Chessy

Classic Locality

Octahedrons (triangular) and dodecahedrons. The brown triangles on the octahedral faces are usually called hopper zones in the original cuprite, variations in the way the crystal grew.

Defining a “classic locality” for minerals is like trying to establish a firm definition for a classic novel, movie, or car. Anyone and everyone can define it however they want; it’s subjective and arbitrary. As Jolyon Ralph put it in a discussion on MinDat, “A classic mineral locality is a locality that a collector believes is a classic mineral locality.”

Having said that, I submit that one locality that many collectors would call “classic” is Chessy, also known as Chessy-les-Mines, near Lyon in eastern France.

Chessy is known for at least 116 mineral species and is the type locality for azurite, blue copper carbonate, which might be enough to establish its “classic” status, but for me it is even more famous for its spectacular pseudomorphs (“false form,” in which one mineral displays the form of another) of malachite (green copper carbonate) after well-formed crystals of cuprite (copper oxide).

The two examples here from my collection are pretty good as these things go; larger and sharper specimens certainly exist though. They both came to me in 1971 from the collection of my mineralogy professor at Indiana University, Dr. Carl Beck.

9 x 8 mm. This dodecahedron is still largely cuprite, with just a light patina of malachite on the surface.

Cuprite is isometric, with crystals showing typical isometric forms like octahedrons and dodecahedrons. Malachite is monoclinic, a crystal system that cannot produce such forms. But the green in the photos here is definitely malachite, but with the crystal form of cuprite – “classic” pseudomorphs, where the malachite faithfully replaces the cuprite while maintaining the crystal form of cuprite, analogous to silica replacing wood at a molecular level so that tree rings and even individual cell structures are sometimes preserved in petrified wood.

In both of my specimens and in many from Chessy, close examination shows that the alteration to malachite is only “skin deep,” with the outer layer of cuprite altered to malachite. The original cuprite is still there underneath.

The onset of the main phase of the Variscan Orogeny about 380 million years ago. From the Tethyan Plate Tectonic working group of the Institut de Mineralogie et Petrographie, Université de Lausanne. Via Wikipedia. Ultimately, I believe this is from Stampfli, G. M., and Borel, G. D., 2002, A plate tectonic model for the Paleozoic and Mesozoic constrained by dynamic plate boundaries and restored synthetic oceanic isochrons: Earth Planet. Sci. Lett. 196, 17–33.

Chessy lies in the Massif Central, a block of complex crust including continental fragments and oceanic material. About 370 million years ago (Late Devonian) it was part of the leading edge of the collision between Gondwana (Africa and more) and Baltica (the core of Europe) called the Variscan Orogeny.

The copper ore (largely chalcopyrite, copper-iron sulfide) was emplaced as a volcanogenic massive sulfide, an ancient analog to today’s black smokers in rift settings in oceanic crust (Marignac, 1999, Ore Deposits of the French Massif Central: Insight into the Metallogenesis of the Variscan Collision Belt: Mineralium Deposita, 34: 472-504).

Long after the Variscan Orogeny, Triassic (about 220-230 million years ago) sediments were deposited, and they are the host of much of the oxidized zone above the original sulfides. Chalcopyrite, copper-iron sulfide, was oxidized to cuprite, copper oxide. Later more intense oxidation and reaction with chemicals from sedimentary limestones produced the azurite (blue copper carbonate) and began the alteration of cuprite to malachite (green copper carbonate).

The story is highly complex and made more so by the much later collision of Italy to rise up the Alps, just east of the Massif Central, and I don’t pretend the summary above to be anything like a comprehensive report.