Internal structure revealed when the “etching” process is applied to that type of meteorite known as a “granular hexahedrite.” See [p. 120].
After a cut section of an iron meteorite has been polished, the flat surface, except for possible inclusions, is mirror-like and resembles stainless steel. It appears to be remarkably uniform and uninteresting, but this appearance is misleading. A characteristic and beautiful structural pattern develops when such a polished nickel-iron surface is treated with, for example, a special mixture of nitric acid, alcohol, and Arabol glue.
This process of treatment is known as “etching.” The different structural patterns brought out by such etching give us the basis for classifying the iron meteorites.
If the etching process reveals certain features from which we can infer a cubic, or 6-faced, crystalline structure, we classify the iron meteorite as a hexahedrite.
If etching produces a certain special pattern from which we can infer an 8-faced, or octahedral, crystalline structure, we recognize the second subdivision of iron meteorites: the octahedrites. This remarkable pattern was discovered and first described by Alois von Widmanstätten, of Vienna, in 1808.
The third subdivision of iron meteorites consists of the “structureless” ataxites. (From the Greek for “without arrangement.”) On an ataxite, etching brings out only a finely granular pattern with a stippled appearance.
The stones are composed chiefly of minerals that are combinations of various elements with silicon and oxygen—for example, olivine (Mg, Fe)₂SiO₄. Meteorites belonging to this division also contain combinations of elements with oxygen—such as magnesium oxide (MgO) and aluminum oxide (Al₂O₃). Usually, the stony groundmass contains scattered specks, grains, and thin veins of the same shiny nickel-iron alloy that makes up the iron meteorites almost in their entirety.
A. BREZINA & E. COHEN PHOTO Widmanstätten pattern which emerges when the carefully polished surface of that type of iron meteorite technically known as a “fine octahedrite” is “etched.”