Charlesite is an uncommon sulfate mineral belonging to the ettringite group, occurring as colorless to pale yellow hexagonal crystals. It is most famous for its occurrence in the zinc mines of Franklin and Sterling Hill, New Jersey, typically found in association with silicate and carbonate minerals.

Hardness

~2.5

Mohs

Luster

Vitreous to dull

Streak

White

Transparency

Transparent, may appear opaque due to surficial alteration

mineralrareEttringite Group family

Charlesite

Hardness: ~2.5
Luster: Vitreous to dull
Streak: White
Transparency: Transparent, may appear opaque due to surficial alteration

Is this charlesite?

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 charlesite with a known reference. Charlesite sits at Mohs ~2.5, 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 to dull.
4
Match the color range
Compare against the expected color range: colorless, light yellow, pink.
5
Look at form & habit
Crystal system: trigonal. Typical habit: crystalline.

Often confused with

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

How to tell apart: Thaumasite is the harder of the two (Mohs 3.5 vs. ~2.5).

Compare →

Often found alongside charlesite

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

All properties

Chemical formula: Ca₆Al₂(SO₄)₂B(OH)₄(OH,O)₁₂·26H₂O
Mohs hardness: ~2.5
Density: 1.77 g/cm³
Colors: Colorless Light yellow Pink
Streak: White
Luster: Vitreous to dull
Transparency: Transparent, may appear opaque due to surficial alteration
Crystal system: Trigonal
Crystal habit: Crystalline
Cleavage: Perfect on 1010
Fluorescence: Light violet or green under SW and LW UV
Rarity: Rare
Uses: Collector
Host rock: Very rare, coating fractures and vugs in a metamorphosed stratiform zinc deposit

Where rockhounds find charlesite

Classic worldwide localities

Franklin, Sussex Co., New Jersey, USA

Field-hunting tip

Where it forms: Very rare, coating fractures and vugs in a metamorphosed stratiform zinc deposit. If you start seeing clinohedrite, ganophyllite, xonotlite in float, you are in the right ground. Typical habit: crystalline. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify charlesite?+

Mohs hardness is ~2.5. Luster: vitreous to dull. The streak is white. Common colors include colorless, light yellow, pink.

Where is charlesite found?+

Notable localities include Franklin, Sussex Co., New Jersey, USA.

What rocks look like charlesite?+

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

What minerals are found with charlesite?+

Charlesite commonly co-occurs with Clinohedrite, Ganophyllite, Xonotlite, Pectolite, Prehnite, Mn-chlorite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does charlesite form in?+

Where it forms: Very rare, coating fractures and vugs in a metamorphosed stratiform zinc deposit. Working float back to the host body is the standard way to chase a fresh occurrence.

What is charlesite used for?+

Charlesite is used in collector.

Find charlesite on the map

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