I’ve previously written about Perthite from Perth, Ontario. In my posting from Monday, January 14, 2013 I mention two outcrops south of Perth where it is believed that Dr. James Wilson found the original specimens, that Dr. Thomas Thomson at the University of Glasgow performed the initial chemical analysis and named Perthite, that T. Sterry Hunt of the Geological Survey of Canada subsequently published a better description of the Perthite and better chemical analysis to correct Dr. Thomson’s "unfortunate want of precision in his mineralogical description" and problems with Dr. Thomson’s reported chemical composition.
I also provided a link to The Hunterian Museum and Art Gallery, University of Glasgow, which has a photograph on its web site of what is likely the original specimen of Perthite sent by Dr. Wilson to Dr. Thomson (Hunterian catalogue number M2361 ). See:
http://www.huntsearch.gla.ac.uk/cgi-bin/foxweb/huntsearch/LargeImage.fwx?collection=all&catno=M2361&mdaCode=GLAHM&filename=M2361a.jpg#caption
In my posting from Wednesday, June 12, 2013 I mentioned a visit by Luis Sánchez-Muñoz and Professor Martin to Perth, discussed Sánchez-Muñoz’s 2012 paper entitled The Evolution of Twin Patterns in Perthitic K-Feldspar from Granitic Pegmatites, and referred to an article by Charles H Warren entitled A Quantitative Study of Certain Perthitic Feldspars, that was published in 1915, which provided a description and analysis of Perthite from the type locality near Perth which Warren estimated contained 51.9% Microcline by weight and 47.3% Albite by weight.
There has been a recent article published on Perthite from the type locality near Perth, Ontario by two Scottish professors: Martin R. Lee of the School of Geographical and Earth Sciences, University of Glasgow, and Ian Parsons of the Grant Institute of Earth Science, University of Edinburgh. The citation for the article follows:
Martin R. Lee, Ian Parsons (2015)
Diffusion-controlled and replacement microtextures in alkali feldspars from two pegmatites: Perth, Ontario and Keystone, South Dakota,
Mineralogical Magazine, December 2015, 79 (7) 1711-1735
DOI: 10.1180/minmag.2015.079.7.21,
http://minmag.geoscienceworld.org/content/79/7/1711
Published online on February 09, 2016
Lee and Parsons compared perthitic microcline from Perth, Ontario (Wards catalogue 46 E 0510) and perthitic microcline from Keystone, South Dakota (Wards 46 E 5125) with the sample of the type perthite from Perth, Ontario (Hunterian Museum, Glasgow, M2361). The two Wards samples were studied using light and electron microscopy while the type perthite from the Hunterian Museum was compared using light microscopy.
Lee and Parsons report that the type specimen differs in bulk composition and microtexture from the Wards sample from Perth (and from the Keystone sample) and conclude that although the Wards sample from Perth and type specimen likely come from the same general locality, the Wards sample described in their paper is not the type material.
Interestingly, they did not analyze the bulk composition of the type specimen or the Wards sample from Perth. They noted that Hunt's two analyses (1851) correspond with a bulk composition of Or41.2Ab55.5An3.4, which they use for the composition of the type specimen M2361. For the bulk composition of the Wards sample from Perth they rely on an analysis conducted by Holdren and Speyer, 1987 which yielded a bulk composition of Or57.4Ab42.0An0.6. They note that the type is more albitic than the Wards’ sample. They don’t mention Warren’s 1915 analysis of Perthite from the type locality (mentioned above), which appears to be in the middle of their two bulk compositions.
Lee and Parsons make some interesting observations on the type specimen: “In thin section ... M2361 is a vein perthite with albite forming periodically distributed lenticular sheets, overall parallel to b , but locally slightly oblique. At low power, lamellae appear relatively smooth-sided, although in detail the interfaces are serrated. ... The average periodicity of the lenses is ~0.5 mm. The albite has a pronounced {110} cleavage , and in places a second feldspar, presumably microcline, forms thin{110} plates partially outlining albite subgrains . In some regions the lenses adopt a more nearly equidimensional, globular shape, and there are also regions in which thick albite lamellae are missing. Fine-scale, film microperthite occurs in these regions. Most of the microcline has very regular, straight-sided tartan twinning, with occasional regions of coarser twinning. It forms a matrix to the albite films, and is crowded with tiny euhedral plates of orange-brown biotite, most abundant in the central parts of the microcline lamellae. This suggests that the formation of biotite was related to the replacement process that produced the macroperthite. The plates of biotite have no visible effect, at the LM scale, on the tartan twinning. The biotite presumably accounts for the dark brown colour of the microcline in this sample and also the golden schiller noted by Hunt and visible in our fragment. The albite has fine-scale Albite twinning, varying in thickness, and sometimes showing lateral off-sets suggesting deformation or the presence of subgrains.”
[References to their figures omitted]
I find it interesting that the golden schiller noted by Hunt (1851) was noted by Lee and Parsons in the thin section of the type specimen M2361 in the Hunterian Museum, Glasgow and attributed to plates of biotite. Charles Warren (1915) for perthite from the type locality near Perth, attributed the red color of the dark reddish-brown microcline “to the presence of exceedingly minute crystal scales of hematite which are chiefly contained to the microcline. They are usually arranged along definite crystallographic directions.”
I’ve collected a number of specimens of Perthite from the outcrop on the west side of Elm Grove Road looking for – but not finding – a golden schiller in any specimens. While it is possible that the Wards specimens of Perthite from Perth also come from this outcrop (I’ve been told by a friend who was born and raised in Lanark County not far from the outcrop that a tremendous volume of Perthite was collected from the outcrop on Elm Grove Road when the road was widened), Lee and Parsons' comment on the Wards specimen that it is "less obviously perthitic" than the type specimen does not accord with the outcrop.
The outcrop on Elm Grove Road is one of the two outcrops of pegmatite containing perthite that are found approximately 8.5 kilometers (five miles) south of the Town of Perth, in the third lot of Concession VI, North Burgess Township, Lanark County. The second outcrop is about 200 metres to the north of the outcrop on Elm Grove Road, on the northeast side of Glenn Drive. I do have one specimen from that location. In my specimen of perthite from the Glenn Drive outcrop the microcline does have a dark brown colour, darker than the microcline from Elm Grove Road, but I attributed it to the fact that the specimen is more weathered and not as fresh as the samples from Elm Grove Road. It would be interesting to have my specimen of Perthite from Glenn Drive analyzed to see if it matches Hunt’s composition. Perhaps Glenn Drive is the type location rather than off Elm Grove Road?
Christopher Brett
Perth, Ontario
