Very dark red, almost black, microcrystals of the rare mineral macaulyite on a contrasting matrix of illite and kaolinite. From: Bennachie, Inverurie, Buchan Grampian, Scotland, United Kingdom. Ex-British Museum Natural History London specimen. — Photo: David Hospital · CC BY-SA 4.0
Macaulayite
A red earthy monoclinic iron-aluminium silicate discovered in the 1970s and named for Aberdeen's Macaulay Institute.
Intro adapted from Wikipedia (CC BY-SA).
Macaulayite is an IMA-recognized mineral species in the silicate class, with the chemical formula Fe³⁺₂₄Si₄O₄₃(OH)₂.
It crystallizes in the monoclinic crystal system.
Key properties: specific gravity 4.41; Earthy luster.
IMA symbol Mcy, IMA status: approved, named in 1981.
A secondary mineral formed in deeply weathered granite.
It is found associated with Kaolinite, illite.
The name refers to the Macaulay Institute for Soil Research, Aberdeen, Scotland.