Perthite from Burgess Ward, Tay Valley Township, Lanark County, Ontario

In an earlier posting I described two outcrops of pegmatite containing perthite, an exsolution texture in feldspar, that are found approximately 8.5 kilometers (five miles) south of the Town of Perth in North Burgess Township, Lanark County. The outcrops are on the west side of Elm Grove Road and on the northeast side of Glenn Drive. The legal description for the land for the purposes of the Land Titles Office is still North Burgess Township, but the municipality is now called Tay Valley Township (which formed through the amalgamation of the three former townships of North Burgess, Bathurst and South Sherbrooke). In Tay Valley Township the names of the three former townships are maintained in the names of Burgess Ward, Bathurst Ward and South Sherbrooke Ward. Both outcrops are in Concession VI of North Burgess township. The outcrop on the west side of Elm Grove Road falls in the third lot of Concession VI, as does  the outcrop on the northeast side of Glenn Drive. 

The original specimens of perthite were collected by Dr. James Wilson in about 1841 from what is now Lot 3, Concession VI of North Burgess Township. Unfortunately the exact locality is not known. However, as the pegmatite outcrops containing Perthite on Elm Grove Road and on Glenn Drive fall in the third lot of Concession VI, it is possible that one of these  was the original location where perthite was found.  Regrettably, the outcrop on Glenn Drive is the front yard of a home and is out of bounds for collecting. However, the outcrop on the west side of Elm Grove Road can be easily accessed and sampled. It can be easily found as it is about 50 meters south of the second (and southerly) intersection of Glenn Drive with Elm Grove Road, and is the only pink outcrop on the west side of Elm Grove Road north of where Long Lake Road intersects Elm Grove Road. I recommend against parking on the west side of Elm Grove Road close to the outcrop as the shoulder of the road is quite narrow (and the ditch is quite deep). The safest parking is on Glenn Drive.

Below are photographs of the outcrop on Elm Grove Road.

It is interesting to trace the development of the term perthite, from its origin to how it is used today, and to look at some of the early mineralogists who studied Perthite. It is also interesting to trace the development of Burgess Township and to look at when North Burgess became part of Lanark County.

Dr. James Wilson (1798-1881), a physician and surgeon in the Town of Perth who had emigrated to Canada from Scotland, found the first occurrence of perthite approximately 8.5 kilometers south of the Town of Perth, on property between Adam Lake and Otty Lake, in what was then Burgess Township, in Upper Canada (now Ontario). Dr. Wilson collected the specimens from an outcrop on property farmed by Thomas Dobbie (also spelt Dobie on some deeds, and later mispelt as Dobey by various members of the Geological Survey of Canada) who acquired the whole of Lot 3 in 1838 and sold the property in deeds dated 1863 and 1872. Dr. Wilson sent the specimens to Dr. Thomas Thomson, Regius Professor of Chemistry at the University of Glasgow in Scotland, who in 1842 named the ‘mineral’ Perthite, after the Town of Perth.

Perthite was found by Dr. Wilson on property that was then in Burgess Township, Leeds County, Johnstown District, Upper Canada. In 1842 Upper and Lower Canada were united to form the Province of Canada, with Upper Canada being renamed Canada West while Lower Canada became Canada East. Upon Confederation in 1867 Canada West was renamed Ontario and Canada East was renamed Quebec. In 1842 Burgess Township was split into North Burgess Township and South Burgess Township. North Burgess Township was attached to the jurisdiction that became the present Lanark County while South Burgess Township remained in what became the United Counties of Leeds and Grenville. The lot and concession numbers of the old Burgess Township were not changed. For example, Lot 3, Concession VI of Burgess Township is now Lot 3, Concession VI of North Burgess Township.

Today perthite is recognized as an exsolution texture in alkali feldspars consisting of two feldspar minerals: a potassium rich alkali feldspar (orthoclase or microcline) and a sodium rich alkali feldspar (albite). The original feldspar crystallized deep within the crust at high temperatures and pressures as homogeneous crystals. On decreasing temperature (and possibly pressure as the rock rose in the earth’s crust) the original homogeneous crystals unmixed (recrystallized) into two feldspar minerals. In perthite orthoclase or microcline makes up a greater proportion of the specimen than albite, and albite is considered to have exsolved from the original homogeneous crystal. (Where albite makes up the greater proportion of the specimen, it is called antiperthite.)

