VALLERIITE
Valleriite – (Fe2+,Cu)4(Mg,Al)3S4(OH,O)6 – is a very rare hydroxysulfide found in certain Cu, Ni, and Cr ores; it does not constitute an ore in itself.
It is classified within the Valleriite Group and may contain Ca, Na, Ni, K, and Si.
It is a mineral that easily goes unnoticed, often being macroscopically confused with pyrrhotite and mackinawite. Under the microscope, it closely resembles mackinawite. For decades, valleriite and mackinawite were believed to be a single mineral; the distinction between them only began to be made in the early 1960s. Consequently, one must exercise caution with the literature, as much of the older information regarding valleriite actually refers to mackinawite. Some expanded mackinawites behave exactly like valleriite under the microscope. Mackinawite is far more common than valleriite.
1. Characteristics
Crystal system: Trigonal, trigonal scalenohedral or ditrigonal pyramidal class.
Color: Dark bronze-yellow-gray. (Similar to pyrrhotite).
Habit: Massive, in nodules up to 5 mm. Fine flakes, crusts. Extremely rare, very fine crystals.
Cleavage: {0001}very good.
Tenacity: No information available.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 1, lower than that of graphite.
Parting: No.
Streak: Black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 3.14
2. Geology and Deposits
Valleriite is typically an alteration product of chalcopyrite in fully brecciated and mineralized chromitites and dunites. It is found in serpentinized, uralitized and brecciated ultramafic rocks containing Cu and Ni sulfides.
It also occurs in copper-bearing carbonatites, especially those containing magnetite, where valleriite replaces magnetite.
It can occur in kimberlites.
3. Mineral Associations
In ultramafic rocks, it is associated with the rock-forming silicates and oxides of the host rock – such as clinopyroxene, plagioclase, olivine, magnetite, ilmenite, graphite, and gahnite (spinel) – as well as minerals resulting from their alteration, such as serpentine and calcite.
These rocks host a sulfide assemblage containing the chalcopyrite that gives rise to valleriite. In addition to chalcopyrite, valleriite may be associated – in this and other parageneses – with other sulfides such as cubanite, pyrrhotite, pentlandite, pyrite, marcasite, galena, sphalerite, bornite, and mackinawite.
More rarely, it occurs with iowaite, vyalsovite, troilite, millerite, sperrylite, and tochilinite.
4. Transmitted Light Microscopy
Not applicable, as valleriite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Valleriite exhibits intermediate hardness, approximately the same as that of pentlandite and cubanite. Valleriite is harder than chalcopyrite but softer than chromite. Under ideal conditions and with orientation perpendicular to the base, valleriite is softer than chalcopyrite.
When valleriite occurs as inclusions within chalcopyrite, both minerals take a good polish, though this mode of occurrence is rarer. Larger aggregates of valleriite do not take a good polish on basal sections. Due to surface smearing during polishing, valleriite may exhibit extremely low reflectivity.
PLANE POLARIZED LIGHT – PPL
Reflection color: The reflection color varies significantly, even in well-polished sections. It ranges from medium brown or bronze to gray or bluish-gray.
Compared to the color of chalcopyrite, the color of valleriite is pink.
Compared to the color of magnetite, the color of valleriite is yellow.
Compared to the color of pentlandite, the color of valleriite is the same.
Pleochroism: Strong, ranging from light grayish-brown or creamy bronze to darker grayish-pink-violet. Or from bluish-gray to dark gray.
Reflectivity: 10,20 – 18,51%
Bireflectance: Strong.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Very strong; ranges between white and bronze-gray. It strongly resembles the anisotropy of graphite and molybdenite.
Exfoliated or expanded mackinawite exhibits anisotropy identical to that of valleriite!
Internal reflections: No.
May be confused with: Graphite and mackinawite, which are very similar. It is practically impossible to distinguish mackinawite from valleriite under the microscope; other analytical techniques are required.
Diagnostic features include low reflectivity, strong pleochroism, and very characteristic anisotropy.
Mackinawite is very similar, and for decades the two minerals were treated as one. Mackinawite is lighter, and its bireflectance and anisotropy are stronger. In mackinawite, the anisotropy colors are essentially white/black, whereas pink and violet hues are associated with valleriite. Paragenesis may offer a clue for identification.
Graphite exhibits much lower reflectivity, stronger bireflectance, and a brownish-gray color.
Molybdenite has a pure white reflection color – lacking yellow-brown tones – and stronger bireflectance.
General Characteristics:
Grain shape: occurs as small veinlets, interstitial fillings, and small blebs within and around associated minerals. It occurs as pisolitic masses in serpentine. Thin, rhythmic crusts in serpentine, resulting from the alteration of olivine or chondrodite, also occur. The veinlets resemble those containing “asbestos” fibers, consisting of fracture fillings with fibers oriented perpendicular to the fracture direction. Valleriite also occurs in magnetite and chromite, filling fractures. Well-developed crystals are extremely rare; they grow suspended in carbonate material amidst masses of serpentine.
Cleavage is not visible.
Twinning is absent, though fine lamellae may occur.
Intergrowths with mackinawite are common, including oriented ones. Well-developed oriented intergrowths with chalcopyrite and sphalerite also occur.
Inclusions of valleriite occur in chalcopyrite, pyrrhotite, pentlandite, chromite, and magnetite. In sulfides, the inclusions are often oriented and exhibit highly variable shapes: needles, vermicular forms, laths, droplets, and stars.
Exsolution textures are rare at best and can be confused with mackinawite.
Substitutions 1: Valleriite can replace various minerals. It may occur enveloping cubanite. Valleriite can replace pyrrhotite, cubanite, and pentlandite. When replacing pentlandite, it is easily confused with mackinawite. Serpentine is frequently replaced by valleriite. In some deposits, valleriite coats, penetrates, and replaces magnetite, partly with the (0001) plane of the valleriite parallel to the (111) plane of the magnetite.
Substitutions 2: Valleriite is sometimes coated and replaced by chalcopyrite.
Alteration: incipient alteration of valleriite leads to the formation of covellite.
