MARCASITE  , a

mineral with the same chemical composition as pyrites, being iron disulphide FeS2, but crystallizing in the orthorhombic instead of in the cubic system . The name is of Arabic origin and was long applied to crystallized pyrites (q.v.); it was restricted to the present species by W . Haidinger in 1845 . The mineral was known to G . Agricola in 1546 under the names Wasserkies or Wesserkies and Leberkies, and it has been variously known as white pyrites, hepatic pyrites, lamellar pyrites, radiated pyrites (German Strahlkies) and prismatic pyrites . The orthorhombic form of the crystals, as distinct from the cubic form of pyrites, was recognized by Rome de l'Isle in 1772, though later R . J . Hauy considered the crystals to be only distorted cubic forms . The crystals are isomorphous with mispickel (q.v.), but only rarely are they distinctly developed and simple (fig.) . Usually. they are twinned on a prism plane, M, producing pentagonal stellate groups of five crystals; twinning on the plain g, in which the crystals intercross at angles of nearly 6o°, is less common . This frequent twinning gives rise to characteristic forms, with many re-entrant angles, to which the names " spear pyrites " and " cockscomb pyrites " are applied . The commonest state of aggregation is that of radially arranged fibres, the external surface of the mass being globular, nodular or stalactitic in form .

Apart from crystalline form, the external characters of marcasite are very similar to those of pyrites, and when distinct crystals are not available the two species cannot always be easily distinguished . The

colour is usually pale bronze-yellow, often rather lighter than that of pyrites; on freshly fractured surfaces of pure marcasite the colour is tin-white, but this rapidly tarnishes on exposure to air . The lustre is metallic and brilliant . The streak is greyish or brownish-black . The hardness (6–62) is the same as that of pyrites, and the specific gravity (4.8–4.9) as a rule rather less . Arsenical varieties of marcasite, containing up to 5% of arsenic, are known as lonchidite and kyrosite . Marcasite readily oxidizes on exposure to moist air, with the production of sulphuric acid and a white fibrous efflorescence of ferrous sulphate, and in course of time specimens in collections often became completely disintegrated . In nature it is frequently altered to limonite with the separation of native sulphur . Marcasite is thus the less stable of the two modifications of iron disulphide . Many experiments have been made with a view to determining the difference in chemical constitution of marcasite and pyrites, but with no very definite results . It is a noteworthy fact that whilst pyrites has been prepared artificially, marcasite has not . Marcasite occurs under the same conditions as pyrites, but is much less common .

Whilst pyrites is found abundantly in the older crystalline rocks and slates, marcasite is more abundant in

clays, and has often been formed as a concretion around organic remains . It is abundant, for example, in the plastic clay of the Brown Coal formation at Littmitz, near Carlsbad, in Bohemia, at which place it has been extensively mined for the manufacture of sulphur and ferrous sulphate . In the Chalk of the south-east of England nodules of marcasite with a fibrous radiated structure are abundant, and in the Chalk Marl between Dover and Folkestone fine twinned groups of " spear pyrites " are common . The mineral is also met with in metalliferous veins, though much less frequently than pyrites; for example the " cockscomb pyrites " of the lead mines of Derbyshire and Cumberland . (L . J . S.) MARCEAU-DESGRAVIERS, FRANCOIS SEVERIN (176)–1796), French general, was born at Chartres on the 1st of March 1769 . His father was a law officer, and he was educated for a legal career, but at the age of sixteen he enlisted in the regiment of Savoy-Carignan . Whilst on furlough in Paris Marceau joined in the attack on the Bastille (July 14, 1789); after that event he took his discharge from the regular army and returned to Chartres, but the embarrassments of his family soon compelled him to seek fresh military enployment . He became drill instructor, and afterwards captain in the depart-mental (Eure-et-Loire) regiment of the National Guard . Early in March 1792 he was elected lieutenant-colonel of one of the battalions of the Eure-et-Loire; he took part in the defence of Verdun in 1792, and it fell to his lot to bear the proposals of capitulation to the Prussian camp . The spiritless conduct of the defenders excited the wrath of the revolutionary authorities, and Marceau was fortunate in escaping arrest and finding re-employment as a captain in the regular service .

Early in 1793 he became with other

officers " suspect, " and was for some time imprisoned . On his release he hurried to take part in the defence of Saumur against the Vendean royalists, and distinguished himself at the combat of Saumur (June to, 1793) by gallantly rescuing the representative Bourbotte from the hands of the insurgents . The Convention voted him the thanks of the country, and thenceforward his rise was rapid . His conduct at Chantonnay (Sept . 5) won him the provisional rank of general of brigade . On the 17th of October he bore a great part in the victory of Cholet, and on the field of this battle began his friendship with Kleber . For the victory of Cholet Kleber was made general of division and Marceau confirmed as general of brigade . Their advice was of the greatest value to the generals in command, and the military talents of each were the complement of the other's . Marceau, who became general of division (Nov . 1o), succeeded to the chief command ad interim, and with his friend won important victories near Le Mans (Dec . 12–13) and Savenay (Dec . 23) .

After the battle of Le Mans, Marceau rescued and protected a

young Royalist lady, Angelique des Mesliers . It is often supposed that he was in love with his prisoner; but the help even of the commander-in-chief did not avail to save her from the guillotine (Jan . 22, 1794) . Marceau had already retired from the war, exhausted by the fatigues of the campaign, and he and Kleber were saved from arrest and execution only by the intervention of Bourbotte . Marceau became affianced about this time to Agathe Lepretre de Chateaugiron, but his constant military employment, his broken health, and the opposition of the comte de ChMeaugiron on the one hand and of Marceau's devoted half-sister " Emira," wife of the Republican politician Sergent, on the other, prevented the realization of his hopes . After spending the winter of 1793–1794 in Paris he took a command in the army under Jourdan, in which Kleber also served . He took part in the various battles about Charleroi, and at the final victory of Fleurus (June 26, 1794) he had a horse shot under him . He distinguished himself again at Jiilich and at Aldenhoven, and stormed the lines of Coblenz on the 23rd of October . With the Army of the Sambre and Meuse he took his share in the campaign of 1795 on the Rhine and the Lahn, distinguishing himself particularly with Kleber in the fighting about Neuwied on the 18th and 19th of October, and at Sulzbach on the 17th of December . In the campaign of 1796 the famous invasion of Germany by the armies of Jourdan and Moreau ended in disaster, and Marceau's men covered Jourdan's retreat over the Rhine . He fought the desperate actions on the Lahn (Sept . 16 and 18), and at Altenkirchen on the 19th received a mortal wound, of which he died on the 21st, at the early age of twenty-seven .

The Austrians vied with his own countrymen in doing

honour to the dead general . His body was burned, and- his ashes, which at the time were placed under a pyramid designed by Kleber, were transferred in 1889 to the Pantheon at Paris . See Maze, Le General Marceau (1889) ; Parfait, Le General Marceau (1892) ; and T . C . Johnson, Marceau (London, 1896) .