The younger Westgarth Forster in his 1821 edition of "A Treatise on a Section of Strata from Newcastle upon Tyne to Cross Fell" (pg 216) states that:
"All the extensive mines in Weardale, in the county of Durham, and Allenheads and Coal-cleugh, in the county of Northumberland, have calcareous and fluor spars for the matrix; and it is from a mine in Weardale, that the beautiful green fluor has been procured lately in such quantities."
In July 1819 Edward Daniel Clarke, Professor of Mineralogy at the University of Cambridge, had reported on the green fluorite find noted by Forster: "Green Fluor Spar, of very uncommon Beauty, and with remarkable properties of Colour and Phosphorescence."
Forster does not cite Clarke and neither was Clarke one of Forster's many subscribers.
The occurrence is given by Clarke as the Middlehope Shields Mine, property of Colonel Beaumont, 1 1/2 miles to the north of West Gate village. He states that "it was only during three months of the autumn of 1818 that this beautiful mineral was obtained in any considerable abundance." He notes that the mine had been "worked with great success for a considerable time" (this is the "Middlehope Old Vein" of Forster 1821), but that excavations had recently been interrupted by miners of the London Lead Company working an intersecting NE-SW trending vein, as shown in Clarke's sketch map below. Forster (1821, pg 308) describes these new workings as follows:
"Middlehope New Vein, two miles and a half N.E. of St. John's Chapel ; much lead, with Amorphous and Crystallized Fluor Spar, in Strata, from the Slate Sills to the bottom of the Great Lime-stone. One Horse Level, one Pressure Engine, and one Crushing Machine. Occupied by the London Lead Company."
"Sketch of the situation of the green fluor mine, showing also the manner of its intersection by the London Lead Company's Mine." from Clarke (1819). Note that NORTH is to the right. Comparison with a modern map reveals the distortions in this rough sketch map. "Green Fluor Mine" in Clarke's map is the equivalent of Forster's "Middlehope Old Vein", whereas Clarke's "London Lead Company's Mine" is the equivalent of Forster's "Middlehope New Vein."
The "rider" of the vein is described as a "buff coloured magnesian limestone." Dolomitization of limestones is well known in Mississippi Valley-type lead zinc deposits in Weardale and the USA.
Clarke states that "It is only in the flats (i.e. large openings or cavities) of what is called the Eleven Fathom, or Great Limestone, that the finely crystallized and transparent fluor-spar is found." "These cavities .. are generally of inconsiderable extent, but sufficiently capacious to admit a man standing upright .."
Middlehope Shields (Old Vein) green fluorite showing tetrahexahedral modifications of cube edges, which were noted by Clarke (1819).
Clarke's enthusiasm about the beauty of these fluorites is still warranted today. The link with the writings of Westgarth Forster and E.D. Clarke from the early 19th century only serves to enhance their interest.
References:
Clarke, E.D. 1819. Annals of Philosophy, Volume XIV, July to December 1819.
Fisher, J.E., 2006, Classic 19th century fluorite specimens from Weardale, Durham: a mineralogical mystery. UK Journal of Mines & Minerals, 27, 25-28.
Forster, W., 1821, A Treatise on a Section of Strata from Newcastle upon Tyne to Cross Fell. The Geological Press, Alston, 422 p.
Symes, R.F. & Young, B. 2008. Minerals of Northern England. National Museum of Scotland in association with Natural History Museum, London.
"All the extensive mines in Weardale, in the county of Durham, and Allenheads and Coal-cleugh, in the county of Northumberland, have calcareous and fluor spars for the matrix; and it is from a mine in Weardale, that the beautiful green fluor has been procured lately in such quantities."
In July 1819 Edward Daniel Clarke, Professor of Mineralogy at the University of Cambridge, had reported on the green fluorite find noted by Forster: "Green Fluor Spar, of very uncommon Beauty, and with remarkable properties of Colour and Phosphorescence."
