Sigloite is a rare phosphate mineral typically found as a secondary oxidation product in tin-bearing hydrothermal deposits. It is most frequently encountered as small, tabular, yellowish crystals often associated with other phosphate minerals like vauxite and paravauxite.

Hardness

Mohs

Luster

Vitreous to pearly

Streak

White

Transparency

Semitransparent

mineralrareParavauxite Group family

Sigloite

Hardness: 3
Luster: Vitreous to pearly
Streak: White
Transparency: Semitransparent

Is this sigloite?

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 sigloite with a known reference. Sigloite sits at Mohs 3, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: white.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous to pearly.
4
Match the color range
Compare against the expected color range: brown, light brown, light yellow, straw yellow.
5
Look at form & habit
Crystal system: triclinic. Typical habit: pseudomorphous.

Often confused with

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

Often found alongside sigloite

Minerals reported to co-occur with sigloite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: FeAl₂(PO₄)₂(OH)₃·7H₂O
Mohs hardness: 3
Density: 2.35 g/cm³
Colors: Brown Light brown Light yellow Straw yellow
Streak: White
Luster: Vitreous to pearly
Transparency: Semitransparent
Crystal system: Triclinic
Crystal habit: Pseudomorphous
Cleavage: On {010}, perfect; on {001}, good
Rarity: Rare
Uses: Collector
Host rock: A rare late-stage secondary mineral, formed by oxidation of vauxite, in open fractures cutting cassiterite veins in a hydrothermal tin deposit

Where rockhounds find sigloite

Classic worldwide localities

Found in the Siglo XX mine, Llallagua, Potosí, Bolivia

Field-hunting tip

Where it forms: A rare late-stage secondary mineral, formed by oxidation of vauxite, in open fractures cutting cassiterite veins in a hydrothermal tin deposit. If you start seeing wavellite, metavauxite, crandallite in float, you are in the right ground. Typical habit: pseudomorphous. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify sigloite?+

Mohs hardness is 3. Luster: vitreous to pearly. The streak is white. Common colors include brown, light brown, light yellow, straw yellow.

Where is sigloite found?+

Notable localities include Found in the Siglo XX mine, Llallagua, Potosí, Bolivia.

What rocks look like sigloite?+

Sigloite is most often confused with Vauxite, Paravauxite, Metavauxite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with sigloite?+

Sigloite commonly co-occurs with Wavellite, Metavauxite, Crandallite, Childrenite, Vauxite, Quartz. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does sigloite form in?+

Where it forms: A rare late-stage secondary mineral, formed by oxidation of vauxite, in open fractures cutting cassiterite veins in a hydrothermal tin deposit. Working float back to the host body is the standard way to chase a fresh occurrence.

What is sigloite used for?+

Sigloite is used in collector.

Find sigloite on the map

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