Mystery trails on Calcite crystal

[DennisAUS]

I have a sample of calcite crystal from Corop in Victoria, Australia. It has a collection of "growths" which I struggle to understand. Given these rocks are Cambrian and were formed in very hot conditions (volcanic, underwater ) no fossils could be expected. It's more likely to be a mineral dissolution feature, but not possible to tell. My problem is the way the "worm holes" butt against each other without joining, suggesting some kind of organic replacement. My mystery appears to follow fractures and isn't evenly tubular. It comes from a road metal quarry in the vicinity of a fault region filled with minerals, where the calcite is among material blasted from a vertical wall so the depth and surrounding rock is not clear. Perhaps it has joined the site over the eons since the rock formed. My local museum geology department has no answer yet. Photographs taken with microscope X10. Any ideas?

[DPS Ammonite]

It looks like the tubular/tabular brownish layered areas formed first in an open cavity probably by mineralized water. Layers of brownish minerals possible coated elongated crystals. Later on, the spaces between the brownish minerals was filled with larger white crystals of calcite. I doubt that the larger white calcite crystals were burrowed by “worms” or dissolved and later the tubular holes were lined with the brownish minerals.

[DennisAUS]

Given the way the brown and white follow fractures around corners and over edges I think the white calcite main crystal mass came first. The brown may then have been laid down in layers with white, but why do the formations not join? Even if there were originally other crystals in the spaces, I would have expected a continuous single edge where the brown formed. On further trips I have looked for more samples of the same material, without success, so I only have one small sample. Thanks anyway for your comments. I will ponder further.

[Ludwigia]

I would concur basically with DPS's evaluation. It could however also be the case that the brown and white bits are broken off from a previous formation from the volcanic spring and were then floating in the mineral gel before it hardened. Perhaps a view of the complete rock would help.

[DennisAUS]

Here are a few un-magnified shots of the whole sample. Total length is about 2.5 inches. Sample was amongst quarry blasting debris when found, so original situe is not known

[DPS Ammonite]

14 minutes ago, DennisAUS said:

Here are a few un-magnified shots of the whole sample. Total length is about 2.5 inches. Sample was amongst quarry blasting debris when found, so original situe is not known

Seeing the whole rock makes me more certain that a single crystal of calcite grew around a layered mineral. The tan mineral is basically an inclusion in the calcite.

[DennisAUS]

So the egg came before the chicken. Thanks. I will be keeping this sample!

 

[Shamalama]

14 hours ago, DPS Ammonite said:

Seeing the whole rock makes me more certain that a single crystal of calcite grew around a layered mineral. The tan mineral is basically an inclusion in the calcite.

 Absolutely agree. You have massive Calcite there, no real crystal faces, just cleavage. the Brown areas really imply to me some sort of matrix that was either heavily eroded host rock, or another, Iron based, mineral that formed first and then the Calcite filled in.

[DennisAUS]

Thanks. Obvious when looking at the larger picture rather than the magnified.

 

[DennisAUS]

I found another piece and cut a slice to expose a non fractured part. Seems to verify the "inclusion" theory. I have had a suggestion from elsewhere that it might be a "wormhole" caused by acid solutions (Google wormholes in calcite) However that would produce a tree pattern with pointed branches, which is not the case. The inclusions are gathered to one side of the calcite layer.

[doushantuo]

Looks POSSIBLY like ambient inclusion trails.

Edit: they have been found in/near Strelley Pool,Btw,as an example for the part of the globe you're coming from

below: image from 

Ambient Inclusion Trails:Their recognition,Age Range,and Applicability to early life on Earth

David Wacey,Matt Kilburn,Crispin Stoakes,Hugh Appleton,Martin Brasier

in:Dilek et al ,editor: Links between geological processes,Microbial activities and evolution of life/Springer publishing/2008