Forster does not cite Clarke and neither was Clarke one of Forster's many subscribers.
A specimen of green fluorite from the 1818 find north of Westgate, at "Middlehope Shields" (Clarke 1819). Crystals are penetration twinned, reach 22 mm on edge, are transparent and display tetrahexahedral edge bevels as well as striated tetrahexahedral vicinal faces. The green fluorites were found in flats in the Great Limestone. (ex Western Minerals [Hoppel #D1135])
"Middlehope New Vein, two miles and a half N.E. of St. John's Chapel ; much lead, with Amorphous and Crystallized Fluor Spar, in Strata, from the Slate Sills to the bottom of the Great Lime-stone. One Horse Level, one Pressure Engine, and one Crushing Machine. Occupied by the London Lead Company."
The "rider" of the vein is described as a "buff coloured magnesian limestone." Dolomitization of limestones is well known in Mississippi Valley-type lead zinc deposits in Weardale and the USA.
Clarke states that "It is only in the flats (i.e. large openings or cavities) of what is called the Eleven Fathom, or Great Limestone, that the finely crystallized and transparent fluor-spar is found." "These cavities .. are generally of inconsiderable extent, but sufficiently capacious to admit a man standing upright .."
Forster (1821, pg 199) showing normal fault, mineralized vein and the flats ("dilated veins") in the Great Limestone in which the green fluorites of the autumn 1818 find were encountered (Middlehope Old Vein).
He describes the crystals as follows:
"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 ; and this remarkable character causes such a play of the green and the blue light to a person regarding these crystals that at first sight he is unable to say which causes the more beautiful appearance, and to which of these hues their real colour ought to be referred."
A large 19th century green fluorite specimen (bought from dealer S. Henson) in the Arthur Russell collection (BM 1964, R1524, illustrated in Plate 52 of Symes & Young 2008), with the distinctive tan coloured "iron stained limestone matrix", gives "Whites Level, Westgate" as place of origin. White's Level is recorded in Forster (1821, pg 310) as follows:
"White's Level, one mile E. by N. of St. John's Chapel ; Lead, with Coloured, Cubical, and Amorphous Fluor Spar, Strata, Coal Sills and Great Lime-stone."
I own a single specimen matching all the distinctive characteristics of the 1818 find. It has a length of 10 cm and consists of a buff-coloured altered limestone matrix hosting transparent green twinned cubes of fluorite to 22 mm on edge. They show internal phantoms, tetrahexahedral edge bevels and striated vicinal faces.
Detail of Middlehope Shields (Old Vein) green fluorite showing thin purple phantoms in a crystal 22 mm on edge. Tetrahexahedral modifications of cube edges are also visible.
Middlehope Shields (Old Vein) green fluorite showing tetrahexahedral modifications of cube edges, which were noted by Clarke (1819).
Middlehope Shields (Old Vein) green fluorite showing striated vicinal modifications of cube faces, as Clarke (1819) puts it, "their surface most beautifully exhibiting the lines of decrement, formed by the laminae of superposition upon the primary nucleus."
Altered porous limestone matrix showing numerous moulds of fossil crinoid stem ossicles and pockets of limonite-rich friable material.
Clarke's enthusiasm about the beauty of these fluorites is still warranted today. The link with the writings of Westgarth Forster and E.D. Clarke from the early 19th century only serves to enhance their interest.
References:
Clarke, E.D. 1819. Annals of Philosophy, Volume XIV, July to December 1819.
Fisher, J.E., 2006, Classic 19th century fluorite specimens from Weardale, Durham: a mineralogical mystery. UK Journal of Mines & Minerals, 27, 25-28.
Forster, W., 1821, A Treatise on a Section of Strata from Newcastle upon Tyne to Cross Fell. The Geological Press, Alston, 422 p.
Symes, R.F. & Young, B. 2008. Minerals of Northern England. National Museum of Scotland in association with Natural History Museum, London.
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