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Epididymite  
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Epididymite is named from from the Greek words epi for near, and didymus for twin referring to its dimorphous relation with Eudidymite.

Discovered in 1893;   IMA status: Valid (pre-IMA; Grandfathered)

Chemistry

Chemical Formula:

NaBeSi₃O₇(OH)

Sodium Beryllium Silicate Hydroxide

Molecular Weight:

245.26 gm

Composition:

Sodium

9.37 %

Na

12.64 %

Na₂O

Beryllium

3.67 %

Be

10.20 %

BeO

Silicon

34.35 %

Si

73.49 %

SiO₂

Hydrogen

0.41 %

H

3.67 %

H₂O

Oxygen

52.19 %

O

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Silicates (Germanates)

Strunz 8th Ed. ID:

8/G.04-20

Nickel-Strunz 10th Ed. ID:

9.DG.55

9 : SILICATES (Germanates)
D : Inosilicates
G : Inosilicates with 3-periodic single and multiple chains

Related to:

Dimorphous with Eudidymite.

Synonyms:

Epidymite

Crystal Data

Crystallography:

Orthorhombic - Dipyramidal

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.

Physical Properties

Cleavage:

Perfect on {001}, Distinct on {100}

Fracture:

Uneven to Conchoidal when massive

Tenacity:

Brittle

Moh's Hardness:

5.5

Density:

2.55 - 2.61 (g/cm³)

Luminescence:

None

Radioactivity:

Not Radioactive

Optical Properties

Color:

Colorless, White, also tinted Violet, Blue, or Yellow when finely crystalline

Transparency:

Transparent to Translucent 

Luster:

Vitreous on fractures, Pearly on cleavages

Refractive Index:

1.536 - 1.546  Biaxial ( + ) or ( - )

Birefringence:

0.0020

Dispersion:

Weak

Occurances

Geological Setting:

A late-stage mineral in nepheline syenite pegmatites.

Common Associations:

Aegirine, Albite, Analcime, Elpidite, Eudialyte, Leucophanite, Natrolite, Neptunite, Tugtupite

Common Impurities:

Al, Fe, Mg, Ca, K

Type Locality:

Narssârssuk pegmatite (Narsarsuk pegmatite), Narssârssuk (Narsarsuk), Igaliku (Igaliko), Narsaq, Kitaa (West Greenland) Province, Greenland

Year Discovered:

1893

View mineral photos:

Epididymite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Epididymite is a rare beryllium silicate mineral that is found in agpaitic pegmatite intrusions, an unusual igneous rock that is high in alkaline metals such as sodium and poor in silica. These intrusions also contain a large number of unusual elements such as beryllium, titanium, fluorine and rare earth metals but are typically low in aluminum and silica. Beryllium is a somewhat exotic element found in unusual concentrations in agpaite pegmatites allowing Epididymite to form.

Epididymite is found in locations that are famous agpaitic pegmatite sites like Kola Peninsula, Russia; Mont Saint-Hilaire, Canada; Arø Island, Langesundfjord, Norway and Narssârssuk, Greenland. These are all well known locations for rare silicate minerals. Other Epididymite locations include Vevja quarry, Tvedalen, Norway; Vezná, Czech Republic; near Saint-Amable, Quebec, Canada; near Quincy, Norfolk County, Massachusetts, USA; and large crystals on Mt. Malosa, Zomba district, Malawi.
 

  
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