Chrysocolla

Chrysocolla is a hydrous copper phyllosilicate mineral and mineraloid with the formula Cu2 – xAlx(H2Si2O5)(OH)4⋅nH2O (x < 1) or (Cu, Al)2H2Si2O5(OH)4⋅nH2O).

The structure of the mineral has been questioned, as a 2006 spectrographic study suggest material identified as chrysocolla may be a mixture of the copper hydroxide spertiniite and chalcedony.

Chrysocolla typically forms amorphously.

Chrysocolla

General

Category Phyllosilicate minerals, mineraloids

Formula Cu2 − xAlx(H2Si2O5)(OH)4•nH2O (x < 1)

IMA symbol Ccl

Strunz classification 9.ED.20

Crystal system Orthorhombic

Unknown space group

Unit cell a = 5.7 Å, b = 8.9 Å,

c = 6.7 Å; Z = 1

Identification

Color Blue, cyan (blue-green), green, dark blue to black, brown, rarely yellow

Crystal habit Massive, nodular, botryoidal

Cleavage none

Fracture Irregular/uneven, sub-conchoidal

Tenacity Brittle to sectile

Mohs scale hardness 2.5–3.5 (7 for chrysocolla chalcedony, high-silica content)

Luster Vitreous to dull

Streak White to a blue-green color

Diaphaneity Translucent to opaque

Specific gravity 1.9–2.4

Optical properties Biaxial (−)

Refractive index nα = 1.575–1.585 nβ = 1.597 nγ = 1.598–1.635

Birefringence δ = 0.023–0.050

History

The name chrysocolla comes from the Ancient Greek χρυσός (khrusós) and κολλα (kolla), meaning "gold" and "glue" respectively, in allusion to the name of the material used to solder gold. The word was first used by Theophrastus in 315 BC.

Classification

In the outdated 8th edition of the mineral classification according to Strunz, chrysocolla belonged to the mineral class of “silicates” and there to the department “sheet silicates (phyllosilicates)”, where it is listed together with ablycite, hydrohalloysite and halloysite in the “halloysite series (dioctahedral)” with the system number VIII/E.11a.

In the Lapis classification by Stefan Weiß, last revised in 2018 and formally based on the old classification by Karl Hugo Strunz in the 8th edition, the mineral was given the system and mineral number VIII/E.21-020. This corresponds to the class of "silicates" and the division "ring silicates", where chrysocolla, together with dioptase, forms an unnamed group with the system number VIII/E.21. 

The 9th edition of Strunz's mineral classification, last updated in 2009 by the International Mineralogical Association (IMA), classifies chrysocolla in the class of "silicates and germanates" and within the division "phyllosilicates". Here, the mineral is found in the subdivision "phyllosilicates with kaolinite layers, composed of tetrahedral and octahedral networks", where it forms an unnamed group with the system number 9.ED.20, together with allophane, imogolite, and neotokite.

In the Dana classification of minerals, which is primarily used in English-speaking countries, chrysocolla has the system and mineral number 74.03.02.01. This corresponds to the class "Silicates" and the division "Phyllosilicates: Modulated Layers." Here, it is found within the subdivision "Phyllosilicates: Modulated Layers with Connected Striations" as the sole member of an unnamed group with the system number 74.03.02.

Crystal structure

Chrysocolla crystallizes orthorhombic in an unspecified space group with the lattice parameters a = 5.72 to 5.92 Å; b = 17.7 to 18.0 Å and c = 8.00 to 8.28 Å and 2 formula units per unit cell. 

Characteristics

Chrysocolla is infusible before the soldering iron. It dissolves in hydrochloric acid, depositing powdered SiO2.

Geology

Chrysocolla has a cyan (blue-green) color and is a minor ore of copper, having a hardness of 2.5 to 7.0. It is of secondary origin and forms in the oxidation zones of copper ore bodies. Associated minerals are quartz, limonite, azurite, malachite, cuprite, and other secondary copper minerals. It is typically found as botryoidal or rounded masses and crusts, or vein fillings.

A 2006 study has produced evidence that chrysocolla may be a microscopic mixture of the copper hydroxide mineral spertiniite, amorphous silica and water.

Deposits

This stone has been found across the world in over 2,700 locations. The main deposits are located in Germany (Kreischa and Schneeberg in Saxony, Lauterberg and Kupferberg in Bavaria), Australia (Broken Hill), the United States (Bisbee and Morenci in Arizona, New Mexico, Utah), Russia (Mednorudnyansk) as well as in England, Congo-Kinshasa, France (Alsace, Pyrénées-Orientales, Loire), Israel, Mexico and the Czech Republic.

Use as a raw material

In localized concentrations, chrysocolla serves as a copper ore. As a copper mineral, it is toxic to certain organisms and is therefore used, among other things, as an antifouling additive for underwater coatings, especially in shipbuilding.

Use as gemstone

Due to being somewhat more common than turquoise, its wide availability, and vivid, beautiful blue and blue-green colors, chrysocolla has been popular for use as a gemstone for carvings and ornamental use since antiquity. It is often used in silversmithing and goldsmithing in place of turquoise and is relatively easy to work and shape. Chrysocolla is a popular gemstone due to its vibrant blue-green spotted surface. However, due to its low hardness, water retention, and tendency to crack, the stone is very sensitive to significant heat and physical and chemical stress.

Chrysocolla exhibits a wide range of Mohs hardness ranging from 2 through 7, which is dependent on the amount of silica incorporated into the stone when it is forming. Generally, dark navy blue chrysocolla is too soft to be used in jewelry, while cyan, green, and blue-green chrysocolla can have a hardness approaching 6, similar to turquoise. Chrysocolla chalcedony is a heavily silicified form of chrysocolla that forms in quartz deposits and can be very hard and approach a hardness of 7.

Two very similar mineral intergrowths (rocks) are also commercially available: Chrysocolla quartz is a mixture of chrysocolla and quartz. Eilat stone is a mixture of chrysocolla, malachite, and turquoise. Another misleading trade name is Azulita for an intergrowth of chrysocolla, azurite, malachite, cuprite, and dioptase.

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