Guimarãesite is a very rare zinc phosphate mineral typically found as small, thin, tabular crystals in phosphate-bearing pegmatites. It is visually similar to other rare phosphates and requires professional analysis for positive identification due to its scarcity and small crystal size.

Hardness

~4.5

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrarePhosphophyllite Group family

Guimarãesite

Hardness: ~4.5
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this guimarãesite?

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 guimarãesite with a known reference. Guimarãesite sits at Mohs ~4.5, 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: brown.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: prismatic.

Often confused with

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

mineral

Phosphophyllite

Mohs 3-3.5

How to tell apart: Guimarãesite is noticeably harder (Mohs ~4.5 vs. 3-3.5).

How to tell apart: Guimarãesite is noticeably harder (Mohs ~4.5 vs. 3.5).

Compare →

Often found alongside guimarãesite

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

All properties

Chemical formula: Ca₂Be₄Zn₅(PO₄)₆(OH)₄·6H₂O
Mohs hardness: ~4.5
Density: 2.963 g/cm³
Colors: Brown
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Monoclinic
Crystal habit: Prismatic
Cleavage: Perfect on {100}, good on {010} (by analogy in the group)
Rarity: Rare
Uses: Collector
Host rock: A late fracture-filling mineral in a phosphate-rich granite pegmatite

Where rockhounds find guimarãesite

Classic worldwide localities

Granite pegmatite near the Piauí river, Itinga county, Minas Gerais, Brazil (type locality)

Field-hunting tip

Where it forms: A late fracture-filling mineral in a phosphate-rich granite pegmatite. If you start seeing albite, microcline, quartz in float, you are in the right ground. Typical habit: prismatic. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify guimarãesite?+

Mohs hardness is ~4.5. Luster: vitreous. The streak is white. Common colors include brown.

Where is guimarãesite found?+

Notable localities include Granite pegmatite near the Piauí river, Itinga county, Minas Gerais, Brazil (type locality).

Is guimarãesite safe to handle?+

Guimarãesite contains beryllium. Contains zinc and phosphorus; wash hands thoroughly after handling and avoid inhaling dust.

What rocks look like guimarãesite?+

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

What minerals are found with guimarãesite?+

Guimarãesite commonly co-occurs with Albite, Microcline, Quartz, Elbaite, Lepidolite, Schorl. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does guimarãesite form in?+

Where it forms: A late fracture-filling mineral in a phosphate-rich granite pegmatite. Working float back to the host body is the standard way to chase a fresh occurrence.

What is guimarãesite used for?+

Guimarãesite is used in collector.

Find guimarãesite on the map

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