Ottrelite, a silicate of alumina with protoxides of iron and manganese and water. It occurs in the form of thin hexagonal plates or tables in certain more or less metamorphosed slates, which are hence termed Ottrelite-slate.
Ottrelite
Chambers's Encyclopaedia, Volume 7: Maltebrun to Pearson, p. 662
Source scan(s): p. 0675