Mineral Information and Data:

Chamosite
(Fe2+,Mg,Al,Fe3+)6(Si,Al)4O10(OH,O)8 [CNMNC approved formula]

Named after its locality: Chamoson, Rhone Valley, Switzerland.

IMA status : Valid species (Pre-IMA 1820) Grandfathered
CLASSIFICATION 
Dana
(8th edition) :
71.04.01.07  
  (71) Phyllosilicate Sheets of Six-Membered Rings
  (71.04) interlayered 1:1, 2:1, and octahedra
  (71.04.01) Chlorite group (Tri-Dioctahedral)
  
Nickel-Strunz
(10th edition) :
09.EC.55
  (09) Silicates
  (09.E) Phyllosilicates
  (09.EC) Phyllosilicates with mica sheets, composed of tetrahedral and octahedral nets
  
Crystal system: Monoclinic System
Point group (H-M): 2/m — prismatic
Space group: C2/m
Unit cell: a = 5.373Å, b = 9.306Å, c = 14.222Å, β = 97.883°, Z=2
Crystal Habit: In scaly aggregates, foliated or granular; oolitic, made up of very minute radiating crystals; massive.
Twinning: Twin and composition plane {001}, axis [310]
  
Color: Green, greenish grey, black, brown; in thin section, colorless, yellowish, green
Diaphaneity: Translucent to almost opaque
Luster: Pearly
Hardness (Mohs): 2 - 3
Measured Density: 3.0 - 3.4 g/cm3
Cleavage: {001}, perfect
Tenacity: Somewhat flexible but inelastic
Streak: Green to gray
  
Polymorphism and Series: Dimorphous with orthochamosite; forms a series with clinochlore.
Geologic Setting: In sedimentary ironstones; authigenically formed under reducing conditions in the presence of decomposed organic material.
Mineral Association: Siderite, kaolinite, quartz, magnetite, pyroxenes, plagioclase, olivine, calcite
  
Other Properties: Paramagnetic (weakly attracted to magnetic fields).

 

Chamosite
Chamosite with minor quartz
Origin: Sleese Creek, British Columbia, Canada
Sample size: 9.6 x 11.1 x 4.6 cm
Photo: Dan Weinrich

 

Chamosite
(zoom-in of above)

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