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The Mineral andesine

Andesine

Andesine was named after its original occurrence in the Andes Mountains of South America. Andesine belongs to the Plagioclase Feldspar group, an isomorphous solid solution series. Albite is one member, containing sodium and no calcium. The other end member, Anorthite, contains calcium and no sodium. Andesine is an intermediary member of this series. Andesine is considered by some authorities as a variety of Albite rather than a separate mineral. The acclaimed Dana's System of Mineralogy lists Andesine as an individual mineral, whereas the IMA does not recognize it as individual mineral species, but rather a calcium-rich variety of Albite. Andesine is the primary feldspar constituent of the igneous rock Andesite.

Chemical Formula

(Na,Ca)Al_1-2Si_3-2O₈

Color

Colorless, white, cream, yellow, pale green, gray

Crystal System

Triclinic

Properties

Streak
White

Hardness
6 - 6.5

Transparency
Transparent to translucent

Specific Gravity
2.66 - 2.68

Luster
Vitreous to pearly

Cleavage
2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal The cleavage angle is about 90º

Fracture
Subconchoidal to uneven

Tenacity
Brittle

Crystal Habits

Crystals, which are uncommon, are tabular and frequently twinned. Most often massive, grainy, as chunky masses, and as compact crystal groupings.

Additional Information

Composition
Sodium calcium aluminum silicate. The ratio of sodium to calcium is about 7:3. The amount of aluminum atoms are between 1 and 2, and the amount of silicon atoms are between 3 and 2.

In the Plagioclase Feldspar series, Andesine contains between 50 and 70 percent Albite (Ab), and between 30 and 50 percent Anorthite (An).

In Group
Silicates; Tectosilicates; Feldspar Group

Striking Features
Crystal habits, cleavage, and hardness

Environment
In basalt rock and in andesites, also in metamorphic rocks.

Rock Type
Igneous, Metamorphic

Uses

There is a new gemstone called Andesine which has recently penetrated the market. Much of the material is Chinese in origin and has been synthetically diffused to enhance color, and is actually produced from Labradorite.

Noteworthy Localities

Andesine is prevalent in volcanic regions throughout the world. Its original locality in the Andes Mountains is the Marmato District, Manizales, Bolivia. Large crystals are found in the San Gabriel Mountains (Los Angeles County), California. Other localities are the Crestmore Quarry, Riverside County, California; Goodall Farm, Sanford, York County, Maine; Mont Saint Hilaire, Quebec, Canada; and Portland, Victoria, Australia.

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 do.
Other Plagioclase Feldspars - Usually cannot be determined by practical means.
Spodumene - Has a splintery cleavage.
Calcite - Much lower hardness.