A group of twinned fluorite crystals, up to 2.5 cm in size, on silicified limestone matrix. Specimen was collected in 2002 from the West Cross Cut.

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Fluorite from the the flats in the Rogerley Mine is a dark emerald green in color with a strong blue-purple daylight fluorescence, and an even stronger purple-white fluorescence under long wave UV light. Under incandescent light, the fluorite tends to be a uniform green, though some larger crystals will occasionally have a pale purple core and/or a thin purple layer at the surface. Crystal size is usually a maximum of around 4 cm, but occassionally larger, mostly opaque crystals are sometimes found. The habit is always cubic and crystals smaller than 4 cm are invariably penetration twins on {111}. Larger crystals are usually untwinned. The smaller twinned crystals also commonly show shallow tetrahexahedral modifications, or "growth patterns" on selected cube faces.

A twinned fluorite crystal showing well developed tetrahexahedral modifications to the cube face. Found in the West Cross Cut, August, 2002.

Crystals that occur oriented facing upward from the floor of cavities are often somewhat etched and dull, but crystals formed on the roof are, for the most part, lustrous. Stalactitic formations of altered limestone matrix coated by fluorite are occasionally found, particularly in the area of the Black Sheep Pocket. Aside from minor amounts of druzy quartz, the only other mineral found in cavities in the flats are octahedral crystals of galena. These crystals are small, usually no larger than 1 cm in size, and invariably coated with a thin alteration layer of cerrusite. Rarely, spinel twins can be found.

Fluorite coated "stalactite", 7 cm high, with crude octahedral galena crystal. Collected from the Black Sheep Pocket, June, 1999.

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Paragenetically, quartz appears to be the earliest mineral to from in vugs in the flats, as fluorite and galena have usually formed on a surface of druzy quartz. The crystallization of galena appears roughly contemporaneous with fluorite, and crystals of galena are often found partially imbedded in fluorite crystals. In vugs found in the Greenbank vein, the earliest mineral to form appears to have been one of the iron-bearing carbonates, which has since been oxidized to limonite. A pale green fluorite formed next, progressing to a pale yellow. A quartz druze was deposited on the surface of the yellow fluorite, which was then overgrown by a final layer of purple fluorite. A second generation of druzy quartz was the last phase to be deposited, and covers the euhedral purple fluorite within the pocket. This sequence of mineralization is not always complete, and appears to have ceased at the early "green" stage in some cavities.

Quartz encrusted purple fluorite crystal group, 10 cm in size, collected from a large cavity in the Greenbank Vein, June, 2000.

Fluorite crystals from the Greenbank Vein are generally untwinned, with individual crystals up to 7 cm in size. Large clusters of crystals are common in some pockets, making for truely massive specimens. In pockets containing purple fluorite, small gemmy twinned crystals are sometimes found on the surface of larger, opaque, untwinned crystals.

Galena is commonly found in vein cavities, and like galena crystals from the flats, shows a fair degree of surface alteration. Crystals from these cavities usually have a cube-octahedral shape, as opposed to galena from the flats cavities which rarely displays cube faces. One cavity was encountered during the summer of 2000 which contained large composite clusters of galena crystals on a crust of white crystalline quartz. No fluorite was found in this cavity.

Opaque green fluorite crystals with galena on quartz, from a cavity in the Greenbank Vein, June, 2000.

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Fluorite and galena-containing cavities are also found on the Sutcliffe Vein, a cross-cutting vein exposed in the western extension of the Rogerley Quarry. The cavities occur at the "High Flats" level near the top of the Great Limestone - the same level as the current workings on the Greenbank Vein. Fluorite crystals found in cavities at this location are generally small, highly lustrous and transparent penetration twins. Most are purple in color, though some bicolor crystals showing outer layers of green have been found as well. This location was worked for specimens by Cumbria Mining and Mineral Co. for a short period during the mid 1970's, though no systematic attempts to mine the vein have happened since that time. Collecting is difficult and dangerous as it requires roping over from the top of the quarry, and collecting while suspended in mid-air.

A twinned purple fluorite crystal, 2.5 cm in size, from the Sutcliffe Vein, western extension of the Rogerley Quarry.

The host limestone in the regions surrounding cavities in the flats has been largely replaced by silica and iron oxides. This material proved very tough, and extracting undamaged specimens is time consuming, even with the aid of the diamond saw. Stalactite-like structures are common in the flats, forming knobs and fingers of matrix covered with druzy quartz, fluorite, and galena crystals. The core of these formations was often hollow or filled with iron oxides. A similar occurrence of these formations has been reported from the flats in the Boltsburn mine by King (1982). On rare occasions fluorite specimens have been found on a matrix containing the fossil remnants of paleozoic corals.

Fossil paleozic corals exposed in the matrix of a fluorite specimen collected from the Rogerley Mine flats, August, 2000.

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The unique colors and daylight fluorescence of Weardale fluorite has long been noted, and it is generally accepted that these properties are related to the presence of rare earth elements (REE) in solid solution (King, 1982). Dunham (1990) reports elevated levels of yttrium (150-1200 ppm) and europium (20-110 ppm) in Weardale fluorite, and suggests that latice defects caused by these elements may create color centers. He also notes that fluorite from the Derbyshire orefields to the south does not fluoresce and has a REE content of an order of magnitude less that Weardale fluorite. Bill et al (1967) suggests that the presence of samarium and gadolinium may be important in green Weardale fluorite as well.

Twinned fluorite crystal, 2.5 cm in size, showing purple highlights caused by daylight fluorescence. Specimen was collected from the flats in July, 2000.

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Fluorite from the Rogerley Mine has recently been analyzed for REE content by direct-coupled plasma spectroscopy (Falster et al, 2000). Elevated REE levels were discovered in all samples from the mine, which included green fluorite from cavities in the flats, and purple, pale green, and yellow fluorite from the Weasel pocket in the Greenbank Vein. Elements which showed particular enrichment were lanthanum, cerium, and yttrium, while europium levels were uniformly low compared to most other REEs.

REE content of fluorite from the Rogerley Mine:

Element Yttrium (Y) Lanthanum (La) Cerium (Ce) Neodymium (Nd) Samarium (Sm) Europium (Eu) Gadolinium (Gd) Dysprosium (Dy) Erbium (Er) Ytterbium (Yb)

Detected Range 93 - 272 ppm 193 - 274 ppm 56 - 150 ppm 36 - 77 ppm 62 - 88 ppm 6 - 31 ppm 48 - 89 ppm 6 - 14 ppm 14 - 24 ppm 12 - 25 ppm

The overall REE content pattern when normalized to chondrite concentrations showed little variation between the various colors of fluorite. This suggests that, while the REE content may be responsible for the strong daylight and UV fluorescence, these elements are probably not the primary chromophores in the Rogerley Mine fluorite

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A chondrite-normalized plot of the rare earth element (REE) concentrations for all colors of fluorite currently found in the Rogerley Mine. Green fluorite is from the cavities in the flats, the other colors are from cavities in the Greenbank vein. Analyses conducted by Al Falster of the University of New Orleans Department of Geology and Geophysics.

A fluorite specimen from the Rogerley Mine flats showing its color in natural light (left), and long wave ultra-violet (right).

This text was adapted from an article by Jesse Fisher and Lindsay Greenbank, which appeared in the January/February 2000 issue of Rocks & Minerals. All photographs by Jesse Fisher, unless otherwise noted.