In perthite from Burgess Township the potassium rich alkali feldspar is a brownish red, while the albite is off white (arguably, Caucasian flesh pink). The albite makes up about 30% to 50% of the perthite. As the potassium rich alkali feldspar is the predominant mineral and dominant colour, the specimens as a whole appear to be a dull, pinkish to brownish red on fresh surfaces and a much darker brownish red on weathered surfaces. In perthite from Burgess Township the potassium rich alkali feldspar and albite occur as parallel interlaminated lamellae generally 1 to 2 mm thick that pinch and swell. Some of the lamellae are up to at least 80 mm long. In four specimens that I collected the lamellae are at about a 70 degree angle to the best cleavage plane. In two specimens additional fine lamellae can be seen (with the aid of a 10x hand lens) within the lamellae of potassium rich alkali feldspar. A number of specimens that I collected show one perfect cleavage plane, while others show two cleavage planes. One specimen shows two cleavage planes and two parting planes. Tilting a cleavage plane in light gives a silvery to golden glow (from both feldspars), much the same as the reflection of light off mica. If cut properly, cabochons made from perthite from this locality should show the silvery to golden glow.

In perthite from Burgess Township the potassium rich alkali feldspar is likely microcline. In the early papers it is called orthoclase. However, microcline was only identified as a separate mineral three and a half decades after perthite was found. Appendix III, 1875 - 1882, by Edward S. Dana (1882), to the Fifth edition of Dana’s Mineralogy, credits Breithaupt (1876) with using Microcline "to designate a new feldspar mineral established by him: a triclinic potash feldspar" that is "near orthoclase in form and in habit" with a composition the same as orthoclase. Dana (1882) also mentions that "A large part of the potash feldspar, previously called orthoclase, is in fact microcline. .... The perthite of Canada is in part microcline (J. Min., 1879, 389)."

August Breithaupt (1791-1873) was a professor of mineralogy at the Freiburg Mining Academy and is credited with the discovery of 47 valid mineral species. He was one of the leading mineralogists of his time and perhaps of all time. He named both orthoclase and microcline, and studied perthite. He developed the concept of mineral paragenesis, made numerous contributions to crystallography and wrote a number of German language text books on mineralogy.

Dr. Thomas Thomson (1773 - 1852), M.D., who named Perthite, was a leading chemist of his day. In 1831 he gave Silicon its current name and in 1820 he identified a new zeolite, named Thomsonite in his honour. However, he botched the analysis of Perthite. Dr. Thomson announced Perthite 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: 



"2. Perthite. – The next mineral I have to notice I distinguish by the name Perthite. It was sent to me by Mr. Wilson, a surgeon in Perth, a township of Upper Canada; hence the name by which I distinguish it. It is very much connected with feldspar in appearance, and was sent as a variety of that mineral.
The colour of the specimen sent to me is white: it consists of a mass of crystals so united together as to form a kind of tesselated pavement. The crystals are obviously four sided prisms, apparently rectangular, but not susceptible of measurement, because they cannot be isolated.
The lustre is vitreous; the hardness is rather less than that of feldspar; but the specific gravity, which is 2.586, is identical with that of some of the varieties of that mineral. It’s constituent elements were found to be
Silica        76
Alumina    11.75
Magnesia    11.00
Protoxide of iron 0.225
Moisture         0.65
                 99.625"See: Thomson, T. (1843), Notice of Some New Minerals, Philosophical Magazine, New Series, Volume XXII, page 188 at page 189.

Today, 170 years later, it would be obvious to anyone that has a taken a mineralogy course, and can remember the formulas for feldspars, that finding that much magnesium in feldspar and finding that little aluminum in feldspar, were errors. Dr. Thomson’s description of the specimen as "a kind of tesselated pavement" confirms that he was looking at Perthite, however his description of the colour as "white" is at odds with specimens from this location unless he was describing only the albite portion.

