Mineral Information and Data:

Margarite
CaAl2(Si2Al2)O10(OH)2 [CNMNC approved formula]

Named from the Greek margaritos - "pearl", for its pearly luster on cleavage. First published in 1823. Approved and defined per nomenclature of the micas (IMA, 1998).

IMA status : Approved Species 1998 ()
CLASSIFICATION 
Dana
(8th edition) :
71.02.02c.01  
  (71) Phyllosilicate Sheets of Six-Membered Rings
  (71.02) with 2:1 Layers
  (71.02.02c) Mica Group (Margarite subgroup)
  
Nickel-Strunz
(10th edition) :
09.EC.30
  (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/c
Unit cell: a = 5.10Å, b = 8.83Å, c = 19.15Å, β = 95.5°, Z=4
Crystal Habit: Rarely forms distinct thin tabular crystals, with pseudohexagonal outline; commonly foliated micaceous aggregates, with lamellae to 3 cm; massive.
Twinning: On composition plane {001}, with twin axis [310]
  
Color: Grayish, pale pink, yellow, or green; colorless in thin section
Diaphaneity: Translucent
Luster: Pearly on cleavage, vitreous on lateral faces
Hardness (Mohs): 3.5 - 4.5
Measured Density: 2.99 - 3.08 g/cm3
Cleavage: {001}, perfect
Tenacity: Laminae brittle
Streak: White
  
Polymorphism and Series: 2M1 polytype
Geologic Setting: From low- to medium-grade metamorphic rocks, as in emery deposits, chlorite and mica schists, and glaucophane-bearing rocks.
Mineral Association: Corundum, diaspore, tourmaline, staurolite, glaucophane, chlorite, magnetite, spinel, andalusite, calcite, quartz.
  

 

Margarite
Origin: Chester Emery Mines, Chester,
Hampshire Co., Massachusetts, USA.
Sample size: 6 x 5 x 4 cm
Photo: Tom Loomis

 

Margarite
(zoom-in of above)

 

Margarite
Origin: Chester, Massachusetts, U.S.A.
Sample size: 5 x 6 cm
Photo: Diederik Visser

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