THE MINERAL EDENITE

Chemistry: NaCa₂(Mg, Fe)₅AlSi₇O₂₂(OH)₂, Sodium Calcium Magnesium Iron Aluminum Silicate Hydroxide.
Class: Silicates
Subclass: Inosilicates
Group: Amphibole
Uses: Only as mineral specimens.
Specimens

Edenite is an uncommon amphibole mineral. It is related to the more well known amphibole, hornblende. Although hornblende is no longer an official mineral, it still serves as a general name for iron, magnesium, aluminum and calcium rich amphiboles of which edenite is one. Edenite had been referred to as "edenitic hornblende" before the name edenite was officially adopted as an official and distinct mineral.

Edenite is placed in the Tremolite Subgroup of the Amphibole Group. It is interesting to compare edenite to the more common tremolite; Ca₂Mg₅Si₈O₂₂(OH)₂. A close look at the formulas of the two minerals shows that edenite contains both a sodium atom and an aluminum atom that is lacking in tremolite (as well as some iron). To compensate, tremolite has an extra silicon. This helps to illustrate edenite's environment of formation which tends to be sodium and aluminum rich granitic pegmatites and metamorphic skarns.

Edenite has a close cousin called ferro-edenite; NaCa₂(Fe, Mg)₅AlSi₇O₂₂(OH)₂. The two minerals form a solid solution series in which the iron and magnesium substitute for each other. Ferro-edenite is the iron rich member hence the ferro designation, while edenite is the magnesium rich member. Edenite's formula is often written without the iron content implying a pure contentration of magnesium, but iron is almost always present in edenite.

Edenite was discovered in the 1830's while ferro-edenite was not discovered until the 1940's. Edenite was discovered in the rocks of the Franklin Marble, a formation that extends from the famous mines of Franklin, New Jersey into New York. Edenite gets its name from Edenville, New York, its type locality. Another similar mineral to edenite has been proposed called fluoro-edenite which is the same as edenite but fluorines would replace the hydroxides.

Edenite can form nice, well formed and sometimes complex monclinic crystals. Its translucent green specimens provide for the best examples of this species. Although not common, edenite is found around the world. It is unfortunately easily confused with other amphibole minerals such as tremolite, hornblende and fluorrichterite. Misidentification is a problem for this species as it has been identified from several localities only to have those specimens later attributed to other amphiboles, sometimes years later. Or possibly not recognized at first, only to be confirmed later if at all. Edenite is quite possibly more widespread than currently believed.

PHYSICAL CHARACTERISTICS:

Color is green to black also dull gray or brown.
Luster is vitreous to pearly to dull.
Transparency: Crystals are generally translucent to opaque.
Crystal System is Monoclinic; 2/m
Crystal Habits include prismatic to stubby crystals with a nearly diamond shaped cross-section the points of which can be truncated by minor prism faces. The typical termination appears to be the two faces of a slightly slanted dome but is actually two of the four faces of a prism. The termination faces are not only slanted toward each other but the two faces are slanted with respect to the long axis of the crystal as well. Some terminations are rather complex and can make the crystal appear pseudo-orthorhombic. Twinning is commonly seen and results in a groove or notch running down the "spine" of the prismatic crystals. A fibrous habit is also seen.
Cleavage is imperfect in two directions at nearly 60 and 120 degrees.
Fracture is uneven.
Hardness is 5 - 6.
Specific Gravity is approximately 3.06 (slightly above average for non-metallic minerals)
Streak is white.
Associated Minerals include tremolite, kaolinite, biotite, pyroxenes, spinel and calcite.
Notable Occurrences include the type locality of Edenville, Orange County, New York, USA as well as Mont Saint-Hilaire, Quebec, Canada; Franklin, New Jersey, USA; Cygnet, Tasmania, Australia; England and Aldan Shield, Siberia, Russia.
Best Field Indicators are crystal habit (especially cross-section), color, luster, locality and cleavage.