Callaghanite

Not azurite

Callaghanite, Gabbs, Nevada USA. Field of view about 3.5 x 3.5 cm; inset cluster is about 3 mm across. Ex. Carl W. Beck collection, photo by R. Gibson.

Callaghanite is a beautiful azurite-blue mineral, hydrous copper-magnesium carbonate, Cu₂Mg₂(CO₃)(OH)₆ · 2H₂O. It was described as a new mineral by Carl Beck in 1954 based on work he did while he was a professor at the University of New Mexico (1946-1954, his first teaching position after receiving his PhD from Harvard) prior to his move back to his native Indiana (he was born in Batesville in 1916), where he was my graduate advisor at Indiana University until he died in 1971.

This specimen was probably collected either by geologist Conrad Martin, who found and supplied most of the original material to Beck, or by Beck himself in 1952 near Gabbs, Nevada. Callaghanite occurs in a contact metamorphic zone in Triassic Luning Formation, which is layers of dolomite (calcium magnesium carbonate) and magnesite (magnesium carbonate).

A granodiorite intrusion metamorphosed the carbonates to brucite (magnesium hydroxide) with other magnesium minerals including forsterite (the magnesium silicate end member of the olivine group). The presence of copper should have produced azurite (copper carbonate) as a hydrothermal deposit, but the abundance of magnesium led to the formation of callaghanite. Beck (and Burns, 1954, Callaghanite, a new mineral: American Mineralogist v. 39, p. 630) named it for their colleague Eugene “Pat” Callaghan (1904-1990), Director of the New Mexico Bureau of Mines, Socorro, New Mexico, USA, for his work on magnesite deposits.

Eugene Callaghan (from Proctor, 1990, Memorial to Eugene Callaghan: Geol. Soc. America Memorials, v. 21).

Callaghan was at Indiana University from 1946 to 1949 and was instrumental in attracting and hiring many of the geology faculty who were still there when my generation was educated in the late 1960s and early 1970s, including Charles Vitaliano, John B. Patton, and Judson Mead, as well as Carl Beck who knew Callaghan in New Mexico but came to Indiana in 1954.

Light blue nakauriite. The large callaghanite crystal at upper center is about 1 mm long.

The type locality for callaghanite at Gabbs, Nevada, also contains the rare mineral nakauriite, Cu₈(SO₄)₄(CO₃)(OH)₆ · 48H₂O. It was not described until 1976 from its type locality in Japan (Suzuki and others, 1976, A new copper sulfate-carbonate hydroxide hydrate mineral, Cu₈(SO₄)₄(CO₃)(OH)₆ · 48H₂O, from Nakauri, Aichi Prefecture, Japan: The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists, 71:7), but it is well known from Gabbs as well. The light blue fibrous material in the specimen above is nakauriite, from Beck’s collection from the early 1950s. I have to wonder why he didn’t work on it as well at the callaghanite, but there is no mention of it in his 1954 paper.

Part of the Gabbs magnesite deposit, from Vitaliano and Callaghan, 1956, USGS Mineral Invest. Field Studies Map MF 35. The granodiorite and related igneous rocks are tan to orange and red; the Luning Formation and its metamorphosed and mineralized zones are in blues, greens, and yellow.

The magnesite mining at Gabbs supplied ore to the largest magnesium metal plant in the world, at Henderson, Nevada, during World War II. The map above suggests some of the complexity of this deposit. The site was first exploited for tungsten in the 1920s, but the open-pit mining for magnesium is evident in the lower part of this map just left of center.