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Brown Cookeite Coating Quartz

The Mineral cookeite

Cookeite (pronounced Cook-ite) is an uncommon member of the Chlorite group, and is best known for its occurrence in granite pegmatites associated with Tourmaline, where it often forms as a growth layer upon the Tourmaline. Cookeite is named in honor of Josiah Parsons Cooke Jr. (1827-1894), a Harvard University scientist who was instrumental in the measurement of atomic weights.
Chemical Formula LiAl5Si3O10(OH)8
Composition Basic lithium magnesium aluminum silicate
Color White, beige, yellow, orange, green, light blue, light brown, light pink
Streak white
Hardness 2.5
Crystal System Monoclinic
Crystal Forms
and Aggregates
Crystals, which are always small, are in tabular pseudohexagonal form. Most often in radiating and botryoidal aggregates, as ball-shaped spheres, and in micaceous crusts, flakes, and small rosettes.
Transparency Translucent
Specific Gravity 2.58 - 2.69
Luster Pearly, waxy
Cleavage 1,1
Fracture Uneven
Tenacity Flexible but not elastic.
Other ID Marks May have a slightly greasy feel.
In Group Silicates; Phyllosilicates; Chlorite Group
Striking Features Crystal habits, localities, and mineral associations
Environment As an alteration mineral in lithium-rich granite pegmatites.
Rock Type Igneous
Popularity (1-4) 3
Prevalence (1-3) 3
Demand (1-3) 2


Cookeite is best known from the pegmatite quarries in Oxford Co., Maine. Specific localities there include Mt. Rubellite, Hebron; Mt Mica, Paris; Plumbago Mountain, Newry; the Tamminen Quarry, Greenwood; and the Bennett Quarry, Buckfield. Cookeite also comes from the Gillette Quarry, Haddam; and at the Strickland Quarry, Portland; both in Middlesex Co., Connecticut.

Some of the best bluish and greenish Cookeite, in interesting ball aggregates with Quartz, comes from Saline Co., Arkansas, especially at the Stand-on-Your-Head mine in Bland. Good Cookeite also comes from the Tourmaline mines around Pala, San Diego Co., California; as well as the Tourmaline mines of of Minas Gerais, Brazil, especially at Conselheiro Pena in the Doce valley.

Quartz, Microcline, Albite, Tourmaline, Apatite, Spodumene

Muscovite and other micas - Are more elastic.
Talc - Softer (1).

cookeite PHOTOS
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