ROSASITE
Rosasite – (Cu,Zn)2(CO3)(OH)2 – is a rarer carbonate, typically occurring as a secondary mineral in polymetallic Cu-Zn deposits. It is part of oxidized ores, but is never one of the main ore minerals in the deposit.
It is classified in the Rosasite Group, may contain Mg and Fe, and has a dark green nickeliferous variety.
Rosasite crystals are typically acicular, very small, up to 0.5 mm, forming radial to spherical aggregates or covering the walls of cavities in the ore in the form of crusts (see below). Due to this habit, the preparation of thin sections and polished sections requires impregnation of the sample.
Macroscopically it is very similar to auricalcite, but has a higher hardness.
It exhibits effervescence in cold dilute hydrochloric acid, like malachite and azurite as well.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Blue, greenish-blue to green, sky blue.
Habit: Crusts with a mammillated, botryoidal, or wart-like appearance, with a fibrous to spherulitic structure.
Cleavage: In two directions, forming a rectangular grid.
Tenacity: Brittle.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 4
Parting: No.
Streak: Light blue to green.
Lustre: Vitreous, silky, matte.
Diaphaneity: Transparent.
Density (g/cm³): 4 – 4.2
2. Geology and Deposits
Rosasite forms as a secondary mineral in the oxidation zone (“iron cap”, “gossan”) of Cu-Zn deposits, with or without associated Pb. It typically forms from solutions containing Zn acting on primary Cu minerals.
It can form after the mining period, in tailings piles or on the walls of abandoned mines.
3. Mineral Associations
In the type locality (Mina Rosas, Sardinia, Italy), it occurs associated with quartz, pyrite, chalcopyrite, calcite, siderite, greenockite, and auricalcite. A total of 25 different minerals are recorded for this locality, also including secondary Pb minerals (cerussite, pyromorphite), secondary Cu minerals (native copper, cuprite, linarite, malachite, azurite, brochantite, chalcantite), and secondary Fe minerals (hematite, goethite).
It is evidently associated with other secondary Zn minerals such as hemimorphite and smithsonite.
It also occurs with dolomite, gypsum, conichalcite, duftite, mimetite, hydrozincite, austinite, and bayldonite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.672 – 1.688 nβ: 1.796 – 1.830 nγ: 1.811 – 1.831
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless to light blue, with strong pleochroism:
x: colorless or pale emerald green.
y = z: pale blue or dark emerald green.
Relief: Very high.
Cleavage: The literature indicates that there are two cleavages that form right angles to each other, which for carbonate is, let’s say, unusual. Since the crystals are very small and acicular, these cleavages will probably not be visible.
Habits: Radial to spherical aggregates of very small prismatic crystals.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.139 to 0.143, very high, resulting in high-order brown to cream colors, up to the 7th order, like all carbonates.
It is generally not possible to determine the interference color precisely.
Extinction: Probably oblique because it is a monoclinic mineral.
Elongation sign: No information available.
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: B(-)
2V angle: 33º
Alterations: No information available.
May be confused with: Aurichalcite is very similar, both macroscopically and probably microscopically.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying rosasite. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with rosasite, such as hematite, pyrite, chalcopyrite, and others.
Sample preparation:
PLANE POLARIZED LIGHT – PPL
Reflection color:
Pleochroism:
Reflectivity:
Bireflectance:
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy:
Internal reflections:
May be confused with:
General Characteristics:
Pseudomorphisms of rosasite on malachite can occur. Pseudomorphisms of rosasite on malachite on azurite are possible.