Magnesio-riebeckite


M
agnesio-riebeckite is a rare gem that is not well known in the gem market. The chemical formula of Magnesio-riebeckite is Na2[(Mg, Fe2+)3Fe3+2]Si8O22(OH)2
(Sodium Magnesium Iron Silicate Hydroxide) and its molecular weight is 876.08 gm. It is also known by the synonym Rhodusite which is named for the Greek island of Rhodes (Rhodus in historic times) where it was found by Bukowski in 1889. Magnesio-riebeckite is usually found as fibrous “asbestiform” masses but very few sources produce solid, opaque, massive material of light to dark blue that can be used to create attractive cabochons.

Magnesio-riebeckite may have originally been discovered by C. Palache in 1928 at the mines at Franklin, New Jersey, USA, but the specimen was incorrectly identified as Crocidolite. The year of discovery is noted as 1949 from the type locality of Alto Chapare District, Chapare Province, Bolivia. Magnesio-riebeckite and Riebeckite are closely related to the mineral Glaucophane. Glaucophane is named for its typical blue color. The name Glaucophane is from the Greek glaukos for “sky-blue” and phainestrai for “to appear”, in allusion to its typical color. The blue color is very diagnostic for the two species and both are the only common amphibole minerals that are typically blue.

There are many localities for finding the fibrous “asbestiform” material but only one source for the attractive solid, deep blue massive material: Krivoy Rog Basin, Dnipropetrovs’k Oblast’ (Dnepropetrovsk Oblast’), Ukraine.

Chemical Formula: Na2[(Mg,Fe2+)3Fe3+2]Si8O22(OH)2
Sodium Magnesium Iron Silicate Hydroxide
Molecular Weight: 876.08 gm
Composition: Potassium 0.13 % K 0.16 % K2O
Sodium 4.09 % Na 5.52 % Na2O
Calcium 1.97 % Ca 2.75 % CaO
Magnesium 5.35 % Mg 8.88 % MgO
Titanium 0.16 % Ti 0.27 % TiO2
Manganese 0.31 % Mn 0.45 % Mn2O3
Aluminum 0.80 % Al 1.51 % Al2O3
Iron 17.47 % Fe 12.06 % FeO / 11.57 % Fe2O3
Silicon 25.65 % Si 54.87 % SiO2
Hydrogen 0.23 % H 2.06 % H2O
Oxygen 43.83 % O
  100.00 % 100.10 % = TOTAL OXIDE

 

Crystallography: Monoclinic – Prismatic
Crystal Habit: Rarely as prismatic crystals. Also columnar, fibrous, or granular aggregates.
Twinning: Simple or multiple twinning || {100}.
   

 

Cleavage: Perfect on {110}; intersecting at ~58° and ~122°; partings on {010}, {001}.
Fracture: Conchoidal to Uneven
Tenacity: Brittle
Moh’s Hardness: 5.0 – 6.0
Density: 3.12 – 3.29 (g/cm3)
Luminescence: None
Radioactivity: Not Radioactive
Health Warning: The asbestiform variety of Magnesio-riebeckite can cause lung disease when inhaled, as in other species of asbestos minerals.

 

Color: Light Blue, Dark Blue, Blackish Blue, Black
Transparency: Translucent to Opaque
Luster: Vitreous, Silky
Refractive Index: 1.668 – 1.680  Biaxial ( – ) or ( + )
Birefringence: 0.0120
Dispersion: r > v or r < v
Pleochroism: Strong; X = dark blue; Y = indigo; Z = yellow-green