Specimens of Peristerite, a variety of plagioclase feldspar with a bluish opalescence, are on display in the mineral cabinets at the Matheson House Museum on Gore Street in Perth, Lanark County, Ontario. It is fitting that specimens of Peristerite should be on display in Perth, because the first specimens of Peristerite to be reported in the scientific literature were found about eleven kilometers from the Town of Perth.
Peristerite was named in a paper published in 1843 by Dr. Thomas Thomson, M.D., Regius Professor of Chemistry in the University of Glasgow. He named Peristerite after the Greek word ‘peristeria’ which means pigeon, from “the colours resembling a pigeons neck.”. Dr. Thomson named the new mineral based on specimens sent to him by two medical doctors and avid mineral collectors in Canada: Dr. James Wilson, who practiced in the Town of Perth in Upper Canada and Dr. A. F. Holmes, who practiced in Montreal in Lower Canada. Both were graduates of the University of Edinburgh. Dr. James Wilson collected his specimens from the nineteenth lot of the ninth Concession of Bathurst Township, in what is now Lanark County. I have not been able to determine where Dr. Holmes collected the specimens that he found and sent to Dr. Thomson.
I have previously commented on Dr. James Wilson’s life in my posting dated October 9, 2012 entitled New Display of Dr. James Wilson’s Mineral And Fossil Collection at the Perth Museum. Dr. James Wilson (1798-1881) emigrated to Canada and practiced as a physician in Perth, Ontario from 1821 to 1869, and then retired to Scotland. Dr. Wilson was an amateur mineralogist and geologist who is credited with being the first to find the trace fossil Climactichnites Wilsoni, and the minerals Perthite and Peristerite. In addition, he found outcrops that later became apatite (phosphate), mica or graphite mines, and found numerous mineral occurrences. He provided locations and specimens to the Geological Survey of Canada and was a friend of Sir William Logan. In October, 2012 the Perth Museum at Matheson House in Perth, Ontario opened to the public its new Geology Exhibition, which features a display of part of the mineral and fossil collection of Dr. James Wilson, including specimens of Climactichnites wilsoni .
Dr. Andrew Fernando Holmes (1797 -1860) had an even more interesting life. He was born at Cadiz, Spain because the vessel in which his British parents sailed had been captured by a French frigate and taken to Spain as a prize. He received his early education in Montreal and received his licence from the Montreal board of medical examiners. He later received a diploma from the Royal College of Surgeons of Edinburgh in 1818 and a Doctor of Medicine degree from University of Edinburgh in 1819. Returning to Canada, he practiced medicine. In 1823 with Dr John Stephenson he founded the first medical school in Canada , which later joined McGill College to become the McGill College Medical Faculty. He was a founding member of the Natural History Society of Montreal in 1827, catalogued the minerals and geological specimens in the society's cabinets, and was a curator of its museum. He was an avid collector of both plants and minerals, and later in life sold his collection of minerals to McGill (where today it forms part of the collection of the Redpath Museum at McGill University). In addition to collecting one of the first specimens of Peristerite, he is credited with collecting the first specimen of Bytownite.
Dr. Thomas Thomson (1773 - 1852), M.D., who named Peristerite, was a leading chemist of his day. Dr. Thomson announced Peristerite to the world in a paper read before the Glasgow Philosophical Society on November 2, 1842, and published in 1843 in Volume XXII of the Philosophical Magazine. Here is the first part of his report:
“Peristerite .–The next mineral which I have to mention was sent to me from Perth in Upper Canada, by Mr. Wilson, and also by Dr. Holmes of Montreal, under the name of Iridescent felspar; but neither its characters nor its composition correspond with that appellation.
The specimens were amorphous masses, and had the appearance of having constituted part of a rock blasted by gunpowder.
It is light brownish red, and exhibits a play of colours, chiefly blue, on the surface. It is translucent on the edges; the lustre is vitreous, and the texture imperfectly foliated: its hardness is only 3.75, which is a good deal less than felspar. Its specific gravity is 2.568.
Before the blowpipe it becomes white but does not melt. With carbonate of soda it melts into a green coloured bead, and on adding nitre the colour becomes red: with borax it fuse into a colourless bead.
Its constituents were found to be
Silica............................................ .72.35
Alumina....... ............................... 7.60
Potash.......................................... 15.06
Lime .............................................. 1.35
Magnesia...................................... 1.00
Oxides of iron and manganese 1.25
Moisture ...................................... 0.50
99.11
The silica is much greater than in felspar, and that alumina much less, while the proportion of potash is nearly the same.”
See: Thomson, T. (1843), Notice of Some New Minerals, Philosophical Magazine, New Series, Volume XXII, page 188 at pages 189-190.
