Ajoite: Occurrence and Properties

Ajoite is a hydrated sodium potassium copper aluminum silicate hydroxide mineral. Ajoite has the chemical formula (Na,K)Cu7AlSi9O24(OH)6.3H2O and minor Mn, Fe and Ca are usually also present in the structure. Ajoite is used as a minor ore of copper.

General information

  • Category: Silicate mineral
  • Formula: (Na,K)Cu7AlSi9O24(OH)6.3H2O
  • Crystal system: Triclinic.

Physical properties

The cleavage is perfect parallel to the plane containing and the hardness is 3.5, between that of calcite and fluorite. It is a little denser than quartz, with specific gravity 2.96. Ajoite is readily decomposed by hydrochloric acid, HCl, and by nitric acid, HNO3, but it does not react with ammonium hydroxide, NH4OH. It has been reported as having barely detectable radioactivity.

  • Transparency: Translucent
  • Color: Bluish green
  • Streak: Greenish white
  • Cleavage: Perfect on {010}
  • Density: 2.96 g/cm3 (Measured) and 2.951 g/cm3 (Calculated)

Environment

Ajoite is a secondary mineral that forms from the oxidation of other secondary copper minerals in copper-rich base metal deposits in massive fracture coatings, in vein fillings, and in vugs. It may form from shattuckite and also it may be replaced by shattuckite.

Identification

  • Color: Bluish green
  • Crystal habit: Sprays of bladed prismatic crystals
  • Mohs scale hardness: 3 ½
  • Luster: Vitreous
  • Streak: Greenish white
  • Diaphaneity: Translucent
  • Specific gravity: 2.96

Occurrence

Ajoite is named after its type locality, the New Cornelia Mine in the Ajo District of Pima County, Arizona. Type material is conserved at the National Museum of Natural History, Washington DC, USA, reference number 113220.

Other localities include Wickenburg and Maricopa County in Arizona, within the United States, and the Messina (Musina) District in South Africa. Quartz specimens from the defunct Messina Mines on the border between Zimbabwe and South Africa are well known for their inclusions of blue copper silicate minerals such as shattuckite, and ajoite, but ajoite from American localities does not occur like this.

 

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