Trilobites with Beekite Rings

[piranha]

The holotype of Anisopyge cooperi Brezinski 1992

 

Brezinski, D.K. (1992)

Permian trilobites from west Texas.

Journal of Paleontology, 66(6):924-943

 

 

Öpik, A.A. (1967)

The Mindyallan Fauna of north-western Queensland.

Bureau of Mineral Resources, Geology and Geophysics, Bulletin, 74(1):1-404  PDF TEXT  74(2):1-167  PDF PLATES

 

Öpik, A.A. (1970)

Nepeid trilobites of the Middle Cambrian of northern Australia.

Bureau of Mineral Resources, Geology and Geophysics, Bulletin, 113:1-47  PDF 

 

[Malcolmt]

Bizzarre

[izak_]

thats interesting.. 

[Ludwigia]

Now that is really cool! I've seen beekite many times before, but never on a trilobite.

[ynot]

So, trilobites had their own version of "hippies" in the cambrian?

 

Neat fossils, thanks for sharing @piranha.

[jpc]

That is cool.  

I know it has been discussed here before, but what is beekite?  

[fossilized6s]

Very neat! I've found a hand full of fossils with beekite, just no bug......yet. 

[doushantuo]

good post,Scott.

 

[Coco]

Hi,

 

5 hours ago, jpc said:

...but what is beekite?  

https://en.wikipedia.org/wiki/Beekite  

 

Very nice trilobite with beekite ! It gives it a healthy look !

 

Coco

[hrguy54]

Those pictures are wild.

 

I immediately thought of the tattoos of the Maori (New Zealand).

[FossilDAWG]

15 hours ago, jpc said:

That is cool.  

I know it has been discussed here before, but what is beekite?  

Beekite is a diagenic deposit that forms during silicification of some fossils.  The silica grows in the form of overlapping rings, rather than completely and seamlessly replacing the fossil.

 

Don C

[piranha]

text from:

 

Holdaway, H.K., & Clayton, C.J. (1982)

Preservation of shell microstructure in silicified brachiopods from the Upper Cretaceous Wilmington Sands of Devon.

Geological Magazine, 119(4):371-382

 

3.b. Beekite Ring Structure

The most characteristic and intriguing type of replacement of calcite is by concentric rings of silica known as 'beekite' (after the Rev. Beek of Bristol who first drew attention to it; Hughs, 1889). It is seen on most bivalve shells, especially Exogyra (PI. 2 A) and pectenids, prior to etching, but in brachiopods the rings are not usually visible until a surface layer of calcite is dissolved away. Beekitized shells, from which all matrix and unsilicified material has been removed, are steely grey to white in colour, hard and brittle.

 

The rings (PI. 2 A) appear as a series of ridges arranged around a central papilla. Where two or more ring systems meet, they do not produce complex interference patterns but abruptly truncate each other. This implies that individual rings of each ring 'nest' have grown sequentially rather than simultaneously. In three dimensions they comprise ellipsoidal to spherical layered systems, rather like the layers of an onion, truncated by the thickness of the shell. In coarse beekite, each layer can be up to 1 mm thick and well rounded where it meets the shell surface. Away from the centre of each system, the rings are usually finer and may coalesce. Since the number of rings in each group on a shell is not constant, the time of initiation and / or the rate of growth was not uniform.

Where a shell is not completely replaced by beekite, the boundary is never adjacent to a beekite ring. The test at the edge of a ring is either more solidly silicified than the beekite area or passes into a milky white nodular material which grades into a white crust, similar to that described above (section 3.a). This distribution would indicate either that the white crust was the precursor of beekite or that replacement was too poorly developed for beekite to have formed here.

In thin section the structure is complex. Silica shows a morphological sequence starting with radiating bundles and sheaths of length-fast chalcedony which exhibit straight, or rarely oblique, extinction up to 20°. This grades into quartzine (length-slow chalcedony with straight extinction), then to lutecite (also length-slow but with oblique extinction up to 30°), and finally indistinct, anhedral grains exhibiting extreme undulose extinction which resemble coalesced bundles of silica fibres (PI. 1B). All phases are usually clear in plane polarized light but may have areas of brown colorization and anomalous optical properties compared with quartz. This was ascribed, by Folk & Weaver (1952), to microscopic, water-filled pores, and by Pelto (1956) to a strained condition of the material with misorientation of the crystallites from fibre to fibre with regions of bad fit, and associated water, between bundles of fibres.

Such a sequence of morphologies has been reported many times before (Jacka, 1974; Orme, 1974; Wilson, 1966; Chowns & Elkins, 1974; Rio & Chalamet, 1980; etc.). Length-fast chalcedony in the form of radiating bundles and sheaths is the usual form of fibrous quartz which occurs as a replacement of skeletal fragments or as chalcedonic overlays or crusts. Quartzine also occurs as colloform overlays or as a replacement of evaporite minerals. Lutecite commonly occurs as a replacement of evaporites (Folk & Pitman, 1971) but may also occur as a replacement of skeletal fragments (Wilson, 1966; Jacka, 1974; Orme, 1974).

Anhedral gains similar to those in the Wilmington material were described by Chowns & Elkins (1974), who also reported a transition to quartzine, and as 'megaquartz' by Jacka (1974) who suggested that this is a solution-precipitation inversion from a metastable silica precursor. This morphology is more usually described as a finer grain size replacement, variously referred to as 'novaculite texture' (Folk & Weaver, 1952), 'granular microcrystalline quartz' (Wilson, 1966; Knauth & Epstein, 1976, and others), and 'silice en petit cristaux polygonaux' (Rio & Chalamet, 1980) and has been found by one of us (C.J.C.) to form a common replacement of opal-CT lepispheres. Indeed, a complete maturation sequence of spherulitic chalcedony to lutecite to quartz was suggested by Orme (1974). Tarr (1938) postulated that microfibrous chalcedony may, in time, pass over into quartz, and White & Corwin (1961) suggested a similar maturation sequence (glass-cristobalitekeatite-chalcedony-quartz) to explain experimental results on the synthesis of chalcedony. A morphological sequence, however, is not evidence of a maturation sequence. In most sedimentary rocks, including the material examined here, there is no clear growth sequence visible, and though some secondary ordering and recrystallization of the material occurs the observed variations in mineral form are probably more a reflection of the original form as precipitated rather than a series of intermediates in a full maturation sequence of opal to quartz.

Beekite replacement almost invariably starts at the thickest part of the shell, in the umbones of brachiopods and bivalves, and spreads outwards, which results in characteristically shaped insoluble residues of partially replaced specimens. This is thought to be the result of two factors, one stochastic and the other mechanistic. Since the beekite is replacing calcite, it is most likely that it will be found where there is most calcite, i.e. at thickest part of the shell. In addition it is possible that partial pressure of carbon dioxide, which is believed to cause replacement (see below, section 5) might build up to a sufficiently high level to initiate silica dissolution more rapidly where the shell is thickest.

[Shamalama]

Way cool. Thanks for the post!