NM4989 : Cliffs and Scree north of Struidh
taken 10 years ago, near to Cleadale, Eigg, Scotland
Mugearite is a type of andesite, which occurs when magma of a specific chemical composition is erupted on the surface of the Earth. The chemical composition of the rock results in different lichens colonising the surface, making mugearite look distinctly paler than the basalt flows above and below.
Mugearite is named after Mugeary in Skye, where it was first described. See Link.
In technical terms, mugearite is defined chemically as the sodic variety of basaltic trachyandesite. What that means in plain English is that it contains more silica than an 'average' basalt and more sodic feldspar and less alkali feldspar than an 'average' andesite. But, as with everything in geology, it's actually far more complicated than the plain English suggests.