COFFINITE
Coffinite – U(SiO4).nH2O – is a relatively rare nesosilicate. It is a uranium ore, common in the Western USA.
It is classified in the Zircon Group, in the Zircon-Thoriogummite Series. It may contain, as impurities, Fe, As, Pb, V, and Al. There is a variety with up to 13.8% ZrO2 and up to 3.4% Y2O3. Macroscopically, it resembles coal or pitch (asphalt).
Metamictization is common in coffinite. It can affect neighboring minerals, forming a rim around the coffinite crystals. Older coffinites (>100 M.a.) are always completely metamictized and, if even older, show a “dust” of galena (the Pb comes from the decay of U).
CAUTION! Coffinite is highly radioactive. Always wash your hands after handling. Do not lick, ingest, or inhale its dust, and do not keep it near your body.
1. Characteristics
Crystal system: Tetragonal, bipyramidal ditetragonal.
Color: Black due to the incorporation of organic matter.
Habit: Coffinite is generally found in powdery form, in aggregates of very fine crystallites, rarely larger than 20 μm. However, crystals up to 5 mm can occur. It can occur massively. Also colloform or botryoidal in encrustations. Botryoidal masses may show well-developed crystals on the surface. It may present radial fibrousness.
Cleavage: No.
Tenacity: Brittle.
Twinning: No.
Fracture: Irregular, subconchoidal.
Mohs Hardness: 5 – 6
Parting: No.
Streak: No information available.
Lustre: Adamantine.
Diaphaneity: Opaque.
Density (g/cm³): 5.1
2. Geology and Deposits
Coffinite is a typical and common mineral in low-temperature, unoxidized U deposits of the Colorado Plateau type, replacing organic material in the matrix of sandstones and conglomerates, associated with pyrite and pitchblende (uraninite). Its deposition occurs from the circulation of groundwater. It can replace fossil bones (of dinosaurs).
It also occurs in other types of sedimentary U deposits, such as in nodules in argillites.
It is relatively frequent in U-rich hydrothermal vein deposits.
3. Mineral Associations
It occurs associated with some common minerals such as quartz, carbonates (calcite, siderite), clay minerals (montmorillonite), oxides (hematite, fergusonite-(Y), rutile), fluorite, sellaite and amorphous organic matter.
With sulfides such as pyrite, marcasite, chalcopyrite, galena, sphalerite, wurtzite, cobaltite, bornite, wittichenite, tennantite and bismuthinite. Also with sulfosalts such as aikinite.
With U minerals such as uraninite (including pitchblende variety), brannerite and tyuyamunite.
With Se minerals such as eucairite, clausthalite, sabatierite and berzelianite.
With V minerals such as rossite, metarossisite, roscoelite, montroseite, paramontroseite and pascoite.
With arsenides such as eskutterudite, nicheline, rammelsbergite, and safflorite.
With native elements such as bismuth and arsenic.
4. Transmitted Light Microscopy
Under transmitted light, coffinite, at the fine edges of small particles, shows a pale brown to very dark brown color. Very young material may be transparent in lilac tones.
Refraction indices: nα = 1.730 – 1.750; nβ = 1.730 – 1.750.
PLANE POLARIZED LIGHT – PPL
Relief: High.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence is 1.730
CONVERGENT LIGHT
Character: U(+/-)
5. Reflected Light Microscopy
Sample preparation: Coffinite generally achieves a good quality polish, but earthy masses are more difficult to polish. Its hardness is similar to, or slightly less than, that of pitchblende.
PLANE POLARIZED LIGHT – PPL
Reflection color: Dark gray.
Pleochroism: Very weak.
Reflectivity: Submicroscopic aggregates show very low reflectivity (~4%), similar to gangue minerals.
Idiomorphic crystals have higher reflectivity, but lower than the reflectivity of uraninite.
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: It appears isotropic. The metamictization that usually affects coffinite prevents observation of its anisotropy. Very young coffinite or artificial preparations show a slight anisotropy.
Internal reflections: It generally does not show, but occasionally displays internal reflections ranging from greenish to yellow and orange. Other literature cites colors ranging from brown to deep bluish-gray.
May be confused with: other materials. Coffinite, even though relatively common, was confused with uraninite for decades. Botryoidal and earthy masses are very similar to some of the alteration stages of pitchblende (a variety of uraninite). They are also similar to some carbonized hydrocarbon materials.
Crystalline coffinite included in siderite may go unnoticed or be confused with cerussite, titanite, and other minerals.
Uraninite, when fresh, is lighter in color.
General Characteristics:
Grain shape: generally powdery, in aggregates of very fine crystallites, rarely larger than 20 µm. Massive, coloform or botryoidal in encrustations. Botryoidal masses may show well-developed crystals on the surface. It may appear radially fibrous or in fine veins surrounded by carbonaceous material. Well-developed crystals may appear prismatic, with a tetragonal appearance, with square basal sections.
Radioactive halos are normal around coffinite grains.
Substitutions: coffinite can replace uraninite, including forming pseudomorphs. Coffinite can replace pitchblende. On the other hand, coffinite is frequently replaced by uraninite, preserving the original crystalline forms. Coffinite is also replaced by sphalerite, pyrite and rammelsbergite. Coffinite replaces native bismuth.
“Galena dust” occurs in older coffinite, with Pb being generated by U decay.
Intergranular films of coffinite between quartz and fluorite may occur.
Inclusions of coffinite in niqueline may occur.
Pseudomorphs are common, usually metamicts.
Corrosion of coffinite by pitchblende may occur and vice versa.
