Ferro-glaucophane is a relatively rare sodic amphibole that acts as an indicator mineral for high-pressure, low-temperature metamorphism. It typically forms prismatic to fibrous crystals in blueschist rocks and is distinguished from common glaucophane by its iron-rich composition. Collectors generally find these in dense, foliated metamorphic assemblages.
Is this ferro-glaucophane?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch ferro-glaucophane with a known reference. Ferro-glaucophane sits at Mohs 6-6.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: blue, bluish gray.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: prismatic crystals, fibrous, massive.
Often confused with
Ferro-glaucophane vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside ferro-glaucophane
Minerals reported to co-occur with ferro-glaucophane. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- □{Na₂}{Fe²⁺₃Al₂}(Si₈O₂₂)(OH)₂
- Mohs hardness
- 6-6.5
- Density
- 3.18-3.34 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Prismatic crystals, fibrous, massive
- Cleavage
- Perfect on {110}, intersecting at ~58° and ~122°
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- In metamorphic rocks of the high-pressure, low-temperature blueschist facies, commonly derived from either siliceous sedimentary, alkalic basaltic, or felsic volcanic predecessors
Where rockhounds find ferro-glaucophane
Classic worldwide localities
- Bouehndep, and near Ouégoa, New Caledonia
- Champ de Praz, Val d'Aosta, Italy
- Menai Bridge, Anglesey, Wales
- Heriot, Scotland
- Ile de Groix, Bay of Biscay, o® Morbihan, France
Field-hunting tip
Where it forms: In metamorphic rocks of the high-pressure, low-temperature blueschist facies, commonly derived from either siliceous sedimentary, alkalic basaltic, or felsic volcanic predecessors. If you start seeing paragonite, lawsonite, albite in float, you are in the right ground. Typical habit: prismatic crystals, fibrous, massive. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify ferro-glaucophane?+
Where is ferro-glaucophane found?+
What rocks look like ferro-glaucophane?+
What minerals are found with ferro-glaucophane?+
What kind of rock does ferro-glaucophane form in?+
What is ferro-glaucophane used for?+
Find ferro-glaucophane on the map
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