Interestingly, The Hunterian Museum and Art Gallery, University of Glasgow, has a photograph on its web site of what may be the original specimen of Perthite sent by Dr. Wilson to Dr. Thomson.   See:

http://www.huntsearch.gla.ac.uk/cgi-bin/foxweb/huntsearch/LargeImage.fwx?collection=all&catno=M2361&mdaCode=GLAHM&filename=M2361a.jpg#caption


Within a decade after Dr. Thomson’s paper was published, T. Sterry Hunt, Chemist and Mineralogist to Canada’s Geological Survey, in three papers (two of which were almost identical), had provided a more accurate analysis and description of perthite, from specimens provided by Dr. Wilson, of Perth, in part to correct Dr. Thomson’s "unfortunate want of precision in his mineralogical description" and that Dr. Thomson’s chemical composition "seemed but little accordant" with perthite’s physical properties.

Thomas Sterry Hunt (1826 - 1892) was the Canadian Geological Survey’s Chemist and Mineralogist from 1847 to 1872, when he resigned to become Professor of Geology at the Massachusetts Institute of Technology. During his career he authored numerous papers on chemistry, mineralogy and geology. Wikipedia also reports that he was the first to link climate change to concentrations of carbon dioxide in the atmosphere.

The following is from T. Sterry Hunt’s report from1847.

"In company with Dr. Wilson I then proceeded to examine the locality from which he had obtained the specimens described by Dr. Thompson of Glasgow as perthite. It is nothing more than a reddish feldspar which makes up a large portion of an intrusive mass of granite in the limestone. The perthite occurs in large individuals of three to four inches in diameter. It is of different shades of reddish-brown, the colours being arranged in bands, and the surfaces of cleavage parallel to P. Present golden reflections like the sunstone. From the analysis of Dr. Thompson it would appear that the mineral, unlike other feldspars, contains no potassium, which is according to him replaced by calcium, and it was upon this chemical difference principally, that he predicated its distinctiveness as a species. It has however been analysed by my pupil Mr. Hartley, in the Laboratory of the Survey, and the results show that it contains both potassium and sodium, and is indeed quite similar in composition to other feldspars. This locality is in the third lot of the sixth concession of Bathurst."  [Comment: Bathurst should be Burgess, and Dr. Thompson’s name should have been spelt as Dr. Thomson.][As quoted in Smith, W. H. (1851), Canada: Past, Present and Future, Volume II, at pages 327-328.]

The following is part of T. Sterry Hunt’s report on perthite from the Report of Progress for 1850 - 1851 (at pages 36 -37):

"Perthite. It is found in the township of Burgess, and, mixed with quartz, forms a pegmatite rock, in which large cleavable masses of the feldspar are occasionally met with. Its cleavage form is apparently monoclinic, and its hardness is 6. upon the scale of Mohs; – specific gravity from 2.576 to 2.579, of a darker fragment 2.583; lustre vitreous inclining to pearly; colour light flesh-red alternating with reddish to pinchbeck-brown, the two colours forming bands from half a line to a line in width, coincident with one of the planes T, often however, interrupted and mingling with one another. The darker bands exhibit on the cleavage surface T, when viewed perpendicularly, a golden reflexion like the variety known as aventurine feldspar, and polished specimens of the mineral in the possession of Dr. Wilson show that it is available for ornamental purposes. The colours of this feldspar become darker on exposure to the action of the weather. The analytical results [follow] ...

                         I                        II
Silica................   66.44............. 66.50
Alumina............... 18.35.........   19.2
Peroxyd of iron..... 1.0
Lime..................... .67 ............. .56
Magnesia.........      .24............... .24
Potash................ 6.37............. 6.18
Soda.................. 5.56.............  5.56
Water, (loss on ignition). .40 ...... .44
                               99.03         98.73

It is evident from these analyses, that the composition of this feldspar is precisely that of orthoclase, to which species it had already been previously referred by Shepard, Dana and myself, (see my Report for 1847-48, p. 135). The proportion of soda is larger than is generally met with in this species, but there are instances of orthoclase in which the greater portion of alkali is soda."

