Yellow crystal aggregates of holtedahlite from the type locality: Tingelstadtjern Quarry, Modum, Viken, Norway. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Holtedahlite
Holtedahlite is an IMA-recognized mineral species in the phosphate class, with the chemical formula Mg₁₂(PO₃OH,CO₃)(PO₄)₅(OH,O)₆.
It crystallizes in the hexagonal crystal system (space group P31m). It is a member of the Holtedahlite structural group.
Key properties: Mohs hardness 4.5–5; specific gravity 2.94; Vitreous luster; optically uniaxial.
IMA symbol Htd, IMA status: approved, named in 1976.
Found in a serpentine–magnesite deposit (Tingelstadtjern quarry, Norway); a common accessory mineral, of likely pneumatolytic origin, in an igneous magnetite–apatite deposit (Gole Gohar, Iran).
It is found associated with Althausite, szaibelyite, apatite, talc, magnetite, magnesite (Tingelstadtjern quarry, Norway).
The mineral was named to honor Olaf Holtedahl (1885–1975), Professor of Geology, University of Oslo, Oslo, Norway.