BROCHANTITE

Brochantite – Cu4(SO4)(OH)6 – is a fairly common sulfate, a typical secondary copper mineral. It is part of oxidized copper ores, but is not an important ore mineral.

There are two monoclinic polytypes. It is soluble in acids.

1. Characteristics

Crystal system: Monoclinic prismatic. 

Color:  Various shades of green (light, dark, emerald, black). 

Habit: Short prismatic, acicular, elongated by [010] or [100], may be tabular. Radial aggregates, massive, earthy, granular, up to 5 cm.

Cleavage: {100} perfect.      

Tenacity: Brittle.        

Twinning: Per {100}, common, pseudoorthorhombic.      

Fracture: Irregular, conchoidal.       

Mohs Hardness: 3.5 – 4

Parting: No.         

Streak: Pale green.         

Lustre: Vitreous, pearly on the cleavages.          

Diaphaneity: Transparent.           

Density (g/cm³): 3.97

           

2. Geology and Deposits

Brochantite is a secondary copper mineral that forms:

– in oxidation zones of copper deposits in arid climates or

– in sulfide copper deposits that undergo rapid oxidation under low acidity conditions, with abundant carbonates.

It is also found in old metallurgical slags, along with other secondary copper minerals and coal remains.

Brochantite is a common corrosion product of bronze sculptures located in urban areas. There, it is formed by the reaction of sulfur dioxide (SO2 – a common pollutant) with the copper used in bronze (an alloy of Cu and Sn, with varying contents of Sb, Al, Zn, Ni, Pb and PO4), because these exposed surfaces do not allow the accumulation of Cu ions or the acidification of water films. On sheltered surfaces, where these processes are possible, antlerite (Cu3(SO4)(OH)4) is formed.

 

3. Mineral Associations

It is associated with a diverse and colorful set of secondary Cu minerals, among which we can recognize predominantly colored minerals:

– in green: malachite, pseudomalachite, antlerite, atacamite, paratacamite, dioptase, chrysocolla,
libethenite and chalcanthite

– in blue: azurite, linarite, cyanotrichite, spangolite, langite, chrysocolla, kaledonite, plancheite, and
chalcoalumite,

– in red: cuprite,

– in black: tenorite.

In addition, it occurs with cerussite, Fe oxides (goethite, lepidocrocite) and quartz.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.728    nβ: 1.771      nγ: 1.800

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Bluish green, with subtle pleochroism in shades of bluish green.

Relief: Very high.           

Cleavage:  No information available.          

Habits: Acicular, short prismatic, can vary from granular to massive.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.072, very high, corresponding to colored interference colors of up to the 4th order, with pink and greenish colors predominating.
However, the strong natural color of the mineral masks these colors, and the colors do not change or change very little between PPL and XPL.

Extinction: No information available, probably oblique.           

Elongation sign: No information available.            

Twins: They may be visible, at least in theory, if the crystals are larger.

Zoning: No information available.             

CONVERGENT LIGHT

Character: B(-)         

2V angle: 72º         

Alterations: Brochantite alters to chrysocolla.          

May be confused with: other green secondary Cu minerals.         

Brochantite 01: Brochantite in ancient metallurgical slag (France, 2000 years old) impregnated with resin. All images, these and the following ones, come from the same sample. Brochantite forms crusts lining cavities in the slag, which is almost entirely opaque due to the opaque minerals that are still present. Generally, these brochantite crusts are massive, sometimes banded. And there is, in addition to brochantite, another Cu mineral, of a yellowish-green color, which is perceived more clearly in XPL.
Brochantite 01: The same section as of the previous image, now in XPL. The strong own color of brochantite masks the interference colors.
Brochantite 02: In PPL, another aspect of brochantite filling spaces in old metallurgical slag. When the cavity is not filled, prismatic crystals tend to develop.
Brochantite 02: The same section as of the previous image, now in XPL. Usually, these secondary colorful copper minerals, in XPL show a color very similar to their color in PPL.
In PPL, prismatic brochantite crystals are found in the cavity wall of this old metallurgical slag.
In PPL, a fragment of coal preserved in ancient metallurgical slag, with the plant cells filled with brochantite. Also in this coal fragment there are at least two secondary Cu minerals filling the plant cells: brochantite and a greenish-citrus-yellow mineral.
In PPL, detail of the same coal fragment whose cells are now filled with brochantite and another secondary Cu mineral.

5. Reflected Light Microscopy

Sample preparation: Polishing brochantite is simple and presents no problems. Due to its often acicular habit and its occurrence in crusts, it is necessary to impregnate the sample.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, as dark as quartz and feldspars.       

Pleochroism: No.      

Reflectivity: Low (<10%).        

Bireflectance: Discreet.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Weak anisotropy.  

Internal reflections: Widespread, dominant, in medium green. 

May be confused with: Dioptase, when it occurs with an acicular habit.

Identification of brochantite is difficult due to the small size of the crystals and because there are several other secondary Cu minerals of green colors that occur in the same paragenesis.       

General Characteristics: 

Grain shape: typically acicular, and they may occur in bands. Other habits are possible.

Pseudomorphs of brochantite on malachite, azurite, and langite may occur.

Alteration of brochantite leads to the formation of chrysocolla.

Brochantite 01: In PPL, ancient metallurgical slag (France, 2,000 years old) is impregnated (resin on top), with a brochantite crust (horizontally between slag and resin). There are no diagnostic features in PPL.
Brochantite 01: The same section as of the previous image, now in XPL. The internal green reflections allow the mineral to be easily recognized. However, other secondary green copper minerals present themselves in the same way; conclusive identification requires other analytical methods.
In PPL, detail of the brochantite crust from the same sample as the previous images, showing the acicular habit of brochantite.