Birnessite
Na₄Mn₁₄O₂₇ · 9H₂O

     Birnessite was originally thought to be black sérandite. It pseudomorphs after sérandite 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.

CLASSIFICATION:

REFERENCES:
MinRec 21:299 (1990), AmMin 82: (1997), Dana 8:315 (1997)

DISTRIBUTION AND RARITY AT MONT SAINT-HILAIRE:

Legend

Birnessite crystals - 10 x 2 cm
© Gilles Haineault