Birnessite was originally thought to be black serandite. It pseudomorphs after serandite and offers some of the most exceptional crystal specimens at Mont Saint-Hilaire in sizes up to 20cm. They commonly form very attractive groups of sharp prismatic to bladed crystals.
PHYSICAL CHARACTERISTICS:
- Color varies from very dark brown to black.
- Luster is dull to greasy.
- Diaphaneity is opaque.
- Crystal System is monoclinic; C2/m.
- Crystal Habits include thin dull coatings and as fine-grained
pseudomorphs after sérandite. - Cleavage: (?).
- Fracture is (?).
- Hardness is 1.5
- Specific Gravity is approximately 3.0 g/cm3.
- Streak is black.
- Associated Minerals include chkalovite, erdite, kogarkoite, makatite,
natrophosphate, rasvumite, revdite, sazhinite-(Ce), sidorenkite,
thalcusite, tugtupite, vitusite-(Ce). - Distinguishing Features: Fine-grained dull black aggregates.
- Origin: Named in 1956, for the locality, Birness, Aberdeenshire, Scotland.
| Dana class # | 7.5.3.1 – Oxides; multiple oxides |
| Strunz class # | IV/F.11-30 – Oxides; hydroxides and oxidic hydrates, water-bearing oxides with layered structure; jiansuiite – birnessite series |