Preface

English fluorites, especially from the famous mining regions in Co. Durham, Cumbria and Northumberland are often regarded as classic examples of fluorescence. The habit of a drastic colour change (or enhancement) when exposed to sunlight, also known as "daylight fluorescence" make these specimens special and treasured in collections world wide. It was soon enough that one discovered that this process wasn't linked to a crystal dependent habit at all. Even the ugliest amorph vein material compared to a flawless gemmy twin from the region showed the remarkable feature. It was soon suggested that REE's (or Rare Earth Elements), were the main cause for this particular phenomenon.

The following scientific analysis was originally published by Dunham^[1]. Since this book was published under crown copyright, this book came in the public domain in 2002. Hence I copied the scientific data in this book for a Mindat article. These data contained some references to more "obscure" localities in England, still accessible or worked around 1950. It could be interesting to look for more information on these historic mining sites and hopefully provide some new information or insights.

However the performed analysis is not only interesting for English specimens and may, up to a certain degree, apply to worldwide fluorite specimens as well. Furthermore these findings may prove a valuable insight in the history of fluorescence study. Dunham warns that the proven percentages of trace elements need to be regarded with the utmost caution and can only be referred to as approximate data. All the samples listed in Appendix I were analysed by spectographical analysis by J.A.C McClelland in 1948-49 with special attention for trace elements of Yttrium and the Lanthanide series (Lanthanum, Cerium, Dysprosium and Ytterbium).

Historical background

According to Dunham^[2] and other researchers, (fluorescence?) colour in fluorite is possibly caused by thin films of organic matter (bitumen) or rare earth elements deposited during a crystals growth process. Later on it was confirmed that there was a close connection between these rare earth element impurities and the unique fluorescence of (North)-English fluorites. It was commonly believed that Europium was related to the intense blue colour under UV, while Ytterbium took care of the green component. It was noted that fluorescence was either "quenched" or "activated" by certain foreign components, like it was proven that the presence of iron neutralized the fluorescent effects.

For this study, the fluorescence of a number of English fluorite specimens were studied using a "Hanovia" ultra-violet lamp fitted with a "Wood's glass filter" (capable of passing trough radiation of 3,000-4,000 A°, with a maximum at 3,600 A°)

Analysis

1/Specimens 1499,1500 & 1504 tested negative on Europium and very low on Yttrium contents, which can be linked with their non-fluorescent nature.
2/Specimens 1501,1502,1503 had a remarkable Rare Earth content where the Yttrium section was predominantly present.
3/Specimen No° 63 has a strong violet fluorescence which is associated with the second highest percentage of Europium content present in the table, which is in turn followed with significant percentages of Lanthanum and Cerium.
4/Specimen N°65 has a slightly lower fluorescent activity than N°64 which could be related to the lack of Lanthanum and the lower Europium contents in N°65. The main reason is probably, according to Dunham, the presence of trace amounts of Nickel which could act as a fluorescence "quencher".
5/The colour in the purple fluorite from Treak Cliff(Dark bands in Blue-John) is possibly related to trace amounts of organic matter in the crystal structure. These organic particles could be obtained with a organic solvent, leaving a colourless fluorite and a yellow-brownish oil behind. Garnett (1920) studied this phenomenon and found out that the "colouring matter" is unevenly distributed as a film over the exposed crystal, which was later on in turn covered by colourless fluorite.

Conclusions

1/ The reactivity of fluorite to UV-light drops with the percantage of Europium present
2/ Yttrium is probably a main cause of the fluorescence, however the Yytrium contents do not decrease evenly with the decrease rate in fluorescence.
3/ It is suggested that only the combination of Yttrium/Europium or Yttrium/Ytterbium gives a decent fluorescent effect. Europium giving the bluish touch, while Ytterbium tends to the green.
4/ Manganese was detected in all the samples where it was sought. It probably plays an important part in the colouring, but to what extend it not sure.
5/ The presence of Lanthanum may cause the purple fluorescence observed in specimens No° 15,63,64 and 69.

