Tetraferriannite is a rare iron-rich mica member of the biotite group often found in alkaline igneous complexes. Collectors look for its dark, pseudohexagonal platy crystals that exhibit perfect basal cleavage typical of mica minerals. It is primarily identified through chemical analysis or occurrence in specific nepheline-syenite environments.
Is this tetraferriannite?
4-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 tetraferriannite with a known reference. Tetraferriannite sits at Mohs 2.5-3, softer than the next harder reference, harder than the previous one.
- 2Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 3Match the color rangeCompare against the expected color range: brown, brownish black.
- 4Look at form & habitCrystal system: monoclinic. Typical habit: massive, scaly.
Often confused with
Tetraferriannite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads vitreous on Tetraferriannite and splendent to submetallic, vitreous, pearly on cleavage on Biotite.
How to tell apart: Luster reads vitreous on Tetraferriannite and pearly to submetallic on cleavage on Phlogopite.
Often found alongside tetraferriannite
Minerals reported to co-occur with tetraferriannite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KFe²⁺₃(Fe³⁺Si₃O₁₀)(OH)₂
- Mohs hardness
- 2.5-3
- Density
- 3.1-3.3 g/cm³
- Colors
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Massive, scaly
- Cleavage
- Perfect on {001}
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- In low-grade metamorphosed banded iron formation
Where rockhounds find tetraferriannite
Classic worldwide localities
- Dales Gorge Member iron-formations, Wittenoom area, Western Australia (type locality)
Field-hunting tip
Where it forms: In low-grade metamorphosed banded iron formation. If you start seeing hematite, magnetite, quartz in float, you are in the right ground. Typical habit: massive, scaly. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify tetraferriannite?+
Where is tetraferriannite found?+
What rocks look like tetraferriannite?+
What minerals are found with tetraferriannite?+
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Find tetraferriannite on the map
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