Cacoxenite: Formation, Properties, Types

Cacoxenite is a rare mineral that typically occurs as radiating needle-like crystals or fibrous masses. Its color ranges from yellow to brownish-yellow.  Cacoxenite is a hydrous iron aluminum phosphate mineral, with the chemical formula Fe³⁺₂₄Al(PO₄)₁₇O₆(OH)₁₂·17(H₂O).

Cacoxenite was first described in 1825 for an occurrence in the Hrbek Mine, Bohemia, Czech Republic. It occurs as a secondary phase in oxidized magnetite and limonite deposits. It also occurs in novaculites and in iron and phosphorus rich sediments.

Often times, it forms as fuzzy brownish yellow, brown, yellow, or gold radiated tufts or strands. It may also appear as reddish orange or greenish yellow. The cacoxenite is occasionally referred to cacoxene or cacoxitite.

The name "Cacoxenite" comes from the Greek words "kakos" meaning "bad" and "xenos" meaning "guest" or "stranger." This name was originally given to the mineral due to its unusual and often unattractive appearance as inclusions within other gemstones. However, over time, the meaning has shifted to reflect the stone's powerful and transformative properties.

Radial Cacoxenite Crystals. La Paloma Mine, Zarza la Mayor, Mancomunidad Rivera de Fresnedosa, Cáceres, Extremadura, Spain. Photo: Rewitzer Christian.

Cacoxenite is a mineral that is commonly an inclusion in quartz, especially amethyst. The inclusions in amethyst often detract from the amethyst as the brown acicular needles dampen the pure purple color to a less desirable brownish hue. The inclusions of cacoxenite will certainly ruin any chance for the host stone to become a gemstone. This is not to say that some specimens of cacoxenite included quartz have no value, for indeed some are actually enhanced with interesting surreal landscaping effects.

Cacoxenite Formation

Cacoxenite is a secondary mineral, meaning it forms through the alteration of other minerals. Its primary source is lithiophilite, a lithium-manganese phosphate mineral found within pegmatites.

The transformation involves two key geochemical processes: oxidation and leaching.

Oxidation: Manganese ions (Mn²⁺) within the lithiophilite crystal lattice undergo conversion to manganese ions (Mn³⁺) by losing an electron. This change in oxidation state alters the mineral's chemical structure and ultimately its color.

Leaching: Simultaneously, lithium ions (Li⁺) are mobilized and transported away from the lithiophilite lattice by hydrothermal fluids, leaving behind vacancies within the crystal structure. This leaching process destabilizes the original mineral.

Transformation:

As oxidation and leaching progress, the altered lithiophilite gradually transforms into cacoxenite.

Vacancies created by lithium leaching are occupied by manganese ions (Mn³⁺), leading to the characteristic iron-aluminum phosphate composition of cacoxenite (Fe³⁺Al₂⁴(PO₄)₁⁷O₆(OH)₁₂·17(H₂O)).

This rearrangement alters the mineral's crystal structure, resulting in a loss of the original lithiophilite color and the emergence of the distinctive reddish-orange, greenish-yellow, or earthy brown hues of cacoxenite.

Cacoxenite from Rio Monti Nieddu, Sarroch, Cagliari Province, Sardinia, Italy

What are the different types of Cacoxenite?

Fibrous Cacoxenite: The most common form, characterized by fine, elongated crystals arranged in a wispy or thread-like pattern within host minerals like quartz or amethyst.

Radial Cacoxenite: This variety exhibits radiating aggregates of acicular crystals, resembling a sunburst with golden or brown needles extending outwards from a central point.

Acicular Cacoxenite: Composed of slender, needle-like crystals, typically golden or brown, occurring as isolated inclusions or forming radiating clusters.

Super Seven: The most well-known type, featuring Cacoxenite inclusions within amethyst, often accompanied by other minerals like Clear Quartz, Lepidocrocite, Goethite, and Rutile.

Cacoxenite crystals inside a rock From Leonie I Mine, Auerbach, Upper Palatinate, Bavaria, Germany

Properties of Cacoxenite