Cuprite: Properties & Facts

Cuprite is a mineral belonging to the oxide group with the chemical formula Cu₂O (copper(I) oxide). Renowned for its deep red to brownish-red hue, cuprite can occasionally appear almost black. It crystallizes in the isometric system, typically forming cubic, octahedral, or dodecahedral shapes, often in combination. Penetration twins are common, adding to its geometric complexity.

Cuprite's dark crystals often exhibit internal reflections of a rich, deep red, creating a striking contrast against their nearly opaque exteriors. Another variety, chalcotrichite, consists of tufts of needle-like crystals that display a brilliant red color and a unique sparkle, making it a popular choice for mineral collectors.

Cuprite, also known as ruby copper due to its vivid red color, has historically been an important ore of copper and continues to be mined in various locations worldwide. Among copper ores—excluding native copper—cuprite yields the highest amount of copper per molecule, as its structure contains two copper atoms for every one oxygen atom. This high copper content makes it a valuable resource in copper extraction.

In its natural state, cuprite forms well-developed cubic, octahedral, or dodecahedral crystals, which belong to the hexoctahedral class of the isometric crystal system. These forms contribute to its aesthetic appeal and make it a favored specimen among mineral enthusiasts.

Cuprite
Stunning Cuprite Crystals. From: Villecun, Olmet-et-Villecun, Hérault, Occitanie, France
Photo by: Laurent Kbaier
It is a secondary mineral which forms in the oxidized zone of copper sulfide deposits. It frequently occurs in association with native copper, azurite, chrysocolla, malachite, tenorite and a variety of iron oxide minerals. It is known as ruby copper due to its distinctive red color.

Properties of Cuprite

Chemical Formula : Cu₂O
Crystal System : Isometric (Cubic)

Physical Properties

  • Color : Ranges from dark red to cochineal red, occasionally appearing almost black. Internal reflections often reveal a deep red hue within darker crystals.
  • Lustre : Adamantine to sub-metallic; earthy in massive forms.
  • Crystal Habits : Cuprite typically crystallizes in cubic, octahedral, or dodecahedral forms, with combinations of these shapes being common. Rarely, it exhibits faces of the obscure gyroid form. A notable variety, chalcotrichite , forms long needle-like crystals or fuzzy crusts. Massive forms also occur.
  • Cleavage : Fair in four directions, forming octahedrons.
  • Fracture : Conchoidal.
  • Hardness : 3.5–4 on the Mohs scale.
  • Specific Gravity : Approximately 6.0 (very heavy for translucent minerals).
  • Streak : Brick red.

Optical and Other Characteristics

  • Transparency : Translucent to opaque.
  • Surface Alteration : Prolonged exposure to strong light may cause cuprite to form a surface film. Crystals are sometimes partially or fully altered to malachite, and rarely to native copper.

Best Field Indicators

The most reliable field indicators for identifying cuprite are its distinctive color , characteristic crystal forms , and relatively low hardness compared to other metallic minerals. Its brick-red streak and high specific gravity further aid in identification.

Cuprite
Outstanding single crystal from the DR Congo.

Varieties

Chalcotrichite : A variety of cuprite characterized by tufts of acicular (needle-like) crystals, displaying a vibrant red color and a sparkling luster.

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