FMF - Minerals Forum, discussion and message board

[Jesse Fisher]

Fluorite crystals, though usually roughly equidimensonal, are sometimes found in mines around the Weardale area in Northern England showing a pronounced tabular appearance due to unequal growth on the three crystallographic axes. This one, found in mid-June 2010 at the Rogerley mine appears to have abandoned a few right angles as well.

[gemlover]

What a beautiful sample, thank you for sharing with us.

John

[James]

Nice one!

James

[Peter Megaw]

Keep digging Jesse!

Say Hi at the pub for me

[chris]

Hi Jesse,

Let's hope you'll find others like this !

Christophe

[Jesse Fisher]

I haven't seen one quite like this in 12 years of digging there. Sadly, a bit dinged up but definitely going into the oddities collection.

[Colleen Thomson]

Cool! i love these oddities. They make you sit up in your seat a bit more and ask questions - like, how did that particular crystal grow like that and not the ones surrounding it from the same pocket? was its growth altered by space restrictions?

I'm no crystallographer - i tend to glaze over after a while :-) but i'm still fascinated by it.
Nice find, Jesse!

[Jesse Fisher]

Not being a crystallographer myself (just a dig in the dirt style geologist) I'm at a loss to explain such things. The North Pennines ore deposits are low temperature MVT, quite similar to the well known ones in Southern Illinois, hence the fluorite is always cubic habit, without octahedral or dodecahedral forms. North Pennines fluorite does show some very unusual characteristics, including the omni-present twinning, well developed vicinal (or growth) faces, and the occasional distortions to the cube. The photo attached is of one of the most extreme examples I have seen, a crystal from the Boltsburn mine now in the NHM London (Russell collection). I am clueless as to what causes these to be so common in the fluorite from the district, and seemingly rare elsewhere, and have never encountered any speculation about it. Anyone care to engage in some speculative arm waving?

[John S. White]

Jesse:

Can't offer any thoughts with respect to how these form and the one you illustrate is by far the most bizarre that I have ever seen. For the benefit of FMF viewers who are not geologists, can I persuade you to define MVT?

Here are a couple that are more typical from the Black Dene mine, Weardale, Durham. The one on the left is 5 cm in length.

[Jordi Fabre]

My contribution to Fluorite's oddities from UK.

In a first view, as a Quartz, it looks rare. And it is rare, because it has the form of octahedral Fluorite, previously existing and substituted by the Quartz. More curious is to observe that the surface is cryptocrystalline Quartz (Chalcedony) but the core of the octahedron, where Fluorite primarily was, is hollow and consists of little geodes covered by Quartz crystals.

[GneissWare]

[Jesse Fisher]

To add a bit of detail - they form at very low temperature (less than 200 C) and typically have a fairly simple mineralogy. The Viburnum Trend and Tri-State district deposits of the central US are primarily galena and sphalerite with lesser pyrite, chalcopyrite, calcite, dolomite, etc. Fluorite-containing varieties are found in Southern Illinois and adjoining areas of Kentucky, and the North Pennine Mountains in England. All are hosted by carbonate rocks, either limestone or dolostone.

[Colleen Thomson]

thanks for the explanations, 'arm waving' and pictures, guys. The NHM crystal is astonishing!
Jordi - the Wh Mary Ann Ps.Chalcedony after Octo Fluorite are pretty cool - Ive got a few of them :-)

[kywawa]

That's great..i always can look such beautiful minerals when i login it this forum..so cool..thanks for sharing.