CRICHTONITE
The mineral currently designated as “crichtonite” – Sr(Mn,Y,U)Fe2(Ti,Fe,Cr,V)18(O,OH)38 – is a very rare oxide of no economic importance.
It belongs to the Crichtonite Group, which comprises 17 other members. It forms a series with senaite. It is radioactive and is macroscopically very similar to ilmenite.
In the past, the name “crichtonite” was applied to “pure” ilmenites – those lacking Fe2O3, MgTiO3 or MnTiO3 and having an exclusive FeTiO3 composition – and later to davidites as well. Therefore, when consulting old labels and texts, it is necessary to verify exactly what the author was referring to.
1. Characteristics
Crystal system: Trigonal rhombohedral.
Color: Black.
Habit: Usually in elongated rhombohedral crystals, up to 2 cm in size.
Cleavage: No. Striations on the primary rhombohedron.
Tenacity: No information available.
Twinning: Polysynthetic, probable.
Fracture: No information available.
Mohs Hardness: 5 – 6
Parting: No information available.
Streak: Dark gray.
Lustre: Metallic to dull.
Diaphaneity: Opaque.
Density (g/cm³): 4.46
2. Geology and Deposits
Crichtonite occurs exclusively in Alpine-type tectonic fissures.
3. Mineral Associations
Crichtonite occurs in the typical assemblage of Alpine-type tectonic fissures, most commonly with quartz (rock crystal, smoky quartz, and “rutilated” quartz), albite, chlorite (clinochlore), magnetite, rutile, and ilmenite.
Also with rarer minerals like cafarsite, dessauite-(Y), and senaite.
4. Transmitted Light Microscopy
Not applicable, as crichtonite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: xx
PLANE POLARIZED LIGHT – PPL
Reflection color: xx
Pleochroism: xx
Reflectivity: xx
Bireflectance: xx
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: xx
Internal reflections: xx
May be confused with: xx
General Characteristics: xx
