THE MINERAL FERRO-EDENITE

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

Ferro-edenite is an uncommon amphibole mineral. It is almost exactly the same as the more common amphibole mineral edenite, but contains more iron than magnesium thus the name ferro-edenite (ferro is latin for iron). The two minerals form a solid solution series in which the iron and magnesium substitute for each other. Edenite is the magnesium rich member. The two are similar in properties except that ferro-edenite is generally darker and denser.

Ferro-edenite 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 ferro-edenite is one. In fact ferro-edenite had been referred to as "ferro-edenitic hornblende" before its adoption as an official and distinct mineral.

Amphiboles like ferro-edenite, edenite and hornblende serve as important petrographic minerals. Their presence allows petrologists (rock scientists) to accurately gauge the pressure of the rock during crystallization. From this information a rock's depth during crystallization can be deduced.

PHYSICAL CHARACTERISTICS:

Color is green to black.
Luster is vitreous to dull.
Transparency: Crystals are generally translucent to opaque.
Crystal System is Monoclinic; 2/m.
Crystal Habits include prismatic to bladed 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.
Cleavage is imperfect in two directions at nearly 60 and 120 degrees.
Fracture is uneven.
Hardness is 5 - 6.
Specific Gravity is approximately 3.46 (above average for non-metallic minerals)
Streak is white.
Associated Minerals include edenite, tremolite, kaolinite, biotite, pyroxenes, spinel and calcite.
Notable Occurrences include Rio Grande de Norte, Brazil and other localities where edenite is found.
Best Field Indicators are crystal habit (especially cross-section), color, density and cleavage.