Mineral Information and Data:

Chesterite
Mg17Si20O54(OH)6 [CNMNC approved formula]

Named for the locality: Chester, Windsor Co., Vermont, USA.

IMA status : Approved Species 1978 (IMA1977-010)
CLASSIFICATION 
Dana
(8th edition) :
68.01.01.01  
  (68) Inosilicate Structures with Chains of More Than One Width
  (68.01)
  (68.01.01)
  
Nickel-Strunz
(10th edition) :
09.DF.05
  (09) Silicates
  (09.D) Inosilicates* Structural terminology per Liebau (1985)
  (09.DF) with 2-periodic multiple chains
  
Crystal system: Orthorhombic System
Point group (H-M): mm2 — pyramidal
Space group: A21ma
Unit cell: a = 18.6140(3)Å, b = 45.306(1)Å, c = 5.2966(3)Å, Z=4
Crystal Habit: Radiating sprays of prismatic crystals, to 5 cm; as intergrowths || {010} in anthophyllite and cummingtonite.
  
Color: Colorless to very light pinkish brown; in thin section, colorless
Diaphaneity: Transparent
Measured Density: 3.08 (calculated) g/cm3
Cleavage: Perfect on {110}, intersecting at ~45° and 135°; breakage along {100} and {010} may be due to parting.
  
Polymorphism and Series: In black wallrock between chlorite and actinolite zones of a metamorphosed ultramafic body.
Geologic Setting: Jimthompsonite, clinojimthompsonite, anthophyllite, cummingtonite, talc
  

 

Chesterite
Green platy chesterite in black, metallic magnetite.
Origin: Near Missoula, Missoula County, Montana, USA.
Photo: Jeff Weissman

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