Mineral Information and Data:

Cookeite
(Al,Li)3Al2(Si,Al)4O10(OH)8 [CNMNC approved formula]

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

IMA status : Valid species (Pre-IMA 1866) Grandfathered
CLASSIFICATION 
Dana
(8th edition) :
71.04.01.02  
  (71) Phyllosilicate Sheets of Six-Membered Rings
  (71.04) interlayered 1:1, 2:1, and octahedra
  (71.04.01) Chlorite group (Tri-Dioctahedral)
  
Nickel-Strunz
(10th edition) :
09.EC.55
  (09) Silicates
  (09.E) Phyllosilicates
  (09.EC) Phyllosilicates with mica sheets, composed of tetrahedral and octahedral nets
  
Crystal system: Monoclinic System
Point group (H-M): 2/m — prismatic
Space group: C1
Unit cell: a = 5.14Å, b = 8.9Å, c = 14.15Å β = 97-98°
Crystal Habit: Pseudohexagonal platy crystals; curved, radial scales, spherulites, barrel-shaped, fibrous
Twinning: Around [310], composition plane {001}.
  
Color: White, yellowish green, pink, brown; in thin section, colorless to pale green or pink.
Diaphaneity: Transparent to translucent
Luster: Pearly or silky
Hardness (Mohs): 2.5 - 3.5
Measured Density: 2.58 - 2.69 g/cm3
Cleavage: {001}, perfect
Tenacity: Flexible but inelastic
  
Geologic Setting: A late-stage hydrothermal alteration product of lithium-bearing minerals in pegmatites; a primary hydrothermal vein mineral.
Mineral Association: Lepidolite, spodumene, tourmaline, petalite, quartz, albite, microcline
  

 

Cookeite
Cookeite occurs as greenish-brown spherical aggregates in cavities between cm-sized albite crystals; lepidolite is associated.
Origin: Bennett Pegmatite, Oxford Co., Maine, U.S.A.
Sample size: ca. 6 x 7 cm
Photo: Diederik Visser

 

Cookeite
Flakes of purple cookeite on quartz matrix
Origin: BB#7 Quarry, Greenwood, Maine, U.S.A.
Sample size: 3 x 3.5 cm
Photo: Diederik Visser

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