Hunt, T.S. (1852), Report of T.S. Hunt, Esq., Chemist and Mineralogist to the Provincial Geological Survey, in Geological Survey of Canada, Report of Progress, 1850-51, at pages 36 -37. He published an almost identical paper in the Philosophical Magazine. See: T. S. Hunt (1851) Examination of some Canadian Minerals, Philosophical Magazine, Volume 1, Fourth Series, Pages 322 -328

T. Sterry Hunt’s 1847 report and his 1851 paper in the Philosophical Magazine unfortunately introduced an error that the specimens were from Bathurst Township, which has been repeated in later papers and texts, including some published within the last twenty years. The type location was in Burgess Township (later North Burgess Township, and now Tay Valley Township).

Sir William E. Logan (1863) describes perthite in his book Geology of Canada, Geological Survey of Canada - Report of Progress from its commencement to 1863, at pages 474 and 833, confirming that the specimens were obtained from "the third lot of the sixth range of Burgess."

It took another ten years for mineralogists to accept that perthite was an inter growth of albite and orthoclase. T. Sterry Hunt (1871) in a paper entitled On the Laurentian Limestones of North America, in a footnote to the perthite of Burgess, mentions "It has since been shown by Gerhard to be made up of thin layers of reddish orthoclase and whitish albite. See Dana’s Mineralogy, fifth edition, page 356." The thesis by Gerhard (1861), entitled Lamellari Orthoclasi et Albitae in Perthite Alisque Feldspathis Observata, is available on the internet from a number of sites. (While it is in Latin, some parts are understandable.) Mineralogy texts and papers published towards the end of the 1800's generally credit Gerhard and/or Breithaupt with determining that Perthite is interlaminated orthoclase and albite. (See, for example: Hoffman, G.C. (1889), Annotated List of the Minerals occurring in Canada, Proceedings and Transactions of the Royal Society of Canada of Canada, Volume VII, page 65 at page 94, where Hoffman
commented "The Perthite of Dr. Thompson (a flesh-red aventurine feldspar, which as shown by Breithaupt, consists of interlaminated albite and orthoclase) occurs in large cleavable masses, in pegmatite veins, cutting Laurentian strata, in the township of North Burgess, Lanark county, province of Ontario." And see: Jukes, J. B. (1872) The Student’s Manual of Geology, Third Edition, at page 76, which contains the comment "But it was Breithaupt’s and Gerhard’s investigation on perthite that clearly showed that many potash feldspars consisted of alternate lamellae of orthoclase and albite.")


In 1888 Eugene Coste introduced some confusion into the source for perthite from Burgess Township when he commented:

"PERTHITE.– Perthite occurs in large cleavable masses in thick pegmatite veins, cutting the Laurentian strata, and is often made up of flesh-red and reddish-brown bands of orthoclase and albite, interlaminated. When cut in certain directions it shows beautiful golden reflections like aventurine, and being susceptible of a high polish, is adapted for an ornamental stone for use in jewellery. It is also found in considerable quantity in Burgess, Ont., about seven miles southwest of the town of Perth, and near Little Adams Lake on what was formerly called Dobey Farm."See: Coste, E. (1888) Report on the Mining & Mineral Statistics of Canada for the year 1887, in Geological and Natural History Survey of Canada, Annual Report, 1887, part S, at page 75S.

Coste (1888) introduced the following errors: "Burgess, Ontario" should be the "Township of North Burgess", "southwest of the town of Perth" should be "south of the town of Perth" or "south, south east of the town of Perth", "Dobey" should be "Dobbie", and "Little Adams Lake" should be "Adam Lake".  (While Adams Lake appears on some maps, Adam Lake is the correct name.  There is no Little Adams Lake.)

The whole of Coste’s description was quoted at page 341 of William A. Parks’ 1912 publication, Report on the Building and Ornamental Stones of Canada, Volume 1, Mines Branch, Report 100. Parks (1912) is the source most often relied upon for perthite being found on the ‘Dobey’ farm.

In the latter half of the 20^th century Ann P. Sabina of the Geological Survey of Canada authored a number of informative books on rock and mineral collecting that contained directions to many of the classic collecting sites in Canada. I have six of her publications. She describes the perthite locations in North Burgess Township in both:

Sabina, Ann P. (1965), Rock and Mineral Collecting in Canada, Volume II, Ontario and Quebec, G.S.C. Miscellaneous Report 8, at page 118 (map at page 116); and

Sabina, Ann P. (1968), Rocks and Minerals for the Collector, Kingston, Ontario to Lac St-Jean, Quebec, G.S.C. Miscellaneous Paper 67-51, at pages 36-38 (map at page 28).

