THE MINERAL FERROGLAUCOPHANE

Chemistry: Na₂ (Fe, Mg)₃Al₂Si₈O₂₂(OH)₂, Sodium Iron Magnesium Aluminum Silicate Hydroxide.
Class: Silicates
Subclass: Inosilicates
Group: Amphibole
Uses: Only as mineral specimens.
Specimens

Ferroglaucophane is an uncommon amphibole mineral. Its name comes from its increased iron (or ferrous) content over that of it close cousin glaucophane. Ferroglaucophane forms a solid solution series with the more common glaucophane; Na₂ (Mg, Fe)₃Al₂Si₈O₂₂(OH)₂. Ferroglaucophane and glaucophane have the same structure and the same chemistry except for their respective iron and magnesium content. Ferroglaucophane is similar to glaucophane but because of more iron in its formula it is slightly denser and generally darker in color with a duller luster. Both minerals are related to riebeckite.

Ferroglaucophane is formed in metamorphic zones known as blueschist facies. This facies forms from material caught under subduction zones in mountain belt regions. This material has undergone intense pressure and relatively low heat as it was subducted downward almost to the mantle.

PHYSICAL CHARACTERISTICS:

Color is blue-gray, gray to black.
Luster is vitreous to dull.
Transparency: Crystals are generally translucent to opaque.
Crystal System is Monoclinic; 2/m
Crystal Habits include scarce prismatic to acicular crystals, usually fibrous, granular or massive.
Cleavage is imperfect in two directions at nearly 58 and 122 degrees.
Fracture is conchoidal to uneven.
Hardness is 6.
Specific Gravity is approximately 3.2 - 3.3 (slightly above average for non-metallic minerals).
Streak is pale gray to blue.
Other Characteristics: Strongly pleochroic from violet to blue to colorless.
Associated Minerals include epidote, paragonite, almandine, glaucophane, lawsonite, omphacite, spessartine, albite and jadeite.
Notable Occurrences include Champ de Praz, Val d'Aosta, Italy; near Menai Bridge, Anglesey, Wales; Scotland; Ile de Groix, France and Syra Island, Cyclades Islands, Greece as well as numerous other localities where glaucophane is found.
Best Field Indicators are crystal habit (especially cross-section), color, pleochroism, environment of formation, associations and cleavage.