My friend Paul and I recently went to The Weardale Mineral Show at St. Johns Chapel, a small village 7 miles west of Stanhope, here in the North East of England. I primarily went there to meet up with e-Rocks boss Mark. Paul went there because I don't drive any more. He's not a collector, but was interested in the minerals on show, particularly the Weardale fluorites. It was a beautiful day, and solar ultraviolet rays were intense outside. Indoors I pointed out to Paul a lovely crystal of emerald-green Rogerley fluorite, about 20mm on the side, which was for sale for £15. I asked to borrow it. The kind lady behind the well stocked mineral table let me take it outside with my friend. He was astonished to see the crystal in sunlight.
It glowed with a beautiful purple light, almost completely masking the verdant green of the fluorite. After returning the specimen, we retired to the Golden Lion pub across the road from the mineral venue, to briefly discuss the diverse colours and daylight fluorescence of Weardale fluorite. And have a pint. And meet Mark.
On the way home I pondered on my inability to concisely answer my friend Paul’s question, “What causes the green and purple colours?” He was aware of what fluorescence was, but the marked degree of colour change? I had lamely replied that the colour and fluorescence were due to trace amounts of rare earth elements. The causes of colour in minerals is both fascinating and highly complex, with fluorescence adding to the difficulty in explanation. Hopefully this post will address the subject of North Pennines green fluorite satisfactorily, without going into such maths-heavy esoterica such as electron spin.
Although we are only looking at daylight fluorescent Weardale green here, the various colours of the fluorite found in the North Pennines - purple, amber, green & (rarely - as phantoms/zones) blue, deserve a mention just as contrast. The following Weardale examples show a some of the diversity of colours, tints & hues, without any UV illumination. The colour of their fluorescence, when UV illuminated, is invariably toward the violet, peaking at 423-425nm.
Frazers Hush Mine - Burtree Slits Mine - West Pasture Mine - Boltsburn Mine
Diana Maria Mine - Eastgate Quarry - Heights Quarry - Eastgate Quarry
Much of the colour and fluorescent properties of North Pennines fluorite, CaF2, is directly attributable to a suite of lanthanide series elements, which replace Ca2+ cations in trace amounts. Here's a Rare-Earth Element (REE) concentration analysis (Jesse Fisher & Prof. John Watkinson) of green Rogerley mine fluorite, using Laser Ablation Inductively Coupled Plasma Mass Spectrometry, or LA ICP-MS
|Rare-Earth Element (REE)
||parts per million
Unlike sunlight, normal indoor lighting doesn't have an ultraviolet component to its spectrum. When indoor white light illuminates a Weardale green fluorite crystal, its colour is apparent to our eyes. It’s green. Sections of the visible light spectrum have been absorbed or transmitted, leaving only a specific range of wavelengths for our eyes to see, and our minds to interpret as green. This selective absorption is due to chromophores.
The chromophore responsible for the green colour of Weardale fluorite is thought to be divalent samarium cations (Sm2+), which cause structural (electronic) lattice defects when they replace a very small proportion of calcium cations (Ca2+) in the CaF2 crystal. The wavelengths of light which are absorbed by the lattice defects become thermal energy within the crystal.
When the crystal is illuminated by sunlight, the invisible highly energetic UV component is also absorbed, this time by divalent europium cations (Eu2+ activators) replacing a few Ca2+ cations in the CaF2 crystal lattice.
An electron associated with the europium cation defect is moved up to a higher energy level by an absorbed incident UV photon. It briefly loses energy by giving some to the kinetic “thermal” energy of the atom, then it decays back down, giving out a photon with less energy (longer wavelength) than the incident UV photon, i.e in the visible spectrum. This mechanism is called the Stokes Shift, and is shown as a Jablonski diagram below. This is why the crystal of Rogerley fluorite I showed my friend Paul didn’t just change colour, it actually appeared to glow purple. More visible light was coming from it than our eyes and brains could instinctively account for, given the level of illumination.
In the above Jablonski diagram (by Jacobkhed), S0 and S1 represent different electronic states. The other numbers (here 0–3 are shown) represent vibrational states.
When trying to visualise electron energy states, it's important not to think in terms of the "classic" orbits of electrons around the nucleus of an atom. In the crystalline lattice of an ionic solid such as fluorite, calcium fluoride, it’s helpful to consider the electron density of the lattice. Electron density is the measure of the probability of an electron being present at a specific location. This is important in CaF2 as there is a significant amount of electron sharing between the Ca2+ cations and the F- anions, i.e. a degree of covalent bonding. If the crystal is pure, and the lattice has no defects, then the electron density between individual atoms has a uniform structural nature within the lattice. When white light is incident on such a crystal, no wavelengths of light are absorbed by the electron cloud of the lattice, from infrared to ultraviolet, so the crystal is colourless and transparent -
The discovery of daylight fluorescent fluorspar in Weardale.
“The finer crystals are perfectly transparent. Their colour by transmitted light is an intense emerald green; but by reflected light, the colour is a deep sapphire blue.”
This intriguing phenomenon was first brought to the attention of scientists from a remarkable find of green fluorite in 1818, at the Whites Level, Westgate, Weardale. From Annals of Philosophy, 1819 - Account of a newly discovered Variety of Green Fluor Spar, of very uncommon Beauty, and with remarkable Properties of Colour & Phosphorescence -
Soon Weardale green fluorite crystals were in demand, not just by collectors and the mineral dealers in nearby Alston, but by scientists investigating the phenomenon. Sir David Brewster looked into the crystals in 1838. This from The British Association for the Advancement of Science report, 1839 - On a New Phenomenon of Colour in certain specimens of Fluor Spar. (note that the mineral is referred to as originating from Alston Moor. This is incorrect, they were from Weardale, the crystals merely being sold by dealers in Alston).
