Minyulite: Properties and Occurrences

Minyulite is a rare phosphate mineral with a chemical formula of KAl2(OH, F)(PO4)2·4(H2O). It is a mineral consisting of a hydrous basic phosphate of potassium and aluminum found in Australia. It occurs as groups of radiating fine fibrous crystals within rock cracks of phosphatic ironstone.

It was first described in 1933 for an occurrence in Western Australia and named after the type locality, Minyulo Well in Western Australia.

General Information

  • Category: Phosphate mineral
  • Formula: KAl2(OH, F)(PO4)24(H2O)
  • Crystal system: Orthorhombic
  • Crystal class: Pyramidal (mm2) (same H-M symbol)
  • Color: Colorless to white, greenish-yellow
  • Formula mass: 372.57 g/mol.

Fig: Minyulite – a rare phosphate mineral

Properties

Minyulite belongs to the orthorhombic crystal system. This indicates that it has three axes of unequal length yet all are perpendicular to each other. As for its optical properties, Minyulite is an anisotropic mineral which means the velocity of light differs when traveling through it depending on the cut of its cross-section which gives it more than one refractive index. Refractive indices are a ratio of the speed of light in a median with respect to the speed of light passing through the mineral.

  • Crystal habit: Radiating fibrous to prismatic crystals
  • Cleavage: {001} Perfect
  • Fracture: Uneven – Flat surfaces (not cleavage) fractured in an uneven pattern.
  • Tenacity: Brittle
  • Mohs scale hardness: 3.5
  • Luster: Vitreous, silky in aggregates
  • Streak: White
  • Diaphaneity: Transparent
  • Specific gravity: 2.45
  • Optical properties: Biaxial (+)

Occurrence: In weathered glauconitic phosphatic ironstone (Australia).

Minyulite is considered as a secondary phosphate since it is formed by the alteration of a primary phosphate. It occurs in association with dufrenite, apatite, fluellite, wavellite, variscite, and leucophosphite.

The mineral can be found in the underlying phosphatized rock zone of ornithogenic soil. Minyulite is not found in abundance, it can be found in the seashore of the maritime Arctic.

Association: Dufr´enite, apatite, fluellite, wavellite, variscite, leucophosphite.