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, 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.