Centennialite is a very rare calcium-copper silicate mineral discovered in the mine dumps of the Centennial Mine in Michigan. It typically occurs as small, delicate, azure blue platy crystals associated with native copper and other secondary minerals in basaltic voids.

Hardness

Mohs

Transparency

Translucent

mineralrare

Centennialite

Hardness: 2
Transparency: Translucent

Is this centennialite?

3-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 centennialite with a known reference. Centennialite sits at Mohs 2, softer than the next harder reference, harder than the previous one.
2
Match the color range
Compare against the expected color range: azure blue, pale.
3
Look at form & habit
Crystal system: trigonal. Typical habit: platy crystals, radial aggregates.

Often confused with

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

mineral

Calcite

Mohs 3

How to tell apart: Calcite is the harder of the two (Mohs 3 vs. 2).

Often found alongside centennialite

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

Calumetite
mineral · Mohs 2

All properties

Chemical formula: CaCu₃Cl₂(OH)₆·nH₂O (n ~ 0.7)
Mohs hardness: 2
Density: 3.100 g/cm³
Colors: Azure bluePale
Transparency: Translucent
Crystal system: Trigonal
Crystal habit: Platy crystals, radial aggregates
Rarity: Rare
Uses: Collector
Host rock: A secondary low-temperature mineral formed by the reaction of acidic water with other copper mineralization and essentially physically indivisible from other copper-containing secondary minerals

Where rockhounds find centennialite

Classic worldwide localities

Analytically confirmed from the Lake Superior native copper district, Michigan, USA, specifically the Ahmeek, Quincy, White Pine, Mohawk, and Franklin Jr. mines

Field-hunting tip

Where it forms: A secondary low-temperature mineral formed by the reaction of acidic water with other copper mineralization and essentially physically indivisible from other copper-containing secondary minerals. If you start seeing calumetite, atacamite family minerals in float, you are in the right ground. Typical habit: platy crystals, radial aggregates. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify centennialite?+

Mohs hardness is 2. Common colors include azure blue, pale.

Where is centennialite found?+

Notable localities include Analytically confirmed from the Lake Superior native copper district, Michigan, USA, specifically the Ahmeek, Quincy, White Pine, Mohawk, and Franklin Jr. mines.

Is centennialite safe to handle?+

Centennialite contains copper. Wash your hands afterwards, never grind or breathe the dust, and store it apart from specimens you handle often.

What rocks look like centennialite?+

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

What minerals are found with centennialite?+

Centennialite commonly co-occurs with Calumetite, Atacamite family minerals. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does centennialite form in?+

Where it forms: A secondary low-temperature mineral formed by the reaction of acidic water with other copper mineralization and essentially physically indivisible from other copper-containing secondary minerals. Working float back to the host body is the standard way to chase a fresh occurrence.

What is centennialite used for?+

Centennialite is used in collector.

Find centennialite on the map

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