Pumpellyite

Chlorastrolite, also known as Greenstone or Green Starstone, is a rare variety of Pumpellyite-(Mg) that was first discovered at Isle Royale, Lake Superior, Michigan, USA by C. T. Jackson and J. D. Whitney in 1847. The chemical formula of Chlorastrolite is Ca₂MgAl₂(SiO₄)(Si₂O₇)(OH)₂ • (H₂O) (Hydrated Calcium Magnesium Silicate Hydroxide) and its molecular weight is 470.70 gm.
Chlorastrolite was once thought to be an impure variety of Prehnite or Thomsonite. Chlorastrolite  has a specific gravity of 3.2 and the refractive index of 1.674 – 1.722 Biaxial ( + ) and ( – ). Beautiful dark green Chlorastrolie cabochons exhibit chatoyancy in a distinctive mosaic or segmented pattern sometimes referred to as “turtleback” due to its similarity to the pattern on a turtle shell. The colors of Chlorastrolite resemble fine Emerald or Imperial Jade. Chlorastrolite is the state gemstone of Michigan.

Category:	Sorosilicate
Chemical Formula:	Ca2MgAl2(SiO4)(Si2O7)(OH)2 • (H2O)
	Hydrated Calcium Magnesium Silicate Hydroxide
Molecular Weight:	470.70 gm

Composition:	Calcium	17.03 %	Ca	23.83 %	CaO
	Magnesium	5.16 %	Mg	8.56 %	MgO
	Aluminum	11.46 %	Al	21.66 %	Al2O3
	Silicon	17.90 %	Si	38.29 %	SiO2
	Hydrogen	0.86 %	H	7.65 %	H2O
	Oxygen	47.39 %	O
		100.00 %		100.00 %	= TOTAL OXIDE

 

Crystallography:	Monoclinic – Prismatic
Crystal Habit:	Crystals are fibrous, flattened plates in clusters or dense mats or random fibers.
Twinning:	Rarely

 

Color:	Green, Bluish Green, Greenish Black, Brown; Colorless, Green, Yellow, Brown in thin section.
Transparency:	Translucent, Opaque
Density:	3.2
Luster:	Vitreous (glassy)
Refractive Index:	1.674 – 1.722  Biaxial ( + ) and ( – )
Birefringence:	0.0140 – 0.0220
Dispersion:	Relatively Strong; r < v  (less commonly r > v)
Pleochroism:	Strong; X = nearly colorless to pale greenish yellow; Y = blue-green; Z = almost colorless to brownish yellow.