Information about Atacamite

Atacamite is a striking copper chloride mineral, highly regarded among collectors for its vivid, deep emerald-green crystals. It is relatively uncommon and forms only under a very specific and unusual combination of conditions, which limits it to comparatively few localities worldwide.

It typically forms as slender, striated prismatic crystals, often grouped in sprays or clusters on a pale matrix. It is typically bright emerald-green to dark blackish-green in colour, with a vitreous to adamantine lustre. Individual crystals can be transparent to translucent, and the combination of their deep colour and bright lustre makes fine specimens particularly impressive.

Atacamite forms as a secondary mineral – that is, it develops through the weathering and alteration of pre-existing copper minerals – but only where the weathering occurs in the presence of both chloride ions (from salt, seawater, or saline groundwater) and very limited moisture.

It is therefore most commonly found in very arid, desert environments, or in coastal locations where copper-bearing rocks have been exposed to sea spray. It also forms more unusually as a volcanic sublimate at active fumaroles such as Mount Vesuvius and Mount Etna, where it deposits directly from hot gases.

It is polymorphous with botallackite, clinoatacamite, and paratacamite – meaning all four minerals share the same chemical formula but have different crystal structures. This can make definitive identification tricky, and many specimens sold or labelled as atacamite prove, on analysis, to be one of its polymorphs instead.

A note for collectors: UK specimens labelled as atacamite should be treated with some caution, as research has shown that many British occurrences of what was historically identified as atacamite are in fact the commoner polymorph paratacamite or clinoatacamite. Specimens from well-documented localities with published references are the most reliable.

Uses and History

Atacamite has a remarkably varied history for a mineral that is relatively rare. Its vivid green colour meant it was used as a pigment by the ancient Egyptians – powdered atacamite has been identified in green-coloured ceramics, glass, and cosmetics – and it was later used in the green inks found in medieval illuminated manuscripts and frescoes across Eurasia.

Perhaps its most famous historical connection is with the Antikythera mechanism – the astonishing ancient Greek mechanical computer recovered from a Roman shipwreck in the Aegean Sea. Much of the bronze of the mechanism had been converted to atacamite by centuries of exposure to seawater, and the mineral’s presence actually helped preserve some of its surface detail. Atacamite has also been identified in the patina of the Statue of Liberty, and in the surfaces of many ancient bronze and copper artefacts recovered from marine or chloride-rich environments.

Atacamite was first formally described in 1802 by the Russian mineralogist and volcanologist Dmitri de Gallitzin, from specimens collected in the Atacama Desert of northern Chile – its type locality – and the mineral takes its name directly from that region. Atacamite was at one time a minor ore of copper at the Chuquicamata Mine in Antofagasta, Chile, one of the largest open-pit copper mines in the world.

Notable localities include the Atacama Desert in Chile, which has produced classic tabular crystals on pale halloysite matrix; South Australia, particularly the Moonta, Wallaroo, and Burra districts, where some of the largest crystals ever found – reportedly exceeding 20 cm – were collected; Tsumeb in Namibia; Bisbee in Arizona, USA; Lavrion in Greece; and the Turinsk copper mine in the Ural Mountains of Russia.

In England, classic localities include Roughton Gill in the Caldbeck Fells, Cumbria; and the Penberthy Croft Mine at St. Hilary and Botallack Mine at St. Just, both in Cornwall. In Wales, atacamite has been recorded from Penrhyn Du on the Llŷn Peninsula in Gwynedd, where deep emerald-green microcrystals coat weathered copper-bearing rock exposed to seawater.

Mineralogy

Hazards and Warnings

Toxic mineral: contains copper and chloride. Mineral collectors should wash their hands after handling specimens, to avoid any exposure to potential toxins.

Almost all rocks, minerals (and, frankly, almost all other substances on earth) can produce toxic dust when cutting, which can cause serious respiratory conditions including silicosis. When cutting or polishing rocks, minerals, shells, etc, all work should be done wet to minimise the dust, and a suitable respirator or extraction system should be used.

Translations

Further Reading / External Links

• https://en.wikipedia.org/wiki/Atacamite
• https://www.mindat.org/min-406.html
• https://museum.wales/mineralogy-of-wales/database/?mineral=44&name=Atacamite
• https://www.handbookofmineralogy.org/pdfs/atacamite.pdf