FMF - Friends of Minerals Forum, discussion and message board

[John Medici]

The midwest USA has produced many elongated fluorites and pyrites, and a few other elongated crystals that are usually not elongated. Below are a couple of examples, maybe a few duplicates of ones posted in USA-Ohio. Halls Gap, Kentucky has pyrite needles (and rings) that are often found in small quartz geodes. I have also found an elongated cube (only one, of around 500 collected over the years) at Fort Wayne, Indiana, which is similar to the Weardale ones shown; it is a yellow one, clear with sharp edges like the other ones found there.

[Ru Smith]

Fascinating specimens, John.

Do you know of any published research that attempts to explain them?

[Duncan Miller]

With rapid crystallization many minerals form dendritic, acicular or skeletal crystals. In slags for example, where crystal growth is stopped by rapid solidification, one commonly finds skeletal olivines and dendritic magnetite. Ice crystals in snow are a more familiar example. Micromounters see habits that are not frequently encountered in hand specimen. For instance, on page 15 of LAPIS 4/2010 there is a photograph of a clearly dendritic fluorite crystal 1,5 mm long. With more prolonged slow growth these dendritic, acicular and skeletal crystals may fill out and assume more equant shapes. It is not hard to imagine the fluorite needles illustrated above being the rapid-growth precursors to the fatter elongated examples, preserving the same aspect ratio. This would point to fast initial dendritic growth followed by a slower more prolonged equant growth. (This is speculative because I am not an expert on crystal growth, although I have studied a lot of slags petrographically).
Duncan Miller

[Pierre Joubert]

Wow! I wish mr. Ploum could get hold of this fascinating little specimen: https://www.mineral-forum.com/message-board/files/0961ohfluorite_delphos87needles_on_pin_headjpg_387.jpg :-)

[bugrock]

The image of the fluorite on the head of a pin from Ohio is not perfectly sharp but the pattern of growth suggests a series of small stacked cubes, perhaps some elongate and more rectangular than cubic.

Have seen elongate micro cuprite with small steps suggesting a series of stacked cubic to rectangular microcrystals. Without seeing fine detail such groups can look like tapering filaments when the diameter of the cubic crystals changes very gradually along the axis of stacking.

So for the smaller specimens of fluorite one could imagine that the component crystals are conventional cubic form. Might this also happen in the larger specimens? If so two or more cubes would have to grow together with the same diameter and with perfect fusion between the components. Seems unlikely but another possibility to consider?

George

[ian jones]

How could I possibly forget Rogerley, the place that I spend my summers drinking beer and digging rocks.

Although atypical, this rather grubby purple fluorite 6x3x3 cm on the top of the specimen came from the vein as we drove forward in 2012 rather than the flats to the side of the vein where the green fluorite occurs. For some reason the vein fluorites are purple and the flats green.

[ian jones]

I'm starting to think that elongated fluorite crystals from the northern Pennines are perhaps a bit more common than previously thought.

In my collection I have a second Blackdene and also a Boltsburn piece, and, not mentioned as a locations elsewhere, also one from Coldberry Mine, Middleton in Teesdale and an elongated and a tabular crystal from the Cement Quarry, Eastgate.

Additionally, the frontispiece in the current issue of the UK Journal of Mines & Minerals shows an elongated crystal from Frazer's Hush Mine.

However, the extremely distorted crystals like the British Museum's Boltsburn piece shown above remain exceptionally rare.

[Jesse Fisher]

Another unusual specimen from the Blackdene Mine. A cluster of uniformly twinned crystals with one larger tabular, untwinned crystal perched on the face. The tabular crystal is 2.5 cm on edge. I haven't seen many that show both habits on the same specimen.

[John Medici]

[Jordi Fabre]

Considering its quality and interest I numbered and moved this topic to the Featured Columns of FMF section.

For more info about how the Featured Columns of FMF section works, please use this link

BTW, Ru Smith the author of the first post in this thread and therefore creator of it is already mentioned in the THANK-YOUs for the reference thread

Jordi

[Ru Smith]

Looking at this Heights Mine fluorite I see a lovely transparent twin (crystals have hexoctahedral corners), with one twin member elongate (L/W ca 2) and the other tabular (L/W ca 0.5).

[Mathieu Chevalier]

Hi,

Two elongated specimens.

The first one is from England (NNHM London : old fashion style museum but some incredible specimens. Have a look if you go to London).

The second one is from France (Fontsante Mine, Tanneron). An amazing locality for diversity of shapes and colors, You will quickly understand I am addicted to this mine.