In his 1852 paper, “On the Change of Refrangibility of Light,” Sir George Stokes (above) described the ability of fluorspar to change invisible light beyond the violet end of the visible spectrum into blue light. He named this phenomenon fluorescence.
Around 1850 another occurrence of very similar highly fluorescent green fluorite was found at the now highly regarded Heights mine, specimens from which were extracted up to the late 1970s, and are in the mineral collections of museums worldwide. Nearby Heights Quarry and Eastgate Quarry also produced splendid specimens.
Then, in the early 1970s Lindsay Greenbank and Mike Sutcliffe discovered the spectacular fluorescent green fluorite of Rogerley mine -
Produce of Rogerley mine
An open-cut enterprise started in 2017, adjacent to the Rogerley adit - The Diana Marie Mine. Here’s a video of the daylight fluorescence of the specimens recently obtained -
Although I was given a splendid tour of Rogerley mine over a decade ago, by a total enthusiast and lovely man, Byron Weege (below), I had to say to him that Heights mine was more spectacular (if a little more lacking in health & safety aspects). He gave me a “1st. light,” beautiful green twin, so there were no hard feelings!
Byron with daylight fluorescent Rogerley green fluorite (© Jesse Fisher, UKMV)
I have spent many, many hours digging in the spoil heaps, rummaging through internal piles of deads, and risking life and limb hammering within Heights mine. My shed is full of Heights specimens. Some I recently unearthed (from the shed), have never been cleaned since they were originally pulled from the mud some 40 years ago!
Most of the specimens I have are damaged, but have interesting features. The particular specimen I’d like to show in this post is intriguing in what it shows of fluorescence. Whilst examining my Heights collection recently (that sounds rather grand, it’s a study collection however, with only a few show specimens), I noticed that several crystals display very bright fluorescence in zones of a brownish drab purple colour by transmitted light. I was using a 395nm UV LED costing under a UK pound.
A once pristine, completely undamaged crystal of green fluorite. 1cm, Heights mine. Now in a sorry state after being carried in pockets with change, and being exposed to the sun.
The crystal was hammered out of a vug by my friend Alan Hornsby in the 70s, wearing a motorcycle helmet and weilding a ball-pein hammer and chisel. While he was having a go at it, I stood back, swigging beer from a tin and smoking a tab, thinking there was no way I’d put my head and upper torso inside that unstable looking mud-filled vug! At least when hammering was happening :)
When the crystal saw the light of day for the first time, the zone of colouration was bright emerald green, with striking violet daylight fluorescence. After several decades of exposure to sunlight, from being “displayed” on various window ledges, along with other detritus, the green has vanished, and the daylight fluorescence markedly diminished. By transmitted light such “bleached” ex-green specimens are usually a pale mauve, graduating to the brownish violet smokey zones in the highly fluorescent examples. Many such specimens could be found on the Heights dumps, having been exposed to sunlight since the mid 1800s.
Here’s a couple of short videos of the crystal, showing what’s left of its daylight fluorescence, and its illumination with a cheap 395nm UV LED. I don’t expect any academy awards for the rushed camerawork…..
All Weardale green fluorite is unstable and will bleach when continually exposed to light. The atoms of samarium and europium responsible for the colour and fluorescence of course remain within the crystal lattice after exposure. However the chromophores associated with these impurities are “annealed,” the electrons moving into more stable energy states, and possibly the cations change oxidation state, from 2+ to 3+. Much is unclear about the mechanism of bleaching, and please regard my synopsis (about the chromophores responsible for colour and fluorescence) as being a stab at being representative of the most likely theories. Indeed, the most common fluorite colour is purple, and it’s not light sensitive, but the jury is out regarding the cause of this colour. It could be due to crystallographic defects such as colloidal calcium and subsequent Mie scattering, or it could be due to missing fluorine anions (F-) in the lattice, causing holes which are filled with one or more electrons: F-centers.
It would be highly interesting to use a state-of-the-art LA ICP-MS rig on the above specimen, or indeed any Weardale zoned fluorite. If the crystal were to be sectioned and polished, then the “strata” of the zones would be exposed for linear elemental analysis. LA ICP-MS is capable of sampling at quite a high spatial resolution. Here’s an image of a garnet crystal with a series of pits blasted out using a Nd:YAG deep UV (213nm) laser, each target over the linear path individually ablating material for element abundance measurement, when sucked into the plasma of the mass spectrometer. The REE concentrations across various colour and fluorescence zones through a fluorite crystal, could be rather enlightening, or muddy the waters further….
To finish, Here’s a video of a corker of a specimen from Heights mine. I wish Mr. Clark had taken it outside!
Glorious Green Fluorite from "Middlehope Shields", County Durham. Ru Smith, 2014
On the Change of Refrangibility of Light. G.G.Stokes, 1852
The origins of color in minerals. Kurt Nassau, 1978
On a New Phenomenon of Colour in certain specimens of Fluor Spar. Sir D Brewster, 1838
Account of a newly discovered Variety of Green Fluor Spar. E.D.Clarke, 1819
And Now For Something Completely Different