(Mg,Fe)<sub>7</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>
Magnesium iron silicate hydroxide
<img src="http://www.manyfacets.com/images/minerals/crocoite2-1.jpg">
Discovered in Cummington, Massachusetts, USA.
Common name: Cummingtonite
(originally wrote this for somewhere else, but figured it'd fit in here too)