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Identity HelpNo clue what this is!

27th Sep 2017 16:54 UTCMike Chontofalsky

I was given this specimen at our club picnic/silent auction. No idea what it is. largest prism is 1.5 cm long. Some prisms have 4, 5 or 6 sides! They all appear to be covered with druzzy quartz? No idea of the location. Any help much appreciated.

27th Sep 2017 17:04 UTCPavel Kartashov Manager

This is petrified colonial corall.

27th Sep 2017 17:25 UTCScott Rider

Could it be aragonite or calcite coated with drusy quartz? Is there a portion of the crsytals not covered by quartz? Can you see any cleavage planes, any broken sections?

What it does remind me of are the columns of the Staffa Cliffs in Scotland in miniature form! But I am sure its not basalt LOL!!!

27th Sep 2017 18:03 UTCRichard Gibson 🌟

Agree with Pavel, looks like Favosites colonial coral. Probably Devonian-Carboniferous age.

27th Sep 2017 18:41 UTCEd Clopton 🌟 Expert

Yes, looks like Favosites coral, commonly known as "honeycomb coral" in Iowa where I'm from. I wouldn't be surprised if your piece is from the upper Midwestern US somewhere. These corals often grow in radiating spheroidal colonies. The number of sides on a column is determined by the arrangement of adjoining columns--there's nothing either crystallographic or genetic about the exact number, and the number of sides can even change from bottom to top in the same column. The columns sticking upward in your photos are pointing toward the center of the colony; the outer growing surface is at the bottom.

27th Sep 2017 19:04 UTCOwen Lewis

Anyone want to suggest how coral can have a hexagonal prismatic form? Favosites colonial coral.... Where's the essential honeycomb?

Like Scott, I'd guess aragonite. So we all have the same mineral in mind. it's only over the formation (by biological excretion or standard hydrothermal growth etc.) that we differ in opinion.

27th Sep 2017 19:06 UTCOwen Lewis

27th Sep 2017 19:28 UTCRichard Gibson 🌟

Hexagonal is an easy 'closest packing' arrangement, but Favosites and other corals more or less do whatever is expedient, so the columns may have almost any (reasonable, 7 or fewer usually) number of sides. (See also Petoskey stone, genus Hexagonaria - but those are not always hexagons, either.). All three Favosites specimens that I have are completely silicified, but certainly the original material would have been carbonate, and it may or may not be silicified.

The"prismatic' aspect reflects the growth of the coral - the individual coral animals (that lived in each cell) grew on platforms (tabulae - hence the name of the group, tabulate), and as the sides grew, the animal needed to move on up, creating successive tabulae, which appear as those "prisms" - they are not really hexagonal prisms. The spaces where the animals lived would be filled with something (carbonate mud, often) after the animal died, so what was originally a living space becomes the solid fill seen in the example here. You can find them that are filled, not filled, and all variations between.

27th Sep 2017 19:49 UTCRichard Gibson 🌟

Here's an example where you can see the tabulae as well as the individual animal spaces = some of which are filled and some not. Everything in this specimen is silica, but would have been calcite or aragonite when the coral colony was alive. Approx. 4 cm wide.

27th Sep 2017 20:18 UTCOwen Lewis

Richard,

Yes, that looks look a geodesic structure. Some wasps build a similar structure from chewed wood pulp to make their communal nest. And then there's the similar design, fashioned in wax, of which bee's make their hives. But, in the OP's pic no cellular structure is apparent. Since this essential is missing, it's enough for me to settle for 'aragonite of geological origin'.

@ Mike,

How does the piece 'heft' in the hand. Light for its size or not?

27th Sep 2017 20:50 UTCEd Clopton 🌟 Expert

Mike's specimen surely originally formed as a solid mass of aragonite columns like the one (now replaced by silica) pictured by Richard. Selective lining/coating or replacement of the aragonite in some of the columns (determined by differing water access due to damage, unequal growth rates of individual coral polyps, etc.) would lead to unequal preservation of some of the columns. If silica-bearing water could reach a given part of the relict colony, that part would be silicified and preserved. When the original aragonite structure dissolves away, the silicified columns remain as free-standing "prisms" as seen in Mike's photos. Partially preserved corals of this sort are a familiar sight in streams and gravel pits in eastern Iowa where I spent most of my life, whereas macroscopically crystallized aragonite is virtually unknown.

27th Sep 2017 21:36 UTCMike Chontofalsky

Thanks for all the input. I will inquire about the location at the next club meeting. It was one of the donations not claimed or bought but thrown in my box! I am leaning toward fossil, origin too. There does seem to be a latter coating of silica. Many thanks.

27th Sep 2017 21:44 UTCMike Chontofalsky

I would describe it as normal or expected SG. Definitely not Barite.

27th Sep 2017 22:18 UTCOwen Lewis

So not substantially consisting of cavities then. And not geodesic in structure either.

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