It is an oxide mineral belonging to with magnesiochromite
(MgCr O ); substitution of aluminum occurs leading to hercynite (FeAl O ).2 4
The Etchison Chrome mine was sunk into a serpentine-talc schist body which yielded magnesiochromite
ore, along with chromian tourmaline, margarite, rutile and magnesite.
Eskolaite occurs as massive green alteration halos around magnesiochromite grains in the silicified serpentinite rock margins with the silica-filled fractures.
Ferroan magnesiochromite is common both as anhedral masses and as octahedral crystals in the serpentinite.
The silica-carbonate rocks contain variable amounts of coarsely crystalline quartz and its varietal forms, chalcedony and opal, with associated carbonate minerals (magnesite, dolomite, ankerite and calcite) and abundant microcrystals and grains of ferroan magnesiochromite and magnetite.
Within the silicified serpentinite margins adjacent to the quartz-rich veins are abundant masses and crystals of ferroan magnesiochromite that show dark green rims.
Birmingham (1987) reported the occurrence of magnesiochromite
as dark brown inclusions in olivine in monchiquite, phonotephrite and tephriphonolite.
When these carbonates are found associated with ultrabasic rocks containing chromium minerals such as magnesiochromite, appreciable amounts of trivalent chromium can substitute for aluminum in their structures during formation.
Chromian dawsonite occurs as isolated spherulites in cavities and as flattened spherulitic masses up to 4 mm in diameter along thin cross seams in a tan, hydrothermally altered (silicified) serpentinite containing dolomite, barite, pyrite and magnesiochromite. The dawsonite varies from nearly white to very light pink (lowest chromium) to a dark reddish violet (highest chromium), which reflects the variable chromium content.
These minerals are found as cavity fillings in a leached vein composed of quartz with minor dolomite, nickeliferous serpentinite and altered grains of magnesiochromite.
The magnesium content of 0.40 to 0.43 atoms per unit formula indicates a composition almost midway between chromite and magnesiochromite