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Cookeite cast after Spodumene

#42564

Cookeite cast after Spodumene

#42564
Availability
Sold
Size
18 x 6 x 4 cm - Cabinet 
Formula
(Al,Li)3Al2(Si,Al)4O10(OH)8 (RRUFF)
Locality
Golconda pegmatite, Governador Valadares, Doce valley, Minas Gerais, Brazil
Tagged
pegmatite mineral

This specimen exhibits a replacement texture of former Spodumene, a lithium silicate. The original Spodumene crystal form has been nicely preserved by the fine-grained Cookeite, another aluminosilicate of lithium. David London in his book "Pegmatites", sites examples of selective replacement, in which some crystals of the pegmatites are completely replaced yet an adjacent mineral is left completely untouched. 

I have not seen this particular replacement before, but is not that unusual. The Bennet pegmatite in Maine and pegmatites in Portugal have very similar examples. In this specimen the Spodumene was altered to Cookeite and Kaolinite. The Kaolinite eventually dissolved out leaving behind the Cookeite cast. A good paper on this process can be read here: Alteration of Spodumene to Cookeite (2007) Clays and Clay Minerals, Bobos, I. et al V55 n3. Although it concerns a Portugal occurrence rather than the Golconda pegmatite, where this specimen was collected. Spodumene does weather easily and has good cleavage, which creates conduits for solutions to penetrate and breakdown the chemistry.

Cookeite

Formula
(Al,Li)3Al2(Si,Al)4O10(OH)8 
Crystal System
Monoclinic 
Crystal Habit
Micaceous, Pseudo Hexagonal 
Cleavage
Perfect, None, None 
Luster
Vitreous (Glassy) 
Color
white, green, brown, yellowish white, pinkish white 
Streak
white 
Class
Monoclinic - Prismatic 
Fracture
Flexible 
Hardness
2.5 

Data courtesy of Webmineral.

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