In my posting last January 14th I set out where to find what is believed to be the type locality for Perthite.
In February I received an email from Luis Sánchez-Muñoz, a researcher with the Institute for Ceramics and Glasses in Madrid, Spain, who identified himself as a geologist who had being doing research on alkali feldspars from pegmatites since 1989, said that he would be visiting Ontario and Quebec to collect specimens of alkali feldspars from famous pegmatites, and that he would be visiting to collect specimens from the type locality. Later I received an email saying he would be joined by Professor Robert Martin of McGill University. On June 5th I was pleased to welcome them to Perth and provide them with a selection of specimens of Perthite from which to choose. Surprisingly, Luis Sánchez-Muñoz only wanted two. Not surprisingly, he chose two specimens showing two directions of good cleavage.
For those interested in perthitic feldspars a recent paper by Luis Sánchez-Muñoz and a number of co-authors is available over the internet. The paper is entitled The Evolution of Twin Patterns in Perthitic K-Feldspar from Granitic Pegmatites, and was published in the August, 2012 edition of The Canadian Mineralogist, Volume 50, No. 4, pages 989-1024. Figure 21, which is a schematic model for a ten stage general evolutionary sequence of transformations in perthitic K-rich feldspar, with indications of twinning and exsolution, and the explanation in the paper, are interesting, as the authors develop a process that ties the development of twinning patterns into the formation of exsolved Albite.
Luis Sánchez-Muñoz and Professor Martin mentioned that they hoped to visit a number of other pegmatite localities in Lanark County, including the famous Bathurst mine. Readers who are mineral collectors will be likely be aware of the mine, as it was Ontario’s second largest feldspar mine (producing 106,018 tons of feldspar), is listed in at least two of Ann Sabina’s mineral collecting guidebooks, and is easily accessible from Perth. (It is on the northeast side of Old Mine Road, which connects McVeigh Road to Bathurst 9th Concession Road.) It has been forty years since I was there, and I can’t help thinking that it’s probably worth a visit, as it is only 12 kilometers from Perth and there are at least four other abandoned feldspar mines in pegmatites in that area.
When they left I could not help wondering if Professor Martin knew what he had signed up for. An article on the web describes a visit in August of 2010 by Dr. Luis Sánchez- Muñoz to Colorado, where he visited 23 pegamites in five days in pursuit of samples of microcline feldspar. (See: Twenty three pegmatites in five Days, a Colorado field trip saga by Peter J. Modreski and Luis Sanchez-Munoz, an Abstract at the 32nd Annual New Mexico Mineral Symposium
http://geoinfo.nmt.edu/museum/minsymp/abstracts/view.cfml?aid=378 )
Since I posted my blog last January, two points on Perthite have caught my eye. First, I was scanning the second edition of Deer, Howie & Zussman’s book Rock-Forming Minerals, Volume 4A, Framework Silcates - Feldspars, and couldn’t help but notice that they mention (at page 5) that "Perthite takes its name from Perth, Quebec, an early locality." That statement is of course wrong, as Perthite takes its name from Perth, Ontario.
Second, I noticed an analysis and description of Perthite from the type locality near Perth, Ontario in an article by Charles H Warren entitled A Quantitative Study of Certain Perthitic Feldspars, that was published in 1915 in Volume 51 of the Proceedings of the American Academy of Arts and Science, starting at page 139. In his paper Charles Warren compared specimens of Perthite from two localities in Canada, four in the United States and two in Finland. This is Charles Warren’s analysis of Perthite from the type locality near Perth:
SiO2 66.50
Al2O3 18.40
Fe2O3 1.05
MnO Trace
MgO .07
CaO .03
K2O 8.77
Na2O 5.40
H2O .20
Total 100.69 Sp. Gr. 2.597
He estimated that it contained 51.9% Microcline by weight and 47.3% Albite by weight.
Charles Warren provided this description of his specimen from Perth, Ontario:
"This specimen, from the original locality, is the richest in albite of any of the feldspars studied. It consists of a rather dark reddish-brown microcline intergrown with about an equal amount of a light red to almost white albite. The red color is due 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. Some hematite is found along fractures in the albite, or is more irregularly placed. ... The albite lamellae seldom exceed 1mm isn width while a commonly observed width is 0.5 mm. The microcline bands will in general average broader than the albite. The orientation of the bands is fairly uniform parallel to the usual direction, but there is a common tendency to bend off toward the direction of the prism and often many short bands coalesce along this same direction. Pinching and swelling, branching and coalescing, often in a very complex fashion, are common particularly in certain areas. The albite forms minutely pointed surfaces of contact with the microcline as a rule. The microcline sometimes shows very distinct polysynthetic twinning but more often this is faintly developed and is often not visible at all."
Charles Warren was limited by the technology of his day: microscopic viewing of thin sections, and chemical analysis of bulk samples. It will be interesting to see what Luis Sánchez- Muñoz finds with current analytical techniques: polarized light optical microscopy, electron probe micro-analysis, scanning and transmission electron microscopy, cathodluminescence imaging, micro-Ramon spectroscopy and nuclear magnetic resonance, all techniques mentioned in his paper that I referred to above. It was nice to assist a visitor from Spain, and it will be interesting to read the results of his research.
Christopher Brett
Perth, Ontario
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