The two perthite locations on Elm Grove Road and on Glenn Drive are likely the two perthite locations mentioned in her publications. However, since she wrote her publications roads have been rerouted, widened, paved and renamed, and what was the Dobbie farm has converted in part to a small subdivision. Glenn Drive did not exist when she wrote her publications and the outcrop presently on Glenn Drive is likely the one that Sabina (1968) describes as "Clearing on farm on left. The dyke containing perthite is exposed in the farm about 100 yards north of this point." The outcrop presently on Elm Grove Road is likely the one that Sabina (1968) describes as "Perthite exposure in woods on right side of road. The dyke has been dynamited and specimens can be obtained from broken blocks near it."

In her description of perthite Sabina (1965) mentions that "One of the first occurrences from which the mineral was obtained is the Dobey farm near the west side of Adam Lake, about 8 miles southeast of Perth. The exact location is not known." referencing Parks (1912).

Sabina (1965) also stated:
"A pegmatite dyke of perthite and quartz outcrops in two places in lot 4, concession 6, North Burgess township, between Otty and Adam lakes. Dugas (1952) mentions that the name "perthite" was first applied to the feldspars from this dyke. The dyke strikes north. It is exposed adjacent to the road and at another place about 1200 feet to the north. ... Reference: Dugas, J.: Dept. Geol. Sci., McGill Univ., Ph.D. Thesis, 1952, p. 78."

Sabina mistakenly placed both occurrences in lot 4.    This was likely a typographical error, as both fall on lot 3.  
It was also an unfortunate turn of phrase for Sabina (1965) to suggest that the perthite "was obtained on the Dobey farm near the west side of Adam Lake." This has been taken by some to mean that perthite was found by Dr. Wilson "on the Dobey farm near the west shore of Adam Lake." The Dobbie farm consisted of 200 acres that abutted on and is west of Adam Lake. The two perthite locations on Elm Grove Road and on Glenn Drive are west of Adam Lake, but the outcrop on Elm Grove Road is at least 600 metres west of Adam Lake while the outcrop on Glenn Drive is at least 700 metres west of Adam Lake.

An Ontario Department of Mines paper on the web entitled Bibliography of Theses On The Precambrian Geology of Ontario, compiled by R. M. Ginn in 1961, provides the following information on the thesis by Dugas that was referred to by Sabina :

Dugas, Jean, 1952, Geology of the Perth map area, Lanark and Leeds Counties, Ontario; Ph. D., McGill Jean Dugas is one of the geologists responsible for the Geological Survey of Canada’s Map 1089A - Geology of Perth, Lanark and Leeds Counties, Ontario, published in 1961 (Geology by M. E. Wilson, 1930 and Jean Dugas, 1949. Descriptive Notes by Jean Dugas). The map can be downloaded free of charge from the GSC’s web site. This map shows the location of the pegmatite outcrop that is now on Elm Grove Road, and shows a pegmatite dyke that may be the one that outcrops on Glenn Drive. Interestingly this map also shows a third outcrop of pegmatite on lot 3, approximately a quarter mile north west of the outcrop on Elm Grove, and shows additional pegmatite outcrops further to the west of Lot 3. I have not yet made time to check those outcrops for perthite.

That pegmatite dykes bearing perthite may be found to the west of lot 3 is confirmed in notes to a mineral exhibit prepared for the Pan-American Exposition, at Buffalo, New York held in 1901, where a number of specimens of perthite from North Burgess township, Lanark County were exhibited by Ontario’s Bureau of Mines. Most were simply said to be from North Burgess township, but others were from Lots 5 and 8, Concession VI, N. Burgess township. (See: Gibson, T. W. (1901), Mineral Exhibit of the Province of Ontario, Collected and Prepared by the Bureau of Mines, Descriptive Catalogue.)


Christopher P. Brett
Perth, Ontario