Dieudonné Sylvain Guy Tancrède de Dolomieu usually known as Déodat de Dolomieu (Dolomieu, Isère, June 23, 1750 - Chateauneuf, November 28, 1801) was a French geologist; the rock dolomite was named after him.

Déodat de Dolomieu was born in Dauphiné, France, one of 11 children of the Marquis de Dolomieu and his wife Marie-Françoise de Berénger. As a child young Déodat showed considerable intellectual potential and special interest in the natural surroundings of his home in the Alps of southeastern France. Dolomieu began his military career with the Sovereign and Military Order of the Knights of Saint John (also called the Knights Hospitaller or the Knights of Malta) at the age of 12. His association with the Maltese Order caused him difficulties throughout his life, beginning with a duel, which he fought at the age of 18, where he killed a fellow member of the order. For this infraction he was sentenced to life in prison but due to the intercession of Pope Clement XIII he was released after 1 year.

During the period prior to the French Revolution Dolomieu took full part in the intellectual ferment of France and the rest of Europe. He maintained numerous social contacts among the nobility and although he never married, Dolomieu had something of a reputation as a ladies' man. Through his friend and mentor, the Duke de la Rochefoucauld, Dolomieu was made a corresponding member of the Royal Academy of Sciences. He spent his spare time taking scientific excursions throughout Europe collecting mineral specimens and visiting mining areas. His particular interests included mineralogy, volcanology, and the origin of mountain ranges. Although Dolomieu was greatly interested in volcanos, he became convinced that water played a major role in shaping the surface of the Earth through a series of prehistoric, catastrophic events. Dolomieu was not a uniformitarian geologist. His contemporary, James Hutton, did not publish the principle of uniformitarianism until 1795. Dolomieu was an observationalist and spent much time collecting and categorizing geological data. Unlike Hutton, no scientific principles or theories are credited to him, although he left his permanent mark on geology in another way.

During one of his field trips to the Alps of Tyrol (today part of northeastern Italy) Dolomieu discovered a calcareous rock which, unlike limestone, did not effervesce in weak acid. He published these observations in 1791 in the Journal de Physique. The following year, in the same journal, the rock was named dolomie (or dolomite, in English) by Nicolas-Théodore de Saussure. Today both the rock and its major mineral constituent bear the name of Dolomieu, as do the Dolomites, the mountain range in northwestern Italy, where he first identified the rock.

In addition to his scientific activities Dolomieu continued to advance in rank in the Knights of Malta and was promoted to Commander in 1780. However, he continued to have difficulties as a result of his liberal political leanings which were unpopular among the conservative nobility who controlled the Order. Dolomieu retired from active military service in 1780 to devote full time to his travels and scientific work.

Dolomieu was at first a strong partisan of the French Revolution, which began in 1789. However, the murder, of his friend the Duc de la Rochefoucauld, a close brush with the guillotine, and the beheading of several of his relatives, turned him against the revolution. During this time Dolomieu became a supporter of Napoleon Bonaparte. In 1795, having lost his fortune in the revolution, Dolomieu accepted the position of Professor of Natural Sciences at the École Centrale Paris and contracted to write the mineralogical portion of the Encyclopédie Méthodique. The following year he was appointed Inspector of Mines and Professor at the École nationale supérieure des mines de Paris, where his portrait still hangs in the library. His extensive mineral collection is today housed at the Muséum National d'Histoire Naturelle de Paris.

By 1798 Dolomieu had developed an international reputation as one of the leading geologists in the world and was invited to join the scientific expedition accompanying Bonaparte's invasion of Egypt, as part of the natural history and physics section of the Institut d'Égypte. In March of 1799 Dolomieu became ill and was forced to leave Alexandria, Egypt, for France. His ship, caught in a storm, sought refuge at the port of Taranto, Italy, where Dolomieu was made a prisoner of war. The city was part of the Kingdom of the Two Sicilies, which was then at war with France. Dolomieu had previously made a powerful enemy of the Grand Master of the Maltase Order when he helped negotiate the surrender of the island of Malta to Napoleon. The Grand Master denounced Dolomieu and he was transferred to Messina, Sicily, and imprisoned under horrible conditions, in solitary confinement, for the next 21 months.

The imprisonment of a world-famous scientist, under such conditions, was abhorrent to the intellectual community of Europe. Even the scientific community of England (who was at war with France) protested the confinement. Talleyrand, the French foreign minister, attempted to negotiate Dolomieu's release through the Pope. Napoleon, who was First Consul of France at the time, felt that asking for such an intervention by the Pope would be dishonorable. The future Emperor's approach to the problem was more direct. In the spring of 1800 Napoleon lead the French army into Italy, delivering a crushing blow to the Austrians and their Italian allies on June 14, at the Battle of Marengo. All of Italy then came within Napoleon's sphere. One of the terms dictated by Napoleon in the peace treaty of Florence (March 1801) was the immediate release of Dolomieu.

Upon his liberation Dolomieu resumed his scientific studies and field excursions. But his health, broken by the long imprisonment in Sicily, gave way during a trip to the Alps. Déodat de Dolomieu died on November 28, 1801 at the home of his sister at Châteauneuf, not far from his boyhood home in Dauphiné.