Quartz with cacoxenite - cut Locality : Minas Gerais - Brasil 15Ct13 — Photo: Didier Descouens · CC BY 4.0
Cacoxenite
An iron-aluminium phosphate associated with iron ores; its name means 'bad guest' — the phosphorus it carries ruins smelted iron.
Intro adapted from Wikipedia (CC BY-SA).
Cacoxenite is an IMA-recognized mineral species in the phosphate class, with the chemical formula Fe³⁺₂₄AlO₆(PO₄)₁₇(OH)₁₂·75H₂O.
It crystallizes in the hexagonal crystal system (space group P6_3/m).
Key properties: Mohs hardness 3.5; specific gravity 2.4; Silky luster; refractive index 1.613; optically uniaxial.
IMA symbol Cac, grandfathered status — known before the IMA’s founding, named in 1826.
A common accessory mineral in oxidized magnetite and "limonite" iron ores; in Fe, Mn-bearing novaculites; a rare constituent of iron-rich sediments and soils.
It is found associated with Dufrenite, rockbridgeite, beraunite, strengite, wavellite, magnetite, "limonite".
Its name comes from the Greek for a bad guest, for the phosphorus content that degrades the quality of iron made from the host ores.