Mineral Information and Data:

[]Fe2+7Si8O22(OH)2 [CNMNC approved formula]

Named for Louis Emmanuel Gruner (1809-1883), Swiss-French chemist who analyzed the mineral. First published in 1853. Species redefined per nomenclature of amphiboles (IMA, 1997).

IMA status : Redefined IMA Approved; 1997 ()
(8th edition) :  
  (66) Inosilicate Double-Width Unbranched Chains, W=2
  (66.01) with P=2 amphibole configuration
  (66.01.01) Group 1, the Mg-Fe-Mn-Li amphiboles (Monoclinic)
(10th edition) :
  (09) Silicates
  (09.D) Inosilicates* Structural terminology per Liebau (1985)
  (09.DE) with 2-periodic double chains, Si4O11; Clinoamphiboles
Crystal system: Monoclinic System
Point group (H-M): 2/m — prismatic
Space group: C2/m
Unit cell: a = 9.5642(7)Å, b = 18.393(2)Å, c =5.3388(3)Å; β=101:892(3)°, Z = 2
Crystal Habit: Commonly as acicular or fibrous radiating crystals; may be asbestiform.
Twinning: Simple or multiple twinning common || {100}, producing narrow twin lamellae.
Color: Dark green to brown, gray to greenish gray; colorless to pale green or brown in thin section
Diaphaneity: Translucent
Luster: Vitreous, silky when fibrous
Hardness (Mohs): 5 - 6
Measured Density: 3.40 - 3.60 g/cm3
Cleavage: Perfect on {110}, with intersections of ~56° and ~124°
Tenacity: Brittle
Polymorphism and Series: Dimorphous with ferro-anthophyllite; forms a series with cummingtonite.
Geologic Setting: Common in medium- to high-grade metamorphosed iron formations. A product of contact metamorphism, and in some blueschist facies metaquartzites.
Mineral Association: Magnetite, hematite, hedenbergite, riebeckite, fayalite, garnet, quartz


Fibrous radial sprays of light green crystals to 8 mm in magnetite matrix.
Origin: Iron pit, Ishpeming, Michigan, United States
Sample size: 4.5 x 2 x 2 cm
Photo: Tom Loomis


(zoom-in of above)


Fibrous radial sprays of light brown crystals in matrix.
Origin: Michigamme, Marquette Co., Michigan, United States
Sample size: 6 x 3.5 x 4 cm
Photo: Tom Loomis


(zoom-in of above)