Mineral Information and Data:

Epididymite
Na2Be2Si6O15 • H2O [CNMNC approved formula]

Named from the Greek epi, "near" and didymos, "twin", for its dimorphous relationship with eudidymite.

IMA status : Valid species (Pre-IMA 1893) Grandfathered
CLASSIFICATION 
Dana
(8th edition) :
66.03.01.04  
  (66) Inosilicate Double-Width Unbranched Chains, W=2
  (66.03) with chains P>2
  (66.03.01) P=3
  
Nickel-Strunz
(10th edition) :
09.DG.55
  (09) Silicates
  (09.D) Inosilicates* Structural terminology per Liebau (1985)
  (09.DG) with 3-periodic single and multiple chains
  
Crystal system: Orthorhombic System
Point group (H-M): mmm (or 2/m 2/m 2/m) — dipyramidal
Space group: Pnma
Unit cell: a = 12.7334(4) Å, b = 13.6298(5) Å, c = 7.3467(3) Å; Z=4
Crystal Habit: As pseudohexagonal crystals, tabular on {001}, also elongated along [010], to 6 cm. Micaceous, spherulitic, fine to coarse granular, porcelaneous massive.
Twinning: On {001}, common, simple and polysynthetic; also as trillings.
  
Color: Colorless, white, also tinted violet, blue, or yellow when finely crystalline.
Diaphaneity: Transparent to translucent
Luster: Pearly on cleavages, vitreous on fractures.
Hardness (Mohs): 5.5
Measured Density: 2.55 - 2.61 g/cm3
Cleavage: Perfect on {001}, distinct on {100}
Fracture: Uneven to conchoidal when massive.
  
Polymorphism and Series: Dimorphous with eudidymite.
Geologic Setting: A late-stage mineral in nepheline syenite pegmatites.
Mineral Association: Albite, aegirine, eudialyte, elpidite, neptunite, tugtupite, leucophanite, natrolite, analcime.
  

 

Epididymite
White epididymite on aegerine.
Origin: Mt. St. Hilaire, Quebec, Canada
Sample size: 3.5 x 1 x 1 cm
Photo: Rob Lavinsky

 

Epididymite
(zoom-in of above)

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