MALACHITE

Malachite – Cu3(CO)2(OH)2 – is a fairly common carbonate, by far the most abundant alteration product of primary Cu minerals such as chalcocite and chalcopyrite. Malachite has been an important copper ore since prehistoric times and usually occurs in subordinate amounts in copper ores. It was used as a pigment and is a decorative stone that fetches very high prices, almost a gem.

Malachite indicates the presence of Cu, but there are two problems: the first is that even minute amounts of malachite create the impression of large amounts of Cu. The second problem is that there are minerals whose color is very similar to malachite. Celadonite, for example, is a clay mineral of similar color and habits. In addition, there are ten other green secondary Cu minerals, generally much rarer than malachite.

Malachite pseudomorphs may occur on azurite or cuprite, more rarely on other minerals (gypsum, calcite, sphalerite, pyrite, etc.). Very rarely, malachite has a well-developed long prismatic habit or even capillary habit. It shows slow effervescence with dilute HCl, which is a simple and important test in its recognition. It may contain Ni, Co and Zn as impurities and has four varieties.

1. Characteristics

Crystal system: Monoclinic prismatic.

Color: Light green, dark green, green-black.

Habit: Massive, mamelonate to botryoidal, stalagtitic, fibrous, acicular, long prismatic, etc.

Cleavage: {-201} perfect, {010} good.

Tenacity: Brittle.

Twinning: On {100} and {201}, common. Polysynthetic.

Fracture: Subconchoidal, irregular.

Mohs Hardness: 3.5 – 4

Parting: No.

Streak: Light green.

Lustre: Adamantine, vitreous, silky, dull.

Diaphaneity: Transparent.

Density (g/cm³): 3.6 – 4

 

2. Geology and Deposits

Malachite occurs in Cu ores and the alteration deposits arising therefrom, even when there are only traces of Cu present.

It is also found in altered volcanic rocks (basalts, diabases, gabbros), formed from the native copper present there, even when there are only very small amounts.

Some care is needed to avoid confusing malachite with celadonite. Celadonite is a green clay mineral whose color is somewhat similar to that of malachite. Glauconite is also green and could, in theory, be confused with malachite. An easy test to identify malachite is with 10% hydrochloric acid, as malachite effervesces with the acid, unlike celadonite and glauconite, which do not effervesce.

 

3. Mineral Associations

Malachite occurs with many other primary (chalcocite, bornite, chalcopyrite, etc.) and secondary Cu minerals. Among the secondary minerals, one is black (tenorite), another is red (cuprite), others are blue (azurite, linarite, chrysocolla, chalcanthite, plancheite, etc.), and many are green (libethenite, chalcantite, brochantite, chrysocolla, etc.).

Several of them can present great color variations, from green to light blue, for example. Furthermore, malachite is associated with quartz, native copper, cerussite, calcite, “limonite”, chalcedony and many rare minerals such as duftite, auricalcite, sphaerocobaltite, kolwezite, shattuckite, chalcophyllite, conicalcite, rosasite, chalcosiderite, clinoclase, graemite, liroconite, mixite and cornetite, among many others.

 

4. Transmitted Light Microscopy

The preparation of a thin section of malachite requires only a rough grinding of the mineral, otherwise the malachite becomes so smooth that it peels off the glass slide. The glue simply does not adhere to the malachite and the thin section is rendered useless during the grinding step.

Refraction indices: nα: 1.655   nβ: 1.875   nγ: 1.909

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Green to yellowish green with visible pleochroism:

X = almost colorless;

Y = yellowish green;

Z = deep green.

Difficult to see in very small crystals.

Relief: Very high.

Cleavage: Generally not noticeable

Habits: Concentric bands, fibrous, acicular, long prismatic. Never in large crystals.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.254, corresponding to higher-order colors, but the intense own green color masks any other color.

Extinction: No information available, probably oblique.

Elongation sign: No information available.

Twins: No information available.

Zoning: No information available.

CONVERGENT LIGHT

Character: B(-), usually impossible to determine because the crystals are too small.

2V angle: 43o

Alterations: Under certain conditions (low H2O, slightly different pH), malachite can transform into azurite. Malachite can break down and form cuprite and tenorite in more oxidizing environments where carbonate is lost. And malachite can alter into chrysocolla (if silica-rich fluids are present) and in brochantite (in sulfate-rich conditions).

May be confused with: other green secondary copper minerals.

 

Thin section of malachite 30 microns thick. The strong color remains and allows you to recognize the mineral even with the naked eye. Part of the sample was lost because the glue simply does not adhere to the malachite when it is minimally polished.
Banded malachite in PPL, with the characteristic bands and typical fine grain. In XPL, the image is practically the same.

5. Reflected Light Microscopy

Sample preparation: Malachite polishing is very simple and is of excellent quality, like calcite and azurite, for example. During grinding of malachite, the abrasive takes on a light green color.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray with a discreet greenish hue. Along imperfections (holes, fractures, etc.) some stronger green internal reflections may be present.

Pleochroism: No.

Reflectivity: Very low (6-8%)

Bireflectance: Distinct, which is important in differentiating malachite from other green copper secondary minerals. This bireflectance is difficult to recognize when the crystals are very small.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Weak grayscale anisotropy, often masked by internal green reflections.

Internal reflections: Generalized, abundant and luminous in green, giving rise to any structures present.

May be confused with: many other copper minerals with green internal reflections (atacamite, brochantite, etc.), but the bireflectance of malachite is very diagnostic.

Polished section of an oxidized copper ore, with malachite (green), chrysocolla (blue) and azurite (blue). The colors of these secondary copper minerals are very evident and facilitate their recognition in the polished section.
Malachite 01: in PPL, malachite aggregate. The banding is almost imperceptible.
Malachite 01: the same section as in the previous image, now in XPL. The structure closely resembles the structure of chalcedony (agate).
Malachite 02: in PPL, malachite aggregate. Banding is more or less visible.
Malachite 02: the same section as of the previous image, now in XPL. Intense green internal reflections are typical for malachite. In yellowish, gangue minerals.
Malachite 03: In PPL, cavities filled with malachite with long prismatic habit, forming radial bundles. The greenish hue of the malachite can be clearly seen. Holes in black.
Malachite 03: The same section as of the previous figure, now in XPL. The habit is very evident and the internal reflections in green dominate the appearance. Anisotropy is difficult to see.
In XPL, radial long prismatic aggregate of malachite (green) enveloped by chrysocolla (blue) in a cavity of a volcanic rock.
In XPL, malachite (green, radiated beams of long prisms or disseminated, massive) associated with azurite in large crystals (blue, strong anisotropy in gray). In PPL, the reflection colors of the two are identical.
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