Arfvedsonite
Mineral NaNa2(Fe4Fe)Si8O22(OH)2
About it
Black prisms in alkali granite and syenite pegmatite; its deep blue-grey streak helps tell it from hornblende. Mont Saint-Hilaire, Quebec, Greenland, and Mount Malosa, Malawi.
- Type
- Mineral
- Group
- Silicate (inosilicate), amphibole group
- Formula
- NaNa2(Fe4Fe)Si8O22(OH)2
- Hardness (Mohs)
- 5–6
- Specific gravity
- 3.37–3.5
- Streak
- deep bluish grey
- Lustre
- vitreous (glassy)
- Crystal system
- Monoclinic
- Colours
- black, dark blue-green
- Transparency
- opaque
- Cleavage and fracture
- perfect in two directions at about 56° and 124°
Typical values: real specimens vary. For more, see mindat.org.