Siderophyllite is an iron-rich mica typically found in alkaline igneous environments like pegmatites. It forms dark, pseudohexagonal tabular crystals that often exhibit the classic micaceous perfect basal cleavage.
Is this siderophyllite?
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 siderophyllite with a known reference. Siderophyllite sits at Mohs 2.5-3, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: greenish gray.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to dull.
- 4Match the color rangeCompare against the expected color range: black, blue green, dark brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: foliated, micaceous.
Often confused with
Siderophyllite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Siderophyllite leaves greenish gray, Biotite leaves white; luster reads vitreous to dull on Siderophyllite and splendent to submetallic, vitreous, pearly on cleavage on Biotite.
How to tell apart: Streak differs, so Siderophyllite leaves greenish gray, Annite leaves brownish white.
How to tell apart: Streak differs, so Siderophyllite leaves greenish gray, Phlogopite leaves white; luster reads vitreous to dull on Siderophyllite and pearly to submetallic on cleavage on Phlogopite.
Often found alongside siderophyllite
Minerals reported to co-occur with siderophyllite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KFe₂Al(Si₂Al₂)O₁₀(OH)₂
- Mohs hardness
- 2.5-3
- Density
- 3.1 g/cm³
- Streak
- Greenish gray
- Luster
- Vitreous to dull
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Foliated, micaceous
- Cleavage
- {001}, perfect
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- In pegmatites in nepheline syenite; in greisens in granite and aplite
Where rockhounds find siderophyllite
Classic worldwide localities
- Pikes Peak, El Paso Co., Colorado, and in the Brooks Range, Alaska, USA
- Mont Saint-Hilaire, Quebec, Canada
- Altenberg, Saxony, Germany
- Reschen Pass, Trentino-Alto Adige, Italy
- Badzhala, Ural Mountains, Russia
- Newcastle, Co. Down, Ireland
Field-hunting tip
Where it forms: In pegmatites in nepheline syenite; in greisens in granite and aplite. If you start seeing microcline, astrophyllite in float, you are in the right ground. Typical habit: foliated, micaceous. Train your eye for that shape before scanning the outcrop.
Common questions
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Find siderophyllite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
