New Mexico Mineral Symposium — Abstracts

History—The Peak district of north central England includes some of the oldest mines in Europe with mining artifacts dating back to the Roman period (2nd Century A.D.). The Peak district is one of the richest mineral districts (debatable) in United Kingdom with more than 280 localized mines, soughs, rakes, and veins. (Soughs are adits or tunnels driven specifically to drain a mine and in which workable veins were found. Rakes are the main type of mineral vein in the Peak district. Veins are orebodies enclosed by the host rock.) The mines in the district were worked exclusively for lead. By 1750 new mining techniques had been introduced, particularly steam power, and mining activity had reached its peak. By 1940 most of the lead veins had been worked out, but the mines were worked for fluorite (fluorspar) so necessary in the war effort. Today lead is produced as a by-product in the mining of fluorspar.

In the district is found one of the two world occurrences of a fibrous banded fluorite known as Blue John. The first source, locality unknown, is between Turkey and Syria. This was reported in the form of two goblets found in a grave. Although of fibrous material, the banding does not resemble that of Blue John. Similar material is now coming from China in association with pyrite. (As yet I have not seen it being used in lapidary items.) The second occurrence of fluorite known as Blue John is found in only one cavern, Treak Cliff, yet there are four caverns at Castleton! The first mention of Blue John is in 1743 when a local "carver" was commissioned to make a vase from a block of fluorite stalagmite! It is recorded by the King's Barmaster (the Crown agent for lead mining) that the source had 16 windlasses set over separate shafts.

Mining of Blue John from Treak Cliff Cavern continues to this day, although the cavern is open to visitors! It is mined during the off season. Average annual production is approximately half a ton, although during the last century up to 20 tons a year were reported! Blue John is used exclusively in lapidary items such as bowls, urns, goblets, inlay tile, and jewelry. The largest one-piece bowl was 20 inches in diameter and was made in 1835. A vase in the Geology Museum in London was made in 1840 and is 30 inches high.

Mining—The mining is done by hand. The veins are approached from the side by removing limestone blocks with hammer and wedges. In a unique mining method, holes are drilled by hand and packed with wooden wedges, and a steel wedge is driven down the middle. If the wooden wedges are dry and are then soaked with water and left overnight to swell, they will crack solid limestone.

Geology—The mineral veins of the Peak district are contained in the Carboniferous limestone and associated basalt lavas. Castleton lies at the northern extremity of the limestone massif. As a result of the action of water over time the area around Castleton is fissured by caverns. The district was part of an Inland Sea as evidenced by many marine fossils. In fact the district is known as the "Paleontologist's Treasure House". There are four caves that can be visited by the public. The great cave is Peak Cavern with an inner chamber 100 ft high and floor length almost a mile. This cavern is noted for its typical wet cave deposits of stalagtitic formations. The second cave, of definite interest to mining historians, is Speedwell Cavern/Mine. This was a working mine in the 1700s, and under-ground movement of waste rock and lead ore was by boat. Tours of this cavern are still by boat. Treak Cliff Cavern, which has the Blue John deposit, consists of a series of rooms that are appropri-ately named Witches Cave, Aladdin's Cave, Fairyland, Dream Cave, and the Dome of St. Paul. At the dome the visitor is some 160 ft below the surface. Veins of Blue John are seen throughout the progression of rooms. The vein does not reach the surface, hence mining is done on the inside.

Mineralogy—Minerals of the district include, galena, sphalerite, barite, calcite, and fluorite. The proportions of these in the various deposits vary considerably, and in Treak Cliff fluorspar is dominant. Blue John is "an unusual variety of fluorspar. When pure it is white or colorless, but varieties of all colors are known - red, pink, yellow, green, blue and purple. In all these one or more impurities included within the fluorspar either stain or influence their light-reflecting properties." In Blue John the impurity is oil. The rough surface of Blue John shows the typical cubic structure. When cut, its banding has shades of light and dark blue, purple, and almost black alternating with colorless, milky-white, yellow, or pale violet. The patterns vary across Treak Cliff and there are some 14 different banding patterns.

A mineralogist states, "it isn't the oil itself which causes the color. It appears to be radiation from minute quantities of uranium adsorbed onto the oil which has caused dislocations in the molecular structure and these in turn affect the way in which light passes through the crystals or is reflected from them."

Supplemental reading
Ford, Trevor D., 1979, Blue John—Derbyshire's Unique Gem: Gems, v. 11, no. 2 (84 High Street, Broadstairs, Kent).

Ford, Trevor D., and Rieuwerts, J. H., 1975, Lead mining in the Peak district: Peak Park Joint Planning Board, Baslow Road, Bakewell, Derbyshire, England, 1st and 2nd ed. with revisions.