Cookeite (inclusions in Quartz)
Current inventory:  0 gems
 

  
Cookeite was named for Josiah B. Cooke, Jr. (1827-1894), American mineralogist and chemist, Harvard University, Cambridge, Massachusetts, USA.

Discovered in 1866; IMA status: Valid (pre-IMA; Grandfathered)

Chemistry

Chemical Formula:

LiAl₄(Si₃Al)O₁₀(OH)₈

Lithium Aluminum Silicate Hydroxide

Molecular Weight:

522.16 gm

Composition:

Lithium

1.33 %

Li

2.86 %

Li₂O

Aluminum

25.84 %

Al

48.82 %

Al₂O₃

Silicon

16.14 %

Si

34.52 %

SiO₂

Hydrogen

1.54 %

H

13.80 %

H₂O

Oxygen

55.15 %

O

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

SILICATES (Germanates)

Strunz 8th Ed. ID:

8/H.23-90

Nickel-Strunz 10th Ed. ID:

9.EC.55

9 : SILICATES (Germanates)
E : Phyllosilicates
C : Phyllosilicates with mica sheets, composed of tetrahedral and octahedral nets

Related to:

Chlorite Group

Members of Group:

Chlorite Group: Baileychlore, Borocookeite, Chamosite, Clinochlore, Cookeite, Corundophilite, Donbassite, Franklinfurnaceite, Nimite, Orthochamosite, Pennantite, Sudoite 

Varieties:

None

Synonyms:

Cookeite (of Brush)

Crystal Data

Crystallography:

Monoclinic - Prismatic

Crystal Habit:

Pseudohexagonal platy crystals; curved, radial scales, spherulites, barrel-shaped, fibrous.

Twinning:

Around [310], composition plane {001}; frequently shows six sector twins by polarized light microscopy

Physical Properties

Cleavage:

Perfecct on {001} 

Fracture:

Micaceous

Tenacity:

Flexible but inelastic

Moh's Hardness:

2.5 - 3.5

Density:

2.58 - 2.69 (g/cm³)

Luminescence:

May fluoresce creamy yellow under SW UV

Radioactivity:

Not Radioactive

Optical Properties

Color:

White, yellowish green, pink, brown; colorless to pale green or pink in thin section. Colorless when pure.

Transparency:

Transparent to translucent

Luster:

Waxy, greasy; pearly or silky on basal cleavage

Refractive Index:

1.572 - 1.600  Biaxial ( + )

Birefringence:

0.014 - 0.024

Dispersion:

r < v

Pleochroism:

Visible; X = Y = pale green to pink; Z = colorless to pale yellow

Occurances

Geological Setting:

A late-stage hydrothermal alteration product of lithium-bearing minerals in pegmatites; a primary hydrothermal vein mineral.

Common Associations:

Lepidolite, Spodumene, Tourmaline, Petalite, Quartz, Albite, Microcline

Common Impurities:

Fe, Mn, Mg, Ca, Na, K

Co-Type Localities:

Mount Rubellite Quarries, Hebron, Oxford County, Maine, USA
Mount Mica Quarry (Mount Mica Mine), Paris, Oxford County, Maine, USA

Year Discovered:

1866

View mineral photos:

Cookeite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Cookeite is a silicate mineral that is a member of the Chlorite Group of minerals that also includes Chamosite and Clinochlore among others. Cookeite was discovered in 1866 and named for Josiah B. Cooke, Jr. (1827-1894), American mineralogist and chemist, Harvard University, Cambridge, Massachusetts, USA. Cookeite has perfect cleavage, micaceoius fracture and is flexible but inelastic with waxy or greasy luster with pearly luster on basal cleavages. It can be found in colors of white, yellowish green, green, pink, brown or colorless when pure. A rare sky blue or bluish green to deep green variety of Cookeite can be found as micaceous, spherulitic balls on colorless Quartz crystals at the Stand-on-Your-Head No. 1 mine (Alum Creek prospect), Bland, Saline County, Arkansas, USA. The picture above shows an inclusion of a single, pale green ball of Cookeite in clear, colorless Quartz from Minas Gerais, Brazil.

Locations for Cookeite: good examples from: in the USA, at Hebron, Mt. Mica, near Paris, and Buckfield, Oxford County, Maine; in the Pala and Rincon districts, San Diego County, California; from the Jeffrey quarry, North Little Rock, Pulaski County, Arkansas. At Waitabit Creek, northwest of Donald, British Columbia, Canada. From Ogofau, Carmarthenshire, England. At Radkovice and Dobrá Voda, Czech Republic. In the Varuträsk pegmatite, 15 km northwest of Skellefteå, Västerbotten, Sweden. From Lipovka, Ural Mountains, Russia. In the Muiâne pegmatite, Alto Ligonha district, Mozambique. At Londonderry, Western Australia. At Itambé, Bahia and Conselheiro Pena, Doce Valley, Minas Gerais, Brazil. Many additional localities are known.
 

  
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