Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Chemical Composition: Lithium aluminium silicate hydroxide
Chemical Formula: LiAl4(Si3Al)O10(OH)8
Method(s) of Verification: XRD & wet chemistry - (Brammall et al., 1937)
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.
- 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.
- 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.