Fluorapophyllite

Fluorapophyllite is a mineral of the apophyllite group, with the chemical formula of KCa4Si8O20(F, OH)·8(H2O). It is a particular apophyllite mineral with more fluorine than hydroxide compared to similar minerals. It gets the first half of its name, “fluor”, from containing more fluorine than hydroxide compared to the other minerals in the apophyllite group. It was given its name because crystals tend to peel or flake apart when they are heated due to the loss of water molecules.

This mineral is found as a secondary mineral in vesicules in volcanic rocks such as basalt. It is popular among many mineral collectors because of the large, well-developed crystals they form and the multiple colors they come in.

General Information

  • Category: Phyllosilicate
  • Formula: KCa4Si8O20(F, OH)·8(H2O)
  • Crystal system: Tetragonal
  • Crystal class: Ditetragonal dipyramidal (4/mmm)

Properties

Fluorapophyllite crystallizes in the tetragonal crystal system. Tetragonal minerals have three axes of different lengths and angles of 90 degrees. Fluorapophyllite is an anisotropic mineral and has low relief. This mineral belongs to the uniaxial (+) optical class, which means its indicatrix has a prolate sphenoid shape with a circular section, principal section, and one optic axis.

  • Color: Colorless, white, pink, pale, yellow, green; in thin section, colorless
  • Crystal habit: Tabular to prismatic crystals; maybe pseudocubic
  • Fracture: Uneven
  • Tenacity: Brittle
  • Mohs scale hardness: 4.5-5
  • Luster: Vitreous, pearly on {001}
  • Diaphaneity: Transparent to translucent
  • Specific gravity: 2.33-2.37
  • Optical properties: Uniaxial (+)

Occurrence: A secondary mineral in amygdules or druses in basalts; in cavities in granite; in tactite and other metamorphic rocks; a late-stage hydrothermal mineral in some mineral deposits.

The most wanted variation of fluorapophyllite is the green colored variant, which is found in India. Fluorapophyllite is also found in New Jersey of the United States.

Association: Zeolites, datolite, pectolite, calcite, quartz.

 

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