Re: ABSRD (FYI) Carbon Crystals and Fossils In Rock

[Marilyn Wegweiser <mdwegweiser@bsuvc.bsu.edu>]

Title: Re: ABSRD (FYI) Carbon Crystals and Fossils In Rock

At 3:14 PM -0400 4/26/01, NJPharris@aol.com wrote:

> In a message dated 4/26/01 4:13:47 AM
> Eastern Daylight Time,
> luisrey@ndirect.co.uk writes:
>
> > So we have specialized "crystal mineral filtrations"
> for different parts of
> > the body that include barbules all along the long fibers and
> soft pliable
> > hair on top of the snout?
> Indeed--and since when do crystals leave
> carbon traces?

I can't resist this one...(please, please forgive me, and read me with a HUGE tongue in Cheek, OK?)

Pure crystals leave carbon traces when they are pure carbon crystals: isometric, predominantly octahedrons of greater or lesser perfection. Also occurs, but less commonly so, as dodecahedrons, cubes, and tetrahedral crystals.  Perfect cleavage is parallel to o[111] (Miller indices)

Graphite is the one we most associate with a perfect hexagonal crystal but most commonly occurring in foliated masses, also scaly, columnar, earthy and in radiate aggregates. It leaves a distinctive shining black streak. (See Sinkakas, any version for additional information on the Carbon Group)

With respect to fossils and partings, in general, the emplacement of an impurity in a rock (a fossil) occurs along a bounding surface that forms more commonly to those who are not sequence stratigraphers as what might be recognizable as a bedding plane.

Note: fluvial deposits will be commonly, and in general, cross bedded to some extent, as will then the fossils (going against the "trend" of the bedding plane - what there is of it) - so they do not fall directly in to this category.

If it (the impurity, aka the Fossil) is fortunate enough to be in a fine grained lithology, and given the right environment of deposition, the fossil will form a natural plane of weakness on the bounding surface in the ionic bonds surrounding it within the rock unit. In a lithology containing many 4-phi or greater (really SMALL) clasts of various clay minerals (plagioclase and orthoclase, along with the micas) - which by the way are the most common things left over in shale - (along with quartz) - then the parting is facilitated by the tendency to cleave along the weak ionic bond forming the cleavage plane in the sheet silicate microstructure of each mica clast.

Whack it (the rock) with a chisel on its edge in the right place and ta-da-boom the rock splits for you as if Moses were parting the waters. Then you have the mold and cast in many cases, or two you have two halves both with some original fossil material attached to them - depending. It's quite lovely to watch, splitting rock like this. With a little practice you can get incredibly skilled at it.  Nearly all invertebrate paleontologists acquire their skill at this in plant containing units or in Grabeau's Trilobite Beds. ;^)

Best regards,

Marilyn W.

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                     =00=  =00=  =00=  =00=
                  Marilyn D. Wegweiser, Ph.D.
             Adjunct Curator of Vertebrate Paleontology

                        Cincinnati Museum Center

                     Geier Collections and Research Center
     
Assistant Professor of Geology
Department of Geology                     mdwegweiser@bsu.edu
Ball State University                        Office: 765-285-8268;765-285-8270
Muncie, Indiana        47306                   FAX:    765-285-8265