Cacoxenite

Chemical formula: Fe<sup>3+</sup><sub>24</sub>AlO<sub>6</sub>(PO<sub>4</sub>)<sub>17</sub>(OH)<sub>12</sub>·75H<sub>2</sub>O

Cacoxenite is a mineral from the phosphate group, known for forming radial, golden-brown inclusions in quartz and amethyst, which significantly increase their collector's value.

## Characteristics Cacoxenite is a hydrated iron and aluminum phosphate that rarely forms independent, well-developed crystals. It most often occurs as radial or fibrous aggregates and acicular inclusions in other minerals, especially in quartz. These inclusions take the form of golden, yellow, or brownish "tufts" and "brushes" scattered within the host mineral. Individual crystals, when found, are very small, acicular, and gathered in radial or spherical clusters. ## Physical Properties This mineral is very soft, with a Mohs hardness of 3-4. It is characterized by a silky or dull luster. It is translucent to opaque. Due to its nature – most often in the form of fine inclusions – its physical properties are rarely possible to examine in isolation. ## Colors and Varieties Cacoxenite occurs in shades from yellow, through golden-brown, reddish-brown, to greenish. Its color is characteristic and is the main identifying feature of the inclusions. No named varieties are distinguished, however, in the ornamental stone trade, the term "amethyst with cacoxenite" or "cacoxenite quartz" is often used for specimens containing its inclusions. It should be noted that many commercial specimens sold under this name actually contain goethite inclusions, not cacoxenite. ## History and Name The mineral's name comes from the Greek words *kakos* (κακός) meaning "bad" or "worthless" and *xenos* (ξένος) meaning "guest" or "stranger". This name, given in 1825 by Karl Cäsar von Leonhard, can be translated as "bad guest" because the presence of phosphorus (originating from cacoxenite) in iron ores lowered the quality of the metal smelted from them. ## Uses Cacoxenite has no industrial application. Its only significance is scientific and collectible, particularly as visually attractive inclusions in other minerals, mainly in quartz and amethyst from Brazil.

Properties

Mohs hardness
3-4
Luster
Silky
Streak
Yellowish-brown
Density
3.394
Cleavage
None
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Hexagonal

Diagnostic features

## Identification Cacoxenite is most easily identified by its characteristic form and color when it occurs as inclusions in quartz. There, it forms radial, brush-like clusters ranging in color from golden-yellow to brown. Independent aggregates are rare and are characterized by a silky luster and low hardness. ## Distinguishing from Similar Minerals It is most often confused with goethite, which also forms acicular, radial inclusions in quartz. Differentiation is often impossible without advanced studies (e.g., Raman spectroscopy or X-ray diffraction). Many commercial specimens labeled as "cacoxenite quartz" actually contain goethite. Another similar mineral can be rutile, which, however, forms inclusions with a much higher, metallic luster and is harder. ## Crystal Forms Cacoxenite crystals are hexagonal, very fine, and appear as needles or hairs. They almost always occur as aggregates: radial, stellate, spherical, or in the form of bundles and brushes.

Geological environment

## Genesis Cacoxenite is a secondary mineral. It forms in oxidation (weathering) zones of ore deposits, in phosphate-rich pegmatites, and in iron-rich sediments. It often forms as a result of low-temperature hydrothermal processes, crystallizing in rock cavities and fissures, especially in quartz. ## Mineral Associations As a secondary mineral, it co-occurs with other phosphates such as variscite, strengite, rockbridgeite, and wavellite. In iron ore deposits, it is accompanied by limonite and goethite. In the form of inclusions, it is most commonly found in quartz (including amethyst). ## Localities The most famous localities from which independent cacoxenite aggregates originate include the Hellertown mine in Pennsylvania (USA) and the area around Rochefort-en-Terre in France. Specimens with cacoxenite inclusions in amethyst and quartz mainly come from the Brazilian states of Rio Grande do Sul and Minas Gerais. It also occurs in the Czech Republic, Germany, Ireland, and Slovakia.

Rarity

Not very common

For collectors

## Quality Criteria The most highly valued by collectors are specimens in which cacoxenite forms distinct, well-defined, radial inclusions in transparent and clear quartz or intensely colored amethyst. The attractiveness is enhanced by the uniform distribution of inclusions and their intense, golden-yellow color. Independent, well-formed, spherical aggregates of cacoxenite, though less visually striking, are much rarer and also sought after by specialized collectors. ## Popular Localities By far the most desired on the collector's market are specimens of "cacoxenite quartz" (often misidentified, actually containing goethite) originating from Brazil. Historical, classic localities for independent cacoxenite aggregates include mines in the Sauerland region in Germany and in Cornwall in the United Kingdom.

Care and storage

## Cleaning Cacoxenite specimens, especially those in delicate, acicular clusters, should be cleaned very carefully. It is best to use compressed air to remove dust. If wet cleaning is necessary, use only distilled water and a very soft brush, avoiding any scrubbing. Specimens embedded in quartz are much more durable and can be washed in lukewarm water with mild soap. ## What to Avoid Cacoxenite is a very soft and brittle mineral. Avoid contact with harder minerals that can scratch it. It is sensitive to acids and strong chemicals. It should not be heated or subjected to ultrasonic cleaning. ## Storage Loose cacoxenite aggregates should be stored in separate, padded boxes to prevent breakage. Specimens embedded in quartz can be displayed, but it is advisable to avoid prolonged exposure to direct sunlight, which theoretically may affect the color of the inclusions.

External references

Sources

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