Centennialite

Chemical formula: CaCu<sup>2+</sup><sub>3</sub>Cl<sub>2</sub>(OH)<sub>6</sub>·nH<sub>2</sub>O (n ~ 0.7)

An extremely rare, blue secondary copper mineral, discovered in the Centennial Mine in Michigan, USA.

## Characteristics Centennialite is a hydrated calcium copper hydroxychloride, occurring as microscopic, bladed crystals. It forms thin, rosetted or spherulitic aggregates of an intense blue color. Due to the small size of the crystals, its features are most often observed under a microscope. It is a secondary mineral, formed by the oxidation of copper deposits. ## Physical Properties Crystals are too small to determine hardness or density in a standard way. The mineral exhibits a blue luster and is transparent to translucent. The streak is light blue. ## Colors and Varieties Centennialite occurs in a uniform, light to intense blue. No varieties of this mineral have been distinguished. ## History and Name The mineral's name comes from its discovery locality – the Centennial Mine in Houghton County, Michigan, USA. It was first described in 1997 by George W. Robinson and John A. Jaszczak. The Centennial Mine, being the type locality, is known for the occurrence of many rare copper minerals. ## Uses Centennialite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals (so-called microminerals) and scientists studying the mineralogy of secondary copper minerals.

Properties

Luster
Vitreous
Streak
Light blue
Transparency
Transparent to translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Identification of centennialite is based on its characteristic blue color, its occurrence as small, spherulitic aggregates, and its origin from a specific locality. Identification is possible almost exclusively under magnification, and final confirmation requires advanced analytical methods such as Raman spectroscopy or X-ray diffraction. ## Distinguishing from similar minerals It can be confused with other secondary, blue copper minerals such as calumetite, anthonite, or cyanotrichite. Distinguishing it from these without specialized equipment is practically impossible. Key factors are its association with the type locality (Centennial Mine) and co-occurring minerals. ## Crystal forms Centennialite forms microscopic, bladed or acicular crystals that combine into radial, spherulitic, or rosetted aggregates. Individual crystals rarely exceed a few tens of micrometers in length.

Geological environment

## Genesis Centennialite is a secondary mineral that forms in the oxidation zone of native copper deposits. It is created by the reaction of chloride-rich waters with copper minerals and calcium-rich rocks, such as basalts. Its formation is associated with the activity of groundwater within old mining workings. ## Mineral associations This mineral co-occurs with other secondary copper minerals. It is most often found in association with calumetite, anthonite, native copper, cuprite, epidote, and quartz. It occurs on weathered basalt fragments. ## Localities The only confirmed locality of centennialite in the world is its type locality: the dumps of the Centennial Mine, located near the town of Calumet, in Houghton County on the Keweenaw Peninsula in Michigan, USA.

Rarity

Extremely rare

For collectors

## Quality criteria The quality of centennialite specimens is assessed based on the size and richness of the aggregates on the rock matrix. Most desirable are specimens with clearly formed, intensely blue spherulites, contrasting with the color of the host rock. The aesthetic arrangement of the aggregates and the absence of damage are also important. ## Popular localities The only source of specimens is the Centennial Mine in Michigan, USA. Material primarily comes from historical collections or is acquired by collectors searching old mine dumps. Specimens from this locality are unique and highly valued among specialized micromineral collectors.

Care and storage

## Cleaning Centennialite specimens are extremely delicate and usually microscopic. They should not be cleaned mechanically or chemically. Any attempts at cleaning can destroy the fragile structure of the aggregates. If dust removal is necessary, a photographic air blower can be used carefully. ## What to avoid Avoid contact with water, acids, bases, and all chemical agents. The mineral is likely unstable under varying humidity and temperature conditions. Avoid vibrations, shocks, and touching. ## Storage Specimens should be stored only under stable conditions, in tightly sealed "micromount" boxes, to protect them from dust, moisture, and mechanical damage. Display should only take place in a controlled environment.

Sources

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