33. In the Siderites the iron generally varies from 80 to 95 per cent., and the nickel from 6 to 10 per cent.; in the Santa Catharina siderite (of which the meteoric origin is somewhat doubtful) 34, and in that of Oktibbeha County 60, per cent. of nickel have been found: the nickel is alloyed with the iron, and several of the alloys have been distinguished by special names. Owing to the presence of the nickel, meteoric iron is often so white on a fractured surface as to be mistaken for silver by its finder; it is also less liable to rust than ordinary iron is. Troilite is frequently present as plates, veins or large nodules, sometimes surrounded by graphite; schreibersite is almost always found, and occasionally also daubréelite.
Evolution of gases on heating.
Further, various chemists have proved that hydrogen, nitrogen, marsh gas, and the carbonic oxides are evolved when meteoric iron or stone is heated; in one case a trace of helium was detected. Probably the gases were not present in the occluded state, but resulted from the decomposition or interaction of non-gaseous constituents during the experiments.
Pane 4l.
Figures produced by action of acids or bromine.
Etched figures.
34. The want of homogeneity and the structure of meteoric iron are beautifully shown by the figures generally called into existence when a polished surface is exposed to the action of acids or bromine; they are due to the inequality of the action on thick or thin plates of various constituents, the plates being composed chiefly of two nickel-iron materials termed kamacite and tænite. A third nickel-iron material, filling up the spaces formed by the intersection of these plates of kamacite and tænite, is termed plessite; it is probably not an independent substance but an intergrowth of the first two kinds.
In the Agram iron, investigated by Widmanstätten in 1808, the plates are parallel to the faces of the regular octahedron; such figures are well shown by the exhibited slice of the Toluca iron; different degrees of distinctness of such "Widmanstätten" figures are illustrated by specimens Pane 4l. of Seneca River, Zacatecas, Charcas, Burlington, Jewell Hill, Lagrange, Victoria West, Nelson County, and Seeläsgen. The large Otumpa specimen, mounted on a separate pedestal, furnishes a good example of the less distinct, and more or less damascene, appearance presented by the etched surface of some meteoric irons of octahedral structure.
The Braunau iron gives no "Widmanstätten" figures, but has cleavages parallel to the faces of a cube; on etching it yields linear furrows which were found (1848) by Neumann to have directions such as would result from twinning of the cube about an octahedral face; as illustrations of the "Neumann lines," etched specimens of Braunau and Salt Pane 4l. River are exhibited.
For meteoric irons of cubic structure the percentage of nickel is lower than 6 or 7; for those of octahedral structure it is higher than 6 or 7, and the plates of kamacite are thinner, and the structure therefore finer, the higher the percentage of that metal. A considerable number of meteoric irons, however, show no crystalline structure at all, and have percentages of nickel both below and above 7; it has been suggested that these masses have been metamorphosed, and that crystalline structure was once present, but has disappeared as a result of the meteorites having been heated, not merely superficially during their passage through the earth's atmosphere, but throughout their mass while travelling in outer space.