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

3-4

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrarePhosphophyllite Group family

Guimarãesite

Hardness: 3-4
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 3-4 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Guimarãesite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Guimarãesite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: colorless, white, pale yellow.
5
Look at form & habit
Crystal system: trigonal. Typical habit: tabular crystals.

Often confused with

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

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: Zn₅(PO₄)₂(OH)₄·H₂O
Mohs hardness: 3-4
Density: 2.83 g/cm³
Colors: Colorless White Pale Yellow
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Trigonal
Crystal habit: Tabular Crystals
Cleavage: Perfect
Rarity: Rare
Uses: Collector
Host rock: Phosphate-rich Pegmatites
Typical price: $100-500 for micro/thumbnail specimens

Where rockhounds find guimarãesite

Classic worldwide localities

Tip Top mine, South Dakota, USA

Field-hunting tip

Look in phosphate-rich pegmatites country — that is the host setting where guimarãesite typically forms. If you start seeing tarbuttite, hydroxylapatite, quartz in float, you are in the right ground. Field specimens usually show a tabular crystals habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify guimarãesite?+

Mohs hardness is 3-4. It typically shows a vitreous luster. The streak is white. Common colors include colorless, white, pale yellow.

Where is guimarãesite found?+

Notable localities include Tip Top mine, South Dakota, USA.

How much is guimarãesite worth?+

Typical asking prices fall in the range of $100-500 for micro/thumbnail specimens. Quality, size, and provenance can move individual specimens well outside that range.

Is guimarãesite safe to handle?+

It contains toxic constituents. Contains zinc and phosphorus; wash hands thoroughly after handling and avoid inhaling dust. Handle with care, avoid grinding or breathing dust, and store separately.

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 Tarbuttite, Hydroxylapatite, Quartz. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does guimarãesite form in?+

Guimarãesite typically forms in phosphate-rich pegmatites. 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.