Adularia  (variety of Orthoclase, Feldspar)
Current inventory:  0 gems
 

  
Adularia is named for the Adula Mountains, Ticino (Tessin), Switzerland which is the type locality for the material. Orthoclase is named from the Greek words for straight and fracture, in allusion to the cleavage angle.

Discovery year unknown;   IMA status: Not Valid (a vaiety of Orthoclase)

Chemistry

Chemical Formula:

KAlSi₃O₈

Potassium Aluminum Silicate

Molecular Weight:

278.33 gm

Composition:

Potassium

14.05 %

K

16.92 %

K₂O

Aluminum

9.69 %

Al

18.32 %

Al₂O₃

Silicon

30.27 %

Si

64.76 %

SiO₂

Oxygen

45.99 %

O

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Silicates (Germanates)

Strunz 8th Ed. ID:

8/J.06-40

Nickel-Strunz 10th Ed. ID:

9.FA.30

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

Related to:

Feldspar Group, Celsian-Orthoclase Series, Hyalophane-Orthoclase Series, dimorphous with Microline

Members of Group:

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

Varieties:

Barium Adularia, Valencianite

Synonyms:

Adulare, Alpine Orthoclase, Orthos

Crystal Data

Crystallography:

Monoclinic - Prismatic

Crystal Habit:

Crystals commonly short prismatic along [100] or [001], tabular on [010], to 20 cm. Cleavable, granular, massive.

Twinning:

Common as simple, contact, or penetration twins according to the Carlsbad, Baveno, or Manebach laws.

Physical Properties

Cleavage:

Perfect on {001} and {010}; partings on {100}, {110}, {110}, and {201}

Fracture:

Conchoidal to Irregular/Uneven

Tenacity:

Brittle

Moh's Hardness:

6.0 - 6.5

Density:

2.55 - 2.63 (g/cm³)

Luminescence:

None

Radioactivity:

Barely Detectable; GRapi = 200.97 (Gamma Ray American Petroleum Institute Units)

Optical Properties

Color:

Colorless, White, Gray, pale Yellow, Colorless in thin section; may exhibit opalescence or schiller iridescence.

Transparency:

Transparent to Translucent

Luster:

Vitreous, Pearly on cleavages

Refractive Index:

1.518 - 1.525  Biaxial  ( - )

Birefringence:

0.004 - 0.005

Dispersion:

Distinct; r > v

Pleochroism:

None

Occurances

Geological Setting:

Generally found in "Alpine-type" parageneses. Orthoclase is the common feldspar of granites, granite pegmatites, and syenites. In cavities in basalts; in high-grade metamorphic rocks and as a result of potassic hydrothermal alteration; also authigenic and detrital.

Common Associations:

Albite, Muscovite, Biotite, Hornblende, Schorl, Beryl

Common Impurities:

Na, Fe, Ba, Rb, Ca

Type Locality:

Adula Mountains, Ticino (Tessin), Switzerland

Year Discovered:

Unknown

View mineral photos:

Adularia Mineral Photos and Locations

More Information

Mindat.org (Adularia)
Webmineral.com (Orothoclase)

Adularia is a variety of Orthoclase which is is a member of the Potassium Feldspars of the Feldspar Group of minerals that includes Adularia, Albite, Amazonite, Andesine, Anorthite, Bytownite, Hyalophane, Labradorite, Moonstone, Oligoclase, Orthoclase, Sanidine and Sunstone. Adularia is generally found in "Alpine-type" formations but rarely as gem quality crystals. Although Adularia is most often known from Alpine (Alps) localities, there are a few "Alpine-type" localities in the Himalayas of Pakistan that also produce some great specimens.

Most Adularia crystals are milky white and opaque to translucent. Rarely, Adularia gems are transparent or even eye clean and may show a light blue color sheen when rotated in the light. This sheen is the inspiration for the term "adularescence" which is named for the gem type.

Adularia is generally found in areas of the Alps including several valleys of the Hohe Tauern Mountains, Salzburg, Austria; the Apuan Alps, Lucca Province, Tuscany, Italy; several valleys in the Alps of Switzerland. Adularia is also found in the Astore District, Northern Areas, Pakistan.
 

  
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