Phosphammite is a rare ammonium phosphate mineral that typically forms as a secondary mineral in guano deposits. It is highly soluble and hygroscopic, meaning it should be stored in a dry, sealed container to prevent deliquescence.

Hardness

1.5-2

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrare

Phosphammite

Hardness: 1.5-2
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this phosphammite?

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 phosphammite with a known reference. Phosphammite sits at Mohs 1.5-2, 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.
4
Match the color range
Compare against the expected color range: colorless.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: crusts, granular, powdery aggregates.

Often confused with

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

mineral

Boussingaultite

Mohs 2

How to tell apart: Luster reads vitreous on Phosphammite and silky on Boussingaultite.

Often found alongside phosphammite

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

All properties

Chemical formula: (NH₄)₂HPO₄
Mohs hardness: 1.5-2
Density: 1.61 g/cm³
Colors: Colorless
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Monoclinic
Crystal habit: Crusts, granular, powdery aggregates
Cleavage: Perfect
Rarity: Rare
Uses: Collector
Host rock: Probably a crystallization product of the liquid fraction of guano

Where rockhounds find phosphammite

Classic worldwide localities

Guañape Island, south of Trujillo, Peru
Toppin Hill Cave, about 320 km northeast of Kalgoorlie, Western Australia
San Pedro, Martin Island, Mexico

Field-hunting tip

Where it forms: Probably a crystallization product of the liquid fraction of guano. If you start seeing ammonian aphthitalite, urea, weddellite in float, you are in the right ground. Typical habit: crusts, granular, powdery aggregates. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify phosphammite?+

Mohs hardness is 1.5-2. Luster: vitreous. The streak is white. Common colors include colorless.

Where is phosphammite found?+

Notable localities include Guañape Island, south of Trujillo, Peru; Toppin Hill Cave, about 320 km northeast of Kalgoorlie, Western Australia; San Pedro, Martin Island, Mexico.

What rocks look like phosphammite?+

Phosphammite is most often confused with Boussingaultite, Mascagnite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with phosphammite?+

Phosphammite commonly co-occurs with Ammonian aphthitalite, Urea, Weddellite, Biphosphammite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does phosphammite form in?+

Where it forms: Probably a crystallization product of the liquid fraction of guano. Working float back to the host body is the standard way to chase a fresh occurrence.

What is phosphammite used for?+

Phosphammite is used in collector.

Find phosphammite on the map

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