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.

Hardness

2.5-3

Mohs

Luster

Pearly

Streak

White

Transparency

Translucent

mineraluncommonMica Group family

Siderophyllite

Hardness: 2.5-3
Luster: Pearly
Streak: White
Transparency: Translucent

Is this siderophyllite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try 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.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Siderophyllite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Siderophyllite typically shows a pearly luster.
4
Match the color range
Compare against the expected color range: black, brown, dark brown.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: pseudohexagonal tabular crystals.

Often confused with

Siderophyllite vs. its common look-alikes — and how to tell them apart in the field.

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: KFe²⁺₂Al(AlSi₂O₁₀)(OH)₂
Mohs hardness: 2.5-3
Density: 3.0-3.1 g/cm³
Colors: Black Brown Dark Brown
Streak: White
Luster: Pearly
Transparency: Translucent
Crystal system: Monoclinic
Crystal habit: Pseudohexagonal Tabular Crystals
Cleavage: Perfect Basal
Rarity: Uncommon
Uses: Collector
Host rock: Alkaline Granite Pegmatites
Typical price: $10-50 per specimen depending on crystal size

Where rockhounds find siderophyllite

Classic worldwide localities

Pikes Peak, Colorado, USA
Khibiny Massif, Russia
Langesundsfjord, Norway

Field-hunting tip

Look in alkaline granite pegmatites country — that is the host setting where siderophyllite typically forms. If you start seeing microcline, quartz, zircon in float, you are in the right ground. Field specimens usually show a pseudohexagonal tabular crystals habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify siderophyllite?+

Mohs hardness is 2.5-3. It typically shows a pearly luster. The streak is white. Common colors include black, brown, dark brown.

Where is siderophyllite found?+

Notable localities include Pikes Peak, Colorado, USA; Khibiny Massif, Russia; Langesundsfjord, Norway.

How much is siderophyllite worth?+

Typical asking prices fall in the range of $10-50 per specimen depending on crystal size. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like siderophyllite?+

Siderophyllite is most often confused with Biotite, Annite, Phlogopite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with siderophyllite?+

Siderophyllite commonly co-occurs with Microcline, Quartz, Zircon, Fluorite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does siderophyllite form in?+

Siderophyllite typically forms in alkaline granite pegmatites. Working float back to the host body is the standard way to chase a fresh occurrence.

What is siderophyllite used for?+

Siderophyllite is used in collector.

Find siderophyllite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.