Unfortunately, Dr. Thomas Thomson’s chemical analysis and much of his description contained significant errors.
Within a decade after Dr. Thomson’s paper was published, T. Sterry Hunt, Chemist and Mineralogist to Canada’s Geological Survey, in two nearly identical papers, provided a much more accurate analysis and description of peristerite, from specimens provided by Dr. Wilson of Perth, in part to correct Dr. Thomson’s “unfortunate want of precision in his mineralogical description” and the fact that Dr. Thomson’s chemical compositions “seemed but little accordant with their general physical characters” . T. S. Hunt commented:
“The second species to be noticed is that described by Dr. Thomson under the name of peristerite, in allusion to the beautiful play of colours analogous to that of Labradorite, which it exhibits. The specimens from Bathurst furnished to me by Dr. Wilson, as duplicates of those sent to Dr. Thomson, are composed of a mixture of quartz grains, readily distinguishable by their lustre, greater hardness and want of cleavage, disseminated through a felspar, which still so far predominates as to give distinct cleavages to the mass; such from his analysis, also would appear to be the substance examined by Dr. Thomson. Specimens of the substance furnished to me from the same locality exhibited the mineral in fine cleavable masses, free from quartz, and occasionally in consequence of an admixture of it, passing into the variety just described.
The crystalline form of the mineral shows it to belong to the triclinic system; the faces of cleavage give apparently the angles of albite, but do not admit to accurate measurement. ... The surface P shows a fine play of colours like Labradorite, in which a delicate cerulean blue predominates, occasionally passing into light green and yellow; the face M is often marked with striae parallel to P. The same play of colours and striation on alternate surfaces are distinguishable in the quartzose masses. The hardness of the mineral is 6 and the specific gravity 2.625 -2.627; lustre vitreous inclining to pearly on P; colour white, passing into pearl-gray, and reddish white or flesh-red in the quartzose specimens; translucent fracture uneven.... the analysis of the pure specimen gave:–
I. II.
Silica........................................... 66.80 ............................. 67.25
Alumina....... ............................... 21,80
Potash.......................................... .58
Soda ........................................ 7.00
Lime ........................................ 2.52 .............................. 2.03
Magnesia...................................... .20
Peroxides of iron ......................... .30
Loss on Ignition............................ .60 .................................. .66
99.80
The results of the analysis, conjoined with its physical characters, show this mineral to be albite. ... Thomson, in his analysis of the peristerite, gives a much larger proportion of silica, but as has been before observed, the specimens examined by him were the quartzose mechanical aggregate.”
[T. S. Hunt (1852), Report of T.S. Hunt, Esq., Chemist and Mineralogist to the Provincial Geological Survey, Addressed to W. E. Logan, Esq., Provincial Geologist, Geological Survey of Canada Report of Progress for the Year 1850 - 1851, at pages 37 -38;
T. S. Hunt (1851), Examinations of Some Canadian Minerals, Philosophical Magazine, Fourth Series, Volume 1, page 322, at pages 323-324; ]
For close to 100 years after T. Sterry Hunt’s paper there was debate in the scientific literature as to whether the “Peristerite of Thomson” was simply albite. Today Peristerite is known to represent a submicroscopically exsolved form of plagioclase feldspar in the range of about An2 to An17 (albite to oligoclase), where because of the submicroscopic intergrowths of two different plagioclase feldspars (one a sodium rich albite and the other a more calcium rich oligoclase), interference effects result in iridescence. Another way of saying this is that the original homogeneous plagioclase feldspar has submicroscopically unmixed into two plagioclase components, with one plagioclase component in the range of An0-5 and the other plagioclase component in the range of An20-35. The blue play of colours, the iridescence or schiller, results from the submicroscopic exsolution, and arises from diffusion of light through adjoining crystals of different optical properties, or from reflection and diffraction arising from diffusion of light through adjoining crystals of different optical properties. The exsolved microstructures can take the form of lamellae, tweeds and blebs, with lamellae in the range of 15 to 35 nm thick.
Last summer I attempted, without success, to locate the original type locality in the nineteenth lot of the ninth Concession of Bathurst Township, in Lanark County. I did find an outcrop displaying a graphic intergrowth of quartz with Perthite, and I located two of the abandoned feldspar mines in the ninth Concession (but on different lots than the nineteenth), but didn’t find what I was looking for.
Below is a photograph [Sam_0532] of Peristerite specimens that are on display in the mineral cabinets at the Matheson House Museum in the Town of Perth, Lanark County, Ontario.
Christopher Brett
Perth, Ontario
No comments:
Post a Comment