Gypsum in the Atacama Desert, northern Chile

Caracoles Mining District, Sierra Gorda

massive gypsum [290 kb]

Fig. 1: A large mass of apparently massive gypsum (alabaster), sculpted by exposure on the desert floor. Photo taken 13 April 1996. The local rocks (limestone and calcareous siltstone) are cut by veinlets, up to 25 mm thick, of fibrous selenite. Local opencut mines worked lead and manganese -bearing veins. The fluted face seen in this mass is typical of weathered surfaces.

"Rock of the Month # 288, posted for June 2025" ---

Gypsum

Here is a short note, to end the 24th year of the Rock of the Month! And a very common mineral, gypsum, calcium sulphate, CaSO4·2H2O, which is soft (2 on the Mohs hardness scale) and very widely distributed. Gypsum can occur in various varieties, including massive (alabaster), and fibrous, lustrous and translucent (selenite or satin spar). It is distinctive under the microscope, within host sediments which may be carbonate or occasionally clastic (Adams et al., 1984, pp. 86-88; p.20). See also examples from Texas and New Brunswick.

This sample was photographed in the Caracoles mining district, northeast of Antofagasta in the Atacama desert of northern Chile. See also another sample, and brief notes on the local geology, concerning secondary manganese oxide crusts (pyrolusite dendrites) in this locality. Quoting that article: "Extensive beds of the common sulphate gypsum (CaSO4·2H2O), some brecciated, are present in the Caracoles area. Some exposed faces of white gypsum develop distinctive surfaces, smooth but fluted on a centimetre scale. The region is noted for gypsum (Ferraris and Di Diase F, 1978). The gypsum (yeso in Spanish) occurs in Jurassic marine sediments of the Caracoles Group, >800 m of sandstone, lutite, gypsum and limestones (Ramirez R and Gardeweg P, 1982, p.23)".

REFERENCES

Adams,AE, MacKenzie,WS and Guilford,C (1984) Atlas of Sedimentary Rocks under the Microscope. Longman, 104pp.

Ferraris B,F and Di Diase F,F (1978) Hoja Antofagasta, Region de Antofagasta. Servicio Nacional de Geologia y Mineria, Carta Geologica de Chile, sheet 30, 1:250,000 scale map and book (48pp.) (in Sp.).

Ramirez R,CF and Gardeweg P,M (1982) Hoja Toconao, region de Antofagasta. Servicio Nacional de Geologia y Mineria, Carta Geologica de Chile, sheet 54, 1:250,000 scale map and book (121pp.) (in Sp.).

Graham Wilson, 30-31 May 2025

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