[meteorite-list] Ablation Zone 5 Layers...Not

From: debfred at att.net <debfred_at_meteoritecentral.com>
Date: Mon, 23 Nov 2009 08:03:34 +0000
Message-ID: <112320090803.23315.4B0A41D60006305000005B1322243323629B0A02D29B9B0EBF0B0A9D000D0A0B_at_att.net>

List, I think Jason has done a fine job with his position with which I agree. I don't want to beat a dead horse but Here are published definitions of fusion crusts on irons.

  ?Fusion Crust? Definition from Glossary of Geology 2nd edition 1980; American Geological Institute, Falls Church, Virginia, 751pp.
 
Fusion crust A thin glassy coating, usually black and rarely exceeding one millimeter in thickness, formed on the surface of a meteorite by frictional heating during atmospheric flight.
Owing to differing effects of the atmosphere upon different meteorite surfaces, fusion crusts may be knobby, striated, ribbed, net, porous, warty, or scoriaceous.

Several points stand out upon careful reading of this definition. The term glassy is an adjective not a noun. Glassy is a term commonly used in describing surface luster in mineralogy. Composition: Silica, silicate, silicon and iron are not mentioned at all. Apparently composition is not pertinent to the matter. Other terms like warty and seem to refer to fusion crust on irons.

Looking further for definitions used specifically by meteoriticists we find a definition on pages 44-45 by Brian Mason in his classic book, ?Meteorites?. Mason presents the classification of Krinov (1960) that also seems to have been a source for the Glossary?s definition. It is rather long but I will post the complete definition because many people have complained there wasn?t a good published definition of fusion crust for meteorites.

Fusion Crust

A freshly fallen meteorite is covered with a fusion crust, except on fracture surfaces produced on or shortly before impact. The nature of the fusion crust varies somewhat, according to the composition of the meteorite. On most meteorites it is black, but on the enstatite achondrites it is colorliess or pale yellow, because of the almost complete absence of iron in these meteorites. The fusion crust on iron meteorites is very thin, only a fraction of a millimeter, and consists of magnetite. The fusion crust on stone meteorites is thicker, and consists of a black glass which includes fragments of the less fusible mineral such as olivine. On chondrites and the calcium-poor achondrites the crust is usually dull, but on calcium-rich achondrites, which are more fusible than other stony meteorites, the crust is shiny and lustrous, and also thicker than on other types. Sometimes the crust shows a network of fine cracks, similar to the ?crazing? which appears in the glaze of pottery. Krinov (1960) has made detailed
studies of the nature of meteorites, and presents the following classification:

Class I. Crust of Frontal Surfaces

Type 1, Close-textured. The crust is perfectly smooth, as though it had been pressed, with practically no indications of structure. Observed mainly upon iron meteorites and covers surfaces, which generally exhibit considerable marks of atmospheric action, regardless of whether the surface has regmaglyptic relief.

Type 2, Nodular. Upon a smooth, close textured crust, fine angular nodules are visible scattered over the surface of the meteorite. Upon stony meteorites, on which this type is mainly found, the nodules represent fused inclusions of nickeliferous iron.

Class II. Crust of Lateral Surfaces

Type 1. Striated. Upon a smooth, close-textured crust thin striae are visible that seem to be flowing across the surface of the meteorite.
Very often the striae are directed toward the rear part of a meteorite, thus clearly indicating the direction of its motion. Upon iron meteorites the striae quite often end in drops, which are sometimes of spherical form. There are also observed curves or even seemingly broken striae which abruptly change direction. Now and then an intricate pattern of the striated crust can be seen. In rarer cases several systems of striae are observed, superimposed one on top of the other and intersecting at various angles. In such cases the striae of the bottom system appear to be wide and flat, while those of the upper system are thin.
Particularly sharply defined striated crustal structure is observed near the sharp edges of lateral surfaces, adjacent to the rear surfaces of iron meteorites. Upon stony meteorites more or less distinct striae are usually observed along the edges of lateral surfaces. Striae are easily distinguishable on the rims between regmaglypts.

Type 2. Ribbed. Represents an intermediate type between the modular and striated crust and is found only upon stony meteorites. The ribs appear like underdeveloped striae.

Type 3. Net. The crust is formed of short striae running together, lending it the appearance of a fine-mesh net. Individual cells of the mesh appear like stitches. Is observed mainly upon more friable stony meteorites, usually near the edges of lateral surfaces or near protuberances.

Type 4. Porous. At magnification of 15-30X tiny pores can be seen clearly upon the crust. Sometimes the crust appears to be sintered. This effect is observed upon iron as well as upon stony meteorites in the vicinity of the edges of the lateral surfaces, which make no sharp borders with the rear surfaces, It is also observed within various depressions on the surfaces.

Class III. Crust of Rear Surfaces

Type 1. Warty. This type of crust is particularly prevalent and sharply defined upon iron meteorites, and on large meteorites it is clearly visible with the naked eye. Under a magnifying glass, a close texture can be seen made up of individual warts, covering the surface of the meteorite. Each node represents a group of finer nodules which are partly fused together. Quite often among the nodes single globules can be seen of perfect spherical form.

Warty structure upon stony meteorites is as a rule considerably less distinct and is encountered only rarely. It is observed primarily within depressions or along the edges of rear surfaces.

Type 2. Scoriaceous. This type of crust is found primarily among the stony meteorites, for which it seems to be characteristic. Upon iron meteorites the scoriaceous crust is observed only on single portions and in the vicinity of sharp edges or near protuberances. The scoriaceous crust has the appearance of clinkered slag.

?Fusion Crust? Definition from: Mason, B.,1962: ?Meteorites? John Wiley and Sons, New York and London, 274pp.

Regards, Fred Olsen
Received on Mon 23 Nov 2009 03:03:34 AM PST