Dark green foliated and platy Clinochlore, variety delessite. It is a phyllosilicate of the chlorite group - basically a mica mineral. It is formerly from the Paul Moore collection with a label matching the numbered specimen. The mineral was named and described by a famous American geologist, William P. Blake, who was the Arizona territorial geologist and connected with the beginnings of the South Dakota tin mining affair. Here's a link to the Mineralogical Record label archives, which gives a bio on Blake. So basically it is a drab specimen with a colorful past.
Clinochlore
Formula
Mg6Si4O10(OH)8
Crystal System
Monoclinic
Crystal Habit
Massive - Fibrous, Pseudo Hexagonal, Granular
Cleavage
Perfect, None, None
Luster
Vitreous - Pearly
Color
blackish green, bluish green, white, yellowish green, olive green