Chlorastrolite
Variety Ca2MgAl2(SiO4)(Si2O7)(OH)2·H2O
Also called Isle Royale greenstone and Greenstone
About it
Green fibrous pumpellyite in radiating patches that form a turtle-back pattern, filling gas holes in basalt. Michigan's state gem, from Isle Royale and the Keweenaw Peninsula; collecting on Isle Royale is illegal.
- Type
- Variety
- Group
- Pumpellyite-(Mg) variety
- Formula
- Ca2MgAl2(SiO4)(Si2O7)(OH)2·H2O
- Hardness (Mohs)
- 5–6
- Specific gravity
- 3.1–3.5
- Streak
- white
- Lustre
- vitreous (glassy), silky
- Crystal system
- Monoclinic
- Colours
- green, blue-green, with a darker turtle-back pattern
- Transparency
- opaque, translucent
- Cleavage and fracture
- none; fibrous, splintery fracture
Typical values: real specimens vary. For more, see mindat.org.