ORCEL, JEAN

(b. Paris France, 3 May 1896; d. Paris, 27 March 1978)

mineralogy.

Orcel’s father, Edouard, and engineer in public works, and his mother, Pauline Lesuisse, a teacher, gave him a taste for exact, methodical, and thorough work. He was finishing his studies at the Lycée Henri IV when World War I began. Nevertheless, he was able to continue his studies at the Sorbonne, having been exempted from military service because of poor health. A few years later he married Jeanne Bianconi; they had four children.

Orcel’s primary interest was in chemistry. While working toward a degree in science (1917), he was noticed by F. Wallerant, professor of mineralogy, who took him into his laboratory at the Faculty of Science as a research assistant.

In 1920 Alfred Lacroix, secrétaire perpétuel of the Académie des Sciences in Paris and professor of mineralogy, offered Orcel a post as teaching assistant in his department at the Muséum National d’Histoire Naturelle, where Orcel was to spend the rest of his career. He became professor in 1937, succeeding Lacroix when the latter retired.

Orcel’s work can be divided into several stages, each characterized by a special orientation of his research, often imposed by the functions entrusted to him. From 1917 to 1927 he studied complex hydrated phyllosilicates, chlorites, which were the subject of his doctoral dissertation. This accurate and detailed study, which contains more than three hundred chemical analyses, was made possible by his improvements in methods of analyzing silicates. The fact that chlorites contain a notable quantity of water in their structure led him to test this element quantitatively and to note the temperature at which OH leaves the lattice. He thought this test was essential to define the different species. He used a new technical method found by Saladin and Le Châtelier, differential thermal analysis, and demonstrated that chlorites lose their water in two stages. This easy and swift method is now used universally in geological sciences, chemistry, metallurgy, and the ceramic industry. It was by using X rays and Orcel’s chemical analysis of chlorites that Charles Mauguin established the first model of atomic order in the structure of these minerals.

Orcel made field trips to collect specimens; at Matra in Corsica, he found beautiful crystals of realgar. In 1920, working in collaboration with the Service de la Carte Géologique de France, he reviewed two geological maps. While working on the revision of the St. Jean de Maurienne map, he discovered, in the Comberousse Valley, a rare aluminous chlorite, sheridanite. The revision of the Roanne map enabled him to collect many specimens from the Charrier mine (Allier) for microscopic study.

This work marked the beginning of a study (1927–1937) of the optic properties of opaque minerals, which cannot be studied by transmitting light through them and which are the principal constituent of metallic ores. Orcel not only observed the color and brightness but also devised a measuring apparatus. He put a photoelectric cellule at the upper part of the microscope in reflected light. The cellule was connected to a galvanometer which measured the reflecting power of the minerals. These optical results led him to identify oxides, sulfides, and other opaque species. Using a polarized microscope in reflected light, Orcel established the order of succession of the different minerals in complex ores and reconstructed the genesis of the ore deposit.

From 1937 until 1944 Orcel devoted his time to teaching and to preserving the finest and most precious specimens of the museum’s collection. He was also active in the French Resistance. After the war, Frédéric Joliot asked Orcel to draw up an inventory of uranium deposits in France and in its overseas possessions. With his colleague L. Barrabé he also trained young prospectors. This was the most exciting period of his career, for the was very happy among his students and passed on to them his passion for well-done work and careful research. Results came rapidly, and a very rich uranium mineral, pitchblende, was discovered at Grury (Saôone et Loire) and at La Crouzille (Haute Vienne).

In 1947, with a group he had formed in his laboratory for the purpose of continuing his research on phyllosilicates, Orcel established the Groupe Français des Argiles and served as its president for many years. He also contributed to forming the Commission Internationale pour l”Étude des Argiles (CIPEA), later the Association Internationale pour l’Étude des Argiles (AIPEA).

During his last active years Orcel became interested in philosophical reflections. He also extended the subjects he taught to include geochemistry and cosmochemistry. This, in turn, led to the study of meteorites. He was fascinated by the explorations made on the moon and by the study of specimens collected on it. Meteorites can vary from a one- microgram particle of dust to a block of several tons. Orcel studied the chemical composition of meteorites very thoroughly. He observed that the most common chondrites (stony meteorites) are chemically similar to earthly rocks such as dunites, basic rocks formed by the anhydrous magnesium silicate olivine. Among the chondrites he studied particularly carbon meteorites that contain not only carbon but also hydrated phyllosilicates. Because of this research Orcel was the French delegate to the permanent international committee established to coordinate investigations and research on meteorites.

In 1968, immediately after he retired, Orcel organized remarkable expositions of meteorites, or messagères du cosmos, as he liked to call them. His purpose was to demonstrate that the history of meteorites reveals a long voyage through the cosmos.

Orcel was very courteous and easy to approach. He devoted his final efforts to his students and to the Académie des Sciences of Paris, of which he had been a member since 1963. His colleagues and students remember him as an untiring worker, fond of innovation in the field of mineralogy, and a remarkable experimenter gifted with a curious and sharp mind.

BIBLIOGRAPHY

I. Original Works.A bibliography is in Poggendorff, VIIb, 3759–3761. Orcel published two books: Les minerais (Paris, 1930), written with C. Berthelot, and Les volcans (Paris, 1953), written with E. Blanquet. Among the most important of his 165 articles are “L examen microscopique des minéraux opaques des roches et des minerais métal-liques” and “Emploi de l’s analyse thermique et des diagrammes de diffraction des rayons X dans l’éude des roches argileuses”, in F. Rinne, ed., La science des roches rev. ed., L. Bertrand, ed. (Paris, 1949), and “Météorites et météores”, in Encyclopédie de la Pléiade. Astronomie (Paris, 1962), 1239–1263.

II. Secondary Literature. Simonne Caillère. “Jean Orcel, 1896–1978”, in Bulletin de minéralogie, 102 (1979), 303–305, with portrait, and Jean Wyart, “Notice nécrologique sur Jean Orcel, membre de la section de sciences des l’ univers”, in Comptes rendus de l’Académie des sciences de Paris, 286 (19 June 1978), 139–143.

Simonne CaillÉre