagioclase feldspars, except 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.

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

Other Names

Soda-lime Feldspar group ,

Lime-soda Feldspar group

Varieties

See individual members of the Plagioclase group for varieties.

In Group

Silicates ; Tectosilicates ; Feldspar group

All About

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.

Uses

The Plagioclase feldspars are used in ceramics. Three varieties have gemstone uses, namely labradorescent Labradorite, and the Moonstone and Sunstone varieties of Oligoclase.

Striking Features

Crystal habits, cleavage, and hardness

Popularity (1-4)

Prevalence (1-3)

Demand (1-3)

Distinguishing
Similar Minerals

Potassium feldspars - don't exhibit striations on twinned crystal surfaces, whereas the Plagioclase feldspars do
Spodumene - has a splintery cleavage
Calcite - much softer (3)

Commonly
Occurs With

Quartz, Muscovite, Biotite, Potassium Feldspar, Hornblende

Noteworthy
Localities

See individual members for localities

Picture Links

See individual members for pictures