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Demonstration at the mineral department, British Museum of Natural History
177 DEMONSTRATION AT THE MINERAL DEPARTMENT, BRITISH MUSEUM OF NATURAL HISTORY. SATURDAY, FEBRUARY 16TH, 1929. REPORT BY L. J. SPENCER, M.A., Sc.D., F.R.S., Keeper of Minerals. !Received March lsi, 1929.]
A PARTY of about thirty members assembled at 2.30 p.m. at the entrance to the Mineral Gallery, and attention was directed to the following new exhibits. LARGE SPECIMENS OF SPAR FROM THE SNAILBEACH MINE, SHROPSHIRE.-These specimens were bequeathed to the Museum last year by Mr. William Oldfield, who for many years was the resident manager of the mine. They were raised by him from the 462-yard and SS2-yard levels about thirty years ago and since then they stood as ornaments in his garden. Having been exposed to the weather for that length of time, they have unfortunately lost their first freshness and brilliancy. The seven specimens, with a total weight of just over 23t cwt., show large crystals of calcite, galena, and quartz, together with a few small crystals of chalcopyrite and zinc-blende. The largest piece is a slab measuring st x 3t feet and weighing S cwt. 73lb. and is completely covered with well-formed crystals of quartz. Different specimens show variations in the size and relative abundance of each kind of crystal, but on all of them the order of deposition may be seen to be (I) calcite, (2) galena, (3) quartz, (4) chalcopyrite and zinc-blende. These specimens have been set up in wall-cases with inside electric lighting and a dark-grey background that harmonises with the shadows. Brief explanatory labels and diagrams to illustrate the formation of mineral veins are also displayed in the cases. (A fuller account of this new exhibit was given in the Natural History Magazine for July, 1928.) FLUORESCENCE OF MINERALS IN ULTRA-VIOLET RAYS.-A wall-case in a dark corner has been put to good use for showing the remarkable luminescent effects that certain minerals display when exposed to ultra-violet rays. Large groups of fluorspar cubes from Weardale, Co. Durham, a 139 lb. block of wille mite from Franklin Furnace, New Jersey, and other minerals, are first seen in ordinary light with electric lighting inside the case. When the visitor presses the two-way spring-switch outside the case the illumination changes over to ultra-violet rays. These are produced by a silica-glass mercury-vapour lamp (the Hanovia artificial sunlight lamp) which is provided
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DEMONSTRATION AT MINERAL DEPARTMENT.
with a screen of dark glass to cut out all the visible rays of light, allowing only the dark ultra-violet rays to fall on the specimens. The fluorspar crystals then shine up with a wonderful bluish-violet glow, wille mite and autunite with a brilliant green, black zinc-blende with a golden yellow, white calcite with a rose-red, etc. When the spring-switch is released, this fairyland of glowing colours suddenly vanishes. Fluorescence in ultra-violet rays is, however, a very variable character of minerals. Some specimens of fluorspar and willemite do not respond, and it is only a granular zinc-blende from Tsumeb, South-West Africa, that shows the golden glow. (See Natural History Magazine, October, 1928.) FINE CRYSTALS OF GEM-MINERALS.-A clear bluish-green crystal of beryl (aquamarine) from a pegmatite vein in Brazil measures 13 X 12 X 10 em. and weighs 2505 grams (= 12,525 carats). It shows well-marked etch-figures on the faces and the edges are slightly rounded by corrosion. A clear transparent crystal of topaz with a pale blue tinge of colour comes from a pegmatite vein in Madagascar. It measures 12 X II X 10 em. and weighs 2290 grams (= IIA50 carats), and the brilliant crystal faces are marked by complex pyramids and lines of growth. This pair of fine crystals affords a good illustration of the growth and solution of crystals. (See Natural History Magazine, October, 1927, and April, 1928.) SIR HANS SLOANE'S COLLECTION.-·After showing some other of the more interesting recent additions, a step was then taken to the other extreme, and attention was drawn to some specimens from Sir Hans Sloane's collection, which formed the nucleus of the British Museum in 1753. A number of these specimens still bear the Sloane numbers, corresponding to entries in the manuscript catalogue written about 1730-1740. The four folio volumes dealing with minerals were shown. These contain such entries as " Stone resembling a dry'd Figg " and references to other stones of odd shapes, which (fortunately or unfortunately) cannot now be traced.
