[meteorite-list] Crust Descriptions...was Ablation

From: MEM <mstreman53_at_meteoritecentral.com>
Date: Mon, 23 Nov 2009 04:07:56 -0800 (PST)
Message-ID: <289144.85038.qm_at_web55208.mail.re4.yahoo.com>

Certainly the best collection of observations set to text I've seen. Thanks for digging up the details Fred. This also goes into detail as to the kind of structures that can be seen over the various faces of the meteoroids orientations.

One exception: Yes "glassy" is a term used to describe luster but "glassy:composed of glass" has a specific meaning in mineralogy. I believe the use in this context is in respect to composition not luster. Otherwise the term "metallic" should be used for irons.

Very few meteorites-- notably eucrites have a glassy-lustered crust, however most all silicate bearing meteorites have a glassy crustal composition in the mineralogical sense. Some of the Silicates are the only class of minerals I can think of which can form glass, although there are likely others. Iron silicates form a dark glass upon melting/quenching however free iron oxidizes and forms magnetite-class minerals and therefore not "glass". Ordinary chondrites have both components. The mention or non mention of the elements is understood by anyone familiar with the mineralogy. Same issue Fe vs Mg in the color of olivine in pallasites.

Regards,
Elton

PS Speaking of simple errors which make it into our reference sources: We know now that ablation and crust formation result from liberation of heat during compression and not because what was long believed to be from "air friction". Sometimes accuracy suffers when one tries to get the main point across. There is always room to ask the source again for clarification.

--- On Mon, 11/23/09, debfred at att.net <debfred at att.net> wrote:

> ? ?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 07:07:56 AM PST