Erdite, a new hydrated sodium iron sulfide mineral

The new mineral erdite, NaFeS₂ · 2H₂O, has been found in abundance in a mafic, alkalic diatreme at Coyote Peak, Humboldt County, California. Crystallization of erdite, the first sodium-bearing sulfìde to be found in nature, was dependent upon unusual, late-magmatic chemistry; its preservation can be ascribed to the youthful age (29 m.y.) and the remarkably dense and impervious nature of the mafic host rock.

Erdite is a striking mineral, showing a copper-red color on fresh surfaces and an extreme reflection pleochroism, brilliant reddish orange to dark gray, in polished surfaces. Erdite is monoclinic, C2/c, a = 10.693(1), b = 9.115(1), c = 5.507(4)A, β = 92° 10(2)′, V = 536.3A³, Z = 4[NaFeS₂ · 2H₂O]; density (calc.) 2.216, (meas.) 2.30(1) g/cm³ for samples containing about 3% magnetite. The six strongest lines of the X-ray diffraction powder pattern are: 6.935,110,(100); 5.342,200,(71); 4.556,020,(41); 3.467,220,(28); 2.310,330,(23); 2.902,221,(15). Reflectance in air at 540 nm is 8.8% for R₁, 20.7% for R₂. Microindentation hardness (VHN) at 15-gram load is 22–67, mean 39.