Appendix I: List of localities^[3]

Specimen No° 1419 was a cleavage of an optical fluorite from Boltsburn East Mine.
Specimen No° 1497 was spar concentration from the gravity mill at Stotfield Burn Mine.
Specimen No° 63-65 were cleavage fragments of green, pale mauve and colourless fluorite found at the dressing plant at Stotfield Burn Mine.
Specimen No° 1489 was a spar concentrate taken from the gravity mill at Sedling Mine.
Specimen No° 1501 came from the tailings at Rispey Mill at Rookhope(?).
Specimen No° 1502 came from the Sharnberry High Level dumps, Co. Durham (?).
Specimen No° 1503 came from the California Dumps, Co. Durham (?).
Specimen No° 1505 came from the South Side Flats, Rotherhope Fell Mine.
Specimen No° 1507 came from the Scoredale Mines (Hilton?).
Specimen No° 66 came form the Brandlehow vein (East Branch) worked in the opencut 800 yards north of Cat Bells, Cumberland.
Specimen No° 67 was a sample of fluorite on dolomite from Florence Mine
Specimen No° 68 was a sample of fluorite on hematite from Ullcoats Mine
Specimen No° 1500 was a fluorite crystal from Greenhow Rake in the Pateley Bridge district.
Specimen No° 1499 was a spar flotation concentrate from Glebe mine, Eyam, Derbyshire.
Specimen No° 1504 were purple crystals from Treak Cliff, Derbyshire.
Specimen No° 14 was a sample from the Magnesian Limestone at Chilton Quarry, Co. Durham
Specimen No° 15 was a heavy concentrate from the Elgin Sandstone from Greenbrae Quarry, Cummingstone.
Specimen No° 69 were pale green crystals from the South Tamar Mine (Bere Alston).
Specimen No° 70 was a specimen of "Chlorophane" associated with Wolfram from the East Pool Mine, Redruth, Cornwall.

Appendix II: Fluorescence and trace-elements in fluorites^[4]

Table 1. REE analysis of fluorite samples as done by J. A. C. McClelland in 1948-49

Lab No° Colour Long-wave UV Fluorescence Y La Eu Ce Dy Yb Other trace elements 1419 Purple Brilliant Bluish 0.03 tr. 0.005 n.d <0.001 X 1497 Grey Brilliant Bluish 0.06 X 0.007 0.001-0.02 X U 0.02 63 Green Strong Pale Violet 0.015 0.03 0.009 0.03 tr. Mn tr.; Gd 0.001 64 Faintly Mauve Moderate Purple 0.035 0.001 0.002 n.d 0.001 Mn tr. Gd 0.0015;Pb tr. 0.02 65 White Purple weaker than 64 0.025 n.d 0.0015 n.d tr. Mn tr.; Ni tr. 0.02 Pb tr. 0.02 1498 Greyish Brilliant Bluish 0.04 X 0.005 0.01 X 1501 Dark Grey Brilliant Bluish (few grains) weaker than 1419-1498 0.08 X 0.007 n.d X 1502 Brown Similar 1502 0.12 n.d 0.01 n.d n.d 1503 Dark particles (some white) Brilliant bluish (only white) 0.11 n.d 0.011 n.d <0.01 n.d U 0.01-0.1 1505 White Brilliant bluish 0.05 n.d 0.003 n.d <0.001 tr. 1507 Amber Bluish (less than 1419) 0.04 n.d 0.002 n.d <0.001 tr. 66 Amber Weak deep purple 0.025 n.d 0.002 n.d tr. Mn tr. 67 Pale blue Very feeble violet 0.007 n.d n.d n.d n.d Mn tr. 68 Blue crystals Very feeble 0.009 n.d n.d n.d n.d Mn tr. 1500 White Not visible 0.02 n.d n.d %0.001 n.d <0.001 n.d 1499 White Faint Brownish 0.005 n.d n.d %0.001 n.d <0.001 n.d 1504 Dark Purple Not visible 0.02 n.d n.d %0.001 n.d <0.001 n.d 14 Yellow Strong Yellow/White 0.007 n.d n.d <0.02 n.d 15 Buff Powder Violet 0.023 0.003 0.0006 <0.01 tr. Nd 0.003 69 Pale Olive Green Moderate Violet 0.02 0.003 tr. n.d Slt. tr. Mn tr.;Pb tr. 70 White Feeble Purple 0.02 n.d Slt. tr. n.d n.d Mn tr.

Footnotes

1.	K.C. Dunham (1952), Fluorspar in Memoirs of the Geological Survey;Special Reports on the Mineral Resources of Great Britain., Vol. IV, 9-12, London.
2.	I found an extensive biography of K.C Dunham. He played an important role in surveying England for the British Geological Survey. His books are standard works for every mine historian or geologist alike. More information on him will follow in an upcoming article.
3.	Where possible the mines and localities were linked to those in mindat. Unknown locations will be looked up and when proven valid, added to the database lateron.
4.	The colour property in this table was originally extended with a small description of the habitus of the specimens. When the specimens were in a crystal state they were nearly always defined as cleavage fragments, while concentrates from a mill have a more powder like form. These extra data are not entered in the table, because they were already described in Appendix 1.

---

In order to leave comments to this article, you must be registered