Mineral Database (Saesneg yn unig)

Mineral Database (Saesneg yn unig)
Mineralogy of Wales

Cookeite

Crystal System: Monoclinic
Formula: LiAl4(Si3Al)O10(OH)8
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Lithium aluminium silicate hydroxide
Method(s) of Verification: XRD & wet chemistry - (Brammall et al., 1937)
Chemical Group:
  • Silicates
Geological Context:
  • Hydrothermal
Introduction: cookeite is a lithium-rich member of the chlorite group. It typically occurs as a late stage alteration product of lithium minerals (e.g. lepidolite, petalite or spodumene) in granite pegmatites. It can also form as a primary hydrothermal vein mineral.
Occurrence in Wales: a single occurrence of cookeite in known from Wales. This was discovered during exploration work for gold (Brammall et al., 1937). The cookeite is interpreted as having formed in the late stages of a low-grade, hydrothermal mineralization event.
Key Localities:
  • Ogofau Mine, Pumpsaint, Dyfed: Brammall et al. (1937) described cookeite as a late-stage coating on other lode minerals (sulphides-quartz-hydromuscovite) in small cavities along shear and fractures planes traversing the ore-body and the host rock shales. The analysed material consisted of ‘a creamy-white, compact but friable mass with a silky sheen on hackly fracture surfaces’. On closer inspection this was observed to have the form of small (<0.5 mm) pale-cream, transparent plates with an irregular outline.
References:
  • Brammall, A. , Leech, J.G.C. & Banister, F.A., 1937 The paragenesis of cookeite and hydromuscovite associated with gold at Ogofau, Carmarthenshire.  Mineralogical Magazine, 24, 507-521.