Fluorannite is a rare mica mineral characterized by a high fluorine content within its octahedral and interlayer structure. It typically appears as brown to greenish-iron-black tabular crystals within alkaline igneous environments, often closely resembling other members of the biotite series.
Is this fluorannite?
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 fluorannite with a known reference. Fluorannite sits at Mohs 3, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: gray.
- 3Read the lusterHold the specimen under a strong light. Luster: submetallic.
- 4Match the color rangeCompare against the expected color range: iron black.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: fibrous, foliated, tabular.
Often confused with
Fluorannite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Fluorannite leaves gray, Biotite leaves white; luster reads submetallic on Fluorannite and splendent to submetallic, vitreous, pearly on cleavage on Biotite.
How to tell apart: Streak differs, so Fluorannite leaves gray, Phlogopite leaves white; luster reads submetallic on Fluorannite and pearly to submetallic on cleavage on Phlogopite.
How to tell apart: Streak differs, so Fluorannite leaves gray, Annite leaves brownish white; luster reads submetallic on Fluorannite and vitreous on Annite.
Often found alongside fluorannite
Minerals reported to co-occur with fluorannite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KFe₃²⁺(AlSi₃O₁₀)F₂
- Mohs hardness
- 3
- Density
- 3.15-3.20 g/cm³
- Colors
- Streak
- Gray
- Luster
- Submetallic
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Fibrous, foliated, tabular
- Cleavage
- Perfect {001}
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- A rock-forming species in the upper part of an A-type granite
Where rockhounds find fluorannite
Classic worldwide localities
- Suzhou, near Shanghai, eastern China
- Katugin Ta-Nb deposit, Chitinskaya Oblast’, Kalar Range, Transbaikalia, eastern Siberia, Russia
Field-hunting tip
Where it forms: A rock-forming species in the upper part of an A-type granite. If you start seeing nepheline, aegirine, microcline in float, you are in the right ground. Typical habit: fibrous, foliated, tabular. Train your eye for that shape before scanning the outcrop.
Common questions
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Find fluorannite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