A PARTY of about thirty members assembled at 2.30 p.m. at the entrance to the Mineral Gallery, and attention was directed to the following new exhibits. LARGE SPECIMENS OF SPAR FROM THE SNAILBEACH MINE, SHROPSHIRE.-These specimens were bequeathed to the Museum last year by Mr. William Oldfield, who for many years was the resident manager of the mine. They were raised by him from the 462-yard and SS2-yard levels about thirty years ago and since then they stood as ornaments in his garden. Having been exposed to the weather for that length of time, they have unfortunately lost their first freshness and brilliancy. The seven specimens, with a total weight of just over 23t cwt., show large crystals of calcite, galena, and quartz, together with a few small crystals of chalcopyrite and zinc-blende. The largest piece is a slab measuring st x 3t feet and weighing S cwt. 73lb. and is completely covered with well-formed crystals of quartz. Different specimens show variations in the size and relative abundance of each kind of crystal, but on all of them the order of deposition may be seen to be (I) calcite, (2) galena, (3) quartz, (4) chalcopyrite and zinc-blende. These specimens have been set up in wall-cases with inside electric lighting and a dark-grey background that harmonises with the shadows. Brief explanatory labels and diagrams to illustrate the formation of mineral veins are also displayed in the cases. (A fuller account of this new exhibit was given in the Natural History Magazine for July, 1928.) FLUORESCENCE OF MINERALS IN ULTRA-VIOLET RAYS.-A wall-case in a dark corner has been put to good use for showing the remarkable luminescent effects that certain minerals display when exposed to ultra-violet rays. Large groups of fluorspar cubes from Weardale, Co. Durham, a 139 lb. block of wille mite from Franklin Furnace, New Jersey, and other minerals, are first seen in ordinary light with electric lighting inside the case. When the visitor presses the two-way spring-switch outside the case the illumination changes over to ultra-violet rays. These are produced by a silica-glass mercury-vapour lamp (the Hanovia artificial sunlight lamp) which is provided
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DEMONSTRATION AT MINERAL DEPARTMENT.
with a screen of dark glass to cut out all the visible rays of light, allowing only the dark ultra-violet rays to fall on the specimens. The fluorspar crystals then shine up with a wonderful bluish-violet glow, wille mite and autunite with a brilliant green, black zinc-blende with a golden yellow, white calcite with a rose-red, etc. When the spring-switch is released, this fairyland of glowing colours suddenly vanishes. Fluorescence in ultra-violet rays is, however, a very variable character of minerals. Some specimens of fluorspar and willemite do not respond, and it is only a granular zinc-blende from Tsumeb, South-West Africa, that shows the golden glow. (See Natural History Magazine, October, 1928.) FINE CRYSTALS OF GEM-MINERALS.-A clear bluish-green crystal of beryl (aquamarine) from a pegmatite vein in Brazil measures 13 X 12 X 10 em. and weighs 2505 grams (= 12,525 carats). It shows well-marked etch-figures on the faces and the edges are slightly rounded by corrosion. A clear transparent crystal of topaz with a pale blue tinge of colour comes from a pegmatite vein in Madagascar. It measures 12 X II X 10 em. and weighs 2290 grams (= IIA50 carats), and the brilliant crystal faces are marked by complex pyramids and lines of growth. This pair of fine crystals affords a good illustration of the growth and solution of crystals. (See Natural History Magazine, October, 1927, and April, 1928.) SIR HANS SLOANE'S COLLECTION.-·After showing some other of the more interesting recent additions, a step was then taken to the other extreme, and attention was drawn to some specimens from Sir Hans Sloane's collection, which formed the nucleus of the British Museum in 1753. A number of these specimens still bear the Sloane numbers, corresponding to entries in the manuscript catalogue written about 1730-1740. The four folio volumes dealing with minerals were shown. These contain such entries as " Stone resembling a dry'd Figg " and references to other stones of odd shapes, which (fortunately or unfortunately) cannot now be traced.