Editing Commercial mica in Scotland — occurrences north of the Great Glen (1943)
From: Kennedy, W Q and Lawrie, T R M. 1943. Commercial mica in Scotland Part 2 Preliminary description of some occurrences north of the Great Glen. London : Geological Survey & Museum.
Article: Mica quarrying and processing in Scotland during the Second World War
Introduction
Mica-bearing pegmatites are common throughout the Highlands, but, although sporadic occurrences containing muscovite of commercial grade and quality have been recorded from time to time, it is only within recent months that serious consideration has been given to the possibility of locating and developing economically valuable deposits. An investigation of the problem is now being undertaken by the Geological Survey, and, as a result of preliminary work, it is possible to indicate the general mode of occurrence of the mineral in Scotland and the prospects within certain areas.
At present, mica deposits of potential economic value are definitely known to occur in the following districts:
- Western Inverness-shire and North-west Argyll, along a north and south belt extending from Loch Sunart across Loch Shiel and Loch Nevis into Knoydart.
- The Strathgarve district of Eastern Ross-shire.
The various deposits all belong to the same type and show identical geological relations. They comprise veins and similar bodies of quartz-rich pegmatite containing large books of white mica (muscovite) and occur within highly altered metamorphic rocks belonging to the so-called Moine Series. The latter, within the districts in question, are highly injected and permeated with granitic material and have been converted into coarsely crystalline gneisses. They are cut by a great suite of veins and bodies of white mica-rich pegmatite in which, for the most part, the muscovite, although extremely abundant, is in the form of comparatively small crystals or books up to about an inch or so in diameter Locally, however, as at the localities specified, the mineral shows an abnormal development and forms books up to 18 inches or more in diameter and weighing many pounds. Such valuable deposits occur always in association with the mica schists of the Moine Series and never with the siliceous granulties or impure quartzites.
Western Inverness-shire and North-west Argyllshire
The mica deposits which have so far been located within this district are shown on the sketch-map (Fig. 3). They comprise five main localities situated along a general north and south line and include two deposits (viz. Dalilea and Knoydart) which are definitely known to yield ruby mica of excellent quality. The area as a whole has not been systematically searched and it is probable that further work may result in the discovery of additional mica-bearing veins.
Ardarie, near Strontian, Argyllshire
Locality 1 P528073
Maps: One-inch to the mile (Geological: unpublished) Sheet 52
Six-inches to one mile: Argyll, Sheet 27 N.W.
Locality
North side of Loch Sunart, 630 yds. N. 20º E. of Ardarie, approximately 4 miles W. of Strontian. On north-east margin of small patch of peat. Altitude: About 700 O.D.
The deposit is situated on the hillside about ⅓ mile from the farm of Ardarie. The intervening ground is rough, but not steep, and offers no difficulty to pack horses and might even be traversed by wheeled vehicles. A road exists from Ardarie to the main Salen-Strontian road.
Nature of the deposit
The mica occurs in a quartz-rich pegmatite vein which runs in a general north and south direction and can be traced along the strike for some thirty yards. The occurrence of mica is pocketly and the veins appears to vary in width up to six feet and apparently dies out to the south where a second parallel vein containing no mica occurs on its western side.
>Some exploration work has been carried out in this deposit which is seen to be extremely rich in mica. Books up to 12 inches in diameter are found and are closely crowed together with the result that there has been considerable mutual interference. This has resulted in bending and distortion of a large percentage of the mica, and flat books are exceptional rather than the rule. There is also some intergrowth of mica and quartz.
The total amount of mica in sight both in the vein and the dumps is considerable, but the nature of the mineral is such that sheet material would probably constitute only a fraction of the output. On the other hand the deposit would certainly provide a considerable amount of material suitable for grinding or pulping, and, in view of its relative accessibility, might be worthy of consideration from this point of view.
Dalilea, Loch Shiel, Inverness-shire
Locality 2, P528073
Maps: One-inch to the mile (Geological: unpublished) Sheet 52
Six-inches to one mile: Inverness, Sheet 157 N.E.
Locality
North side of Loch Shiel, 1⅜ miles N. 35º E. of Dalilea House: midway between summit (1,071 ft.) of Ceann Loch Uachdrach and Lochan a’ Mhuilinn. Altitude: About 900 O.D.
The deposit is situated about ¾ mile from the old track across the hill from Austinscroft by way of General Ross’s Cairn to Kinlochmoidart. This track might take wheeled traffic and, in any case, joins the main road from Dalilea to Acharacle and thence to Fort William (by way of Corran Ferry) near Austinscroft. The intervening ¾ mile between the deposit and the old track consists of comparatively rough but not steep ground, which would offer no obstacle to pack horses. The total maximum distance to be traversed by this means would not exceed 1½ miles.
Nature of the deposit
The deposit occurs in highly injected pelitic mica schists of the Moine Series and comprises a narrow zone 8 ft. or so in width within which the rocks are cut by veins and lenticular bodies of quartz-rich pegmatite containing books of white mica up to 12 inches or more in diameter and two or three inches in thickness. The individual veins within the zone are comparatively narrow and do not exceed a maximum of three feet in width. Usually they are less and tend to be lenticular. The mica books are not crowded and are, almost without exception, perfectly flat.
The deposit forms a ridge which stands up above the surrounding rocks and will permit of working by opencast methods.
Some recent exploration work has been carried out at this locality by the Eastern Mica Company, Arbroath, who are now working the deposits, and a quantity of excellent mica has been extracted.
The mica is a ruby muscovite of fair-stained quality and is free from spotting and magnetic staining. Sizes from No. 1 downwards are obtained but no detailed figures of percentages are available.
The observed length of the mica-bearing zone is at least twenty-five yards, but the deposit may extend further to the north under superficial deposits. Further exploration and systematic work is necessary to prove the deposit which, although not very extensive, so far as present observations go, can nevertheless supply a small but useful quantity of good mica.
Austinscroft, near Dalilea, Loch Shiel, Inverness-shire
Locality 3, P528073
Maps: One-inch to the mile (Geological: unpublished) Sheet 52
Six inches to one mile: Inverness, Sheet 157 N.E.
Locality
North side of Loch Shiel, about ½ mile W. 20º N. of Dalilea House and 300 yds. due E. of Austinscroft. Altitude: 125 O.D.
The deposit is situated within the wooded area about 100 yds. N. of the road from Dalilea to Acharacle and some 4 miles from the latter village.
Nature of the deposit
This represents a minor occurrence of mica but is worthy of mention as it indicates the possibilities of the area.
The mica occurs in a dyke-like vein of white muscovite pegmatite, about 4 to 5 ft. in width, which cuts the injected pelitic Moine schists in a direction parallel to the strike. The vein is exposed for a distance of 50 ft. but may extend further to the north and south below thin superficial deposits.
Some exploration work has been carried out, and a certain amount of mica can be seen on the dump. The mica is a silvery to greenish, rather opaque muscovite and is almost invariably heavily ruled and ribboned. The books vary in size up to 8 inches across but the total amount of mica in sight is comparatively small.
The pegmatite itself is rather quartz-rich and contains masses of relatively pure, cream-coloured albite.
The value of this deposit as a source of mica is doubtful, but it would provide some excellent albite. The mica, however, being soft, might prove useful for certain purposes where this quality is desirable. Only a small output can be envisaged.
Diollaid, West of Glenfinnan, Inverness-shire
Locality 4, P528073
Maps. - One-inch