FMF - Friends of Minerals Forum, discussion and message board

[James Catmur]

Just attended a talk about impact craters during which a photo was shown of a diamond pseudomorph after graphite, from the Popigai impact crater in Russia. Has anyone seen one? The speaker had the photo and said they are used as industrial diamonds and not available easily. None on MinDat, but diamonds are mentioned.

[Jesse Fisher]

Is this what is referred to as "bort"? I think this is a diamond heavily included with graphite. Usually ground up for industrial purposes. I've never heard of it called a pseudomorph, however.

[James Catmur]

He showed a photo of a graphite shaped diamond.

[Pete Richards]

Could it have been a flat macle twin of diamond? I don't know about this in diamonds, but some analogous twins of spinel are very flat and have six or even 12 sides, and look very strongly hexagonal.

[Matt_Zukowski]

Along the lines of where Pete R was going, why do they think it is a pseudomorph? Is it just that it outwardly looks hexagonal? Is it near and aligned with graphite? What are the dimensions?

[Bob Carnein]

It seems there is not enough information to discuss this.

[alfredo]

May perhaps be off topic, but pseudomorphs the other way around, graphite after diamond, exist in two locations, in northern Morocco and southern Spain, geologically related to each other despite being on different continents. Supposedly this happens when a diamond-bearing kimberlite rises to the surface slowly instead of the more typical explosively rapid eruption.

[James Catmur]

He gave almost no information about it, but says they are found at two locations, both of which are impact craters. As the best location for large ones (Popigai) is in Russia they are almost impossible to obtain. At the other location (which he did not mention) they are very small

Found them with a search:

In the Popigai impact, the conditions needed to form diamond were present for just an instant of time. This flash formation converted flakes of graphite in the Archean graphite-garnet gneiss into diamond. Many of the diamonds produced were small polycrystalline stones that are approximately the same size and shape as the graphite flakes in the gneiss. Most are tiny stones under 2.0 millimeters in size that might be suitable for producing diamond abrasives. Because these diamonds were formed in a flash of heat and pressure, there was insufficient time for large, single-crystal stones of great clarity and purity to develop.

[ https://geology.com/articles/popigai-crater-diamonds/ ]
(link normalized by FMF)

These diamonds are usually 0.5 to 2 mm (0.020 to 0.079 in) in diameter, though a few exceptional specimens are 10 mm (0.39 in) in size. The diamonds inherited the tabular shape of the original graphite grains and also the original crystals' delicate striations.

[ https://www.wikiwand.com/en/Popigai_impact_structure ]
(link normalized by FMF)

So are they true pseudomorphs?

[James Catmur]

This says:

paramorphoses on graphite crystals

[ https://link.springer.com/article/10.1007/s00710-022-00778-y ]
(link normalized by FMF)

It also mentions other locations

So it looks like I have an answer. I found his quick comment in the talk interesting

James