14.7.4: Other Carbonate Minerals

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Malachite Cu₂CO₃(OH)₂

Origin of Name
From the Greek word moloche, meaning “mallows,” referring to the green color of mallow leaves.

14.390.png — Figure 14.390: Malachite from near Bisbee, Arizona; the specimen is almost 13 cm across

14.391.png — Figure 14.391: Malachite (green) with a lesser amount of azurite (blue), from Morocco

Hand Specimen Identification
Distinctive light to dark green color, occurrence as a secondary mineral that often replaces or coats other minerals, habit, and association with other copper minerals (especially azurite) help identify malachite. It may be confused with other secondary copper minerals but effervesces in cold dilute HCl, unlike most of the others.

Figure 14.390 shows typical green malachite. It has crystallized on top of a rusty mix of iron oxides/hydroxides, forming as an alteration product of other (primary) Cu- and Fe-minerals. Malachite and azurite, both hydrated copper carbonates, commonly occur together. Figure 14.391 shows a specimen from Morocco that contains both minerals. Figure 1.4 is a photo of another sample that contains malachite and azurite. Figure 14.348, above, shows malachite with cuprite (Cu-oxide).

Physical Properties

hardness	3.5 to 4
specific gravity	3.7 to 4.0
cleavage/fracture	perfect (201)/subconchoidal
luster/transparency	adamantine/transparent to translucent
color	bright green
streak	pale green

Properties in Thin Section
Malachite is biaxial (-), a = 1.655 , β = 1.875, γ = 1.909, δ = 0.254, 2V = 43°.

Crystallography
Malachite is monoclinic, a = 9.48, b = 12.03, c = 3.21, β = 98.0°, Z = 4; space group \(P\dfrac{2_1}{a}\); point group \(\dfrac{2}{m}\).

Habit
Coarse malachite crystals are rare. Malachite is usually fine-grained and massive; frequently colloform or banded, and often intergrown with other secondary copper minerals.

Structure and Composition
Malachite‘s structure resembles those of other carbonates. Triangular (CO₃)^2- groups are surrounded by three Cu(O,OH)₆ octahedra. The octahedra share edges to form chains. Zn and Co are commonly present in small amounts.

Occurrence and Associations
Malachite is a secondary copper mineral typically found in carbonate rocks with azurite, cuprite, native copper, limonite, and chrysocolla.

Related Minerals
Similar minerals include other hydrated Cu and Zn carbonates, including azurite, Cu₃(CO₃)₂(OH)₂; hydrozincite, Zn₅(CO₃)₂(OH)₆; aurichalcite, (Zn,Cu)₅(CO₃)₂(OH)₆; and a number of other rare hydrated Cu and Zn carbonates.

Azurite Cu₃(CO₃)₂(OH)

Origin of Name
From the French word azur, meaning “sky-blue color.”

14.392.png — Figure 14.392: Azurite from the Guangdong Province, China

14.393.png — Figure 14.393: Azurite with malachite from Oujda, Morocco

Hand Specimen Identification
Azurite’s softness, characteristic blue color, association with malachite, effervescence in cold HCl, and habit are distinctive.

Figure 14.392 is a photo of spectacular blue azurite, but samples like this are uncommon. The photo in Figure 14.393 shows a more typical example. The azurite is not as blue as in the previous photo, and the crystals are not as coarse. This specimen also contains minor amounts of green malachite. The azurite and malachite are on top of rusty iron oxides/hydroxides.

Physical Properties

hardness	3.5 to 4
specific gravity	3.77
cleavage/fracture	brittle, perfect {011}, good {100}/conchoidal
luster/transparency	normally dull and earthy/transparent to translucent
color	blue
streak	blue

Properties in Thin Section
Azurite is biaxial (+), a = 1.730 , β = 1.756, γ = 1.836, δ = 0.106, 2V = 68°.

Crystallography
Azurite is monoclinic, a = 4.97, b = 5.84, c = 10.29, β = 92.4°, Z = 2; space group \(P\dfrac{2_1}{a}\); point group \(\dfrac{2}{m}\).

Habit
Azurite is typically massive and earthy. It may form as a crust on other copper minerals. Rarer individual crystals are tabular or prismatic.

Structure and Composition
Usually nearly pure copper carbonate, azurite has a complex structure consisting of Cu²⁺ ions coordinated to two oxygens of adjacent (CO₃)^2- groups and to two OH^– groups, making a square planar arrangement. The square groups are linked to form chains.

Occurrence and Associations
Azurite, like malachite, is a secondary copper mineral formed by alteration of copper oxides and sulfides. It is less common than malachite.

Related Minerals
Chemically similar minerals include malachite, Cu₂CO₃(OH)₂; hydrocerussite, Pb₃(CO₃)₂(OH)₂; hydromagnesite, Mg₅(CO₃)₄(OH)₂; and aurichalcite, (Zn,Cu)₅(CO₃)₂(OH)₃.