Oligoclase (a variety of Albite (Feldspar))
Current inventory:  0 gems
 

  
Oligoclase was named in 1826 by August Breithaupt from the Greek words όλίγος and κλάσις meaning little and to break because the mineral was thought to have a less perfect cleavage than Albite.

Discovered in 1826; IMA status: Not Valid (variety name)

Chemistry

Chemical Formula:

(Na,Ca)[Al(Si,Al)Si₂O₈]

Sodium Calcium Aluminum Silicate

Molecular Weight:

263.02 gm

Composition:

Sodium

8.30 %

Na

11.19 %

Na₂O

Calcium

0.76 %

Ca

1.07 %

CaO

Aluminum

10.77 %

Al

20.35 %

Al₂O₃

Silicon

31.50 %

Si

67.39 %

SiO₂

Oxygen

48.66 %

O

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Silicates (Germanates)

Strunz 8th Ed. ID:

8/J.07-30

Nickel-Strunz 10th Ed. ID:

9.FA.35

9 : SILICATES (Germanates)
F : Tektosilicates without zeolitic H₂O
A : Tektosilicates without additional non-tetrahedral anions

Related to:

Feldspar Group. Plagioclase series.

Members of Group:

Feldspar Group: Albite, Amazonite, Andesine, Anorthite, Anorthoclase, Banalsite, Buddingtonite, Bytownite, Celsian, Dmisteinbergite, Hyalophane, Labradorite, Microcline, Oligoclase, Orthoclase, Paracelsian, Reedmergnerite, Sanidine, Slawsonite, Stronalsite, Svyatoslavite 

Varieties:

Lazur-Feldspath, Oligoclase-Albite

Synonyms:

Adventurine Oligoclase, Amansite, Amantice, Amantite, Amausite, Lluvia, Oligoalbite, Oligoclasite, Soda-Spodumene (of Dana)

Crystal Data

Crystallography:

Triclinic - Pinacoidal

Crystal Habit:

Crystals uncommon, flattened and tabular along [010], to 6 cm; more commonly cleavable to compact, massive.

Twinning:

Typically twinned according to the Albite, Carlsbad, and Pericline laws.

Physical Properties

Cleavage:

Perfect on {001}, less perfect on {010}, imperfect on {110}, (001) ^ (010) ~94º

Fracture:

Irregular/Uneven, Sub-Conchoidal, Step-Like

Tenacity:

Brittle

Moh's Hardness:

6.0 - 6.5 

Density:

2.63 - 2.66 (g/cm³)

Luminescence:

Not usually fluorescent; may fluoresce faint red to faint white in SW UV

Radioactivity:

Not Radioactive

Optical Properties

Color:

Colorless, white, pale greenish, blue; may be tinted gray, green or red due to inclusions; colorless in thin section; may be opalescent or iridescent

Transparency:

Transparent to translucent

Luster:

Vitreous to pearly

Refractive Index:

1.533 - 1.552  Biaxial ( + ) or ( - )

Birefringence:

0.008 - 0.009

Dispersion:

Weak; r > v

Pleochroism:

n/a

Occurances

Geological Setting:

Common in granite, syenite, nepheline syenite, and their pegmatites; in diorite, rhyolite, andesite. In serpentinite and gneiss; in amphibolite facies metamorphic rocks; as clastic grains in sedimentary rocks.

Common Associations:

Quartz, Orthoclase, Sanidine, Tourmaline, Corundum

Common Impurities:

n/a

Type Locality:

Danvikstull, Stockholm, Södermanland, Sweden

Year Discovered:

1826

View mineral photos:

Oligoclase Mineral Photos and Locations

More Information

Mindat.org (Oligoclase)
Mindat.org (Albite)
Webmineral.com (Albite)

Oligoclase is a somewhat rare variety of Albite and a member of the Plagioclase Feldspars of the Feldspar Group of minerals that includes Albite, Amazonite, Andesine, Anorthite, Bytownite, Hyalophane, Labradorite, Moonstone, Oligoclase, Orthoclase, Sanidine and Sunstone. The Plagioclase Feldspars form a series between Albite and Anorthite. Albite is the sodium-rich end member while Anorthite is the calcium-rich end member and Oligoclase is the intermediate member. Oligoclase is about 90-70% Albite and 10-30% Anorthite.

Oligoclase was named in 1826 by August Breithaupt from the Greek words όλίγος and κλάσις meaning little and to break because the mineral was thought to have a less perfect cleavage than Albite. It had previously been recognized as a distinct species by J. J. Berzelius in 1824. He named it Soda-Spodumene (Natron-Spodumen) because of its resemblance in appearance to Spodumene.

Oligoclase is found as colorless to pale yellow or pale green, and rarely blue, crystals in only a few locations around the world. Colorless, transparent crystals have been found at the Hawk Mine, Bakersville, North Carolina and occasionally faceted as gems. "Confetti" Sunstone, from Arusha, Tanzania is a variety of Oligoclase with a colorless, yellow, orange or light green background and containing reddish metallic flashes, due to the presence of many small red plateletes of Hematite oriented within the Feldspar structure. A similar type of Sunstone has been found at Coimbatore, Tamil Nadu State, India. A bright blue Oligoclase has rarely been available from Madawaska, Bancroft District, Hastings County, Ontario, Canada.

Oligoclase distribution: A widespread mineral. Fine material from Danviken, near Stockholm, Sweden. At Arendal, on Tromöy Island at Alve, Tvedestrand, and elsewhere in Norway. In Sweden, at Ytterby. From Kemiö (Kimito) Island, Finland. At Silberberg, near Bodenmais, Bavaria, Germany. On Tenerife, Canary Islands. In the USA, at Fine and Macomb, St. Lawrence County, New York; on Corundum Hill, Chester County, and at Media, Delaware County, Pennsylvania; near Hawk, east of Bakersville, Mitchell County, North Carolina. At Lake Harbour, Baffin Island, Northwest Territories, Canada.
 

  
Oligoclase gems for sale:

We have not photographed our Oligoclase gems yet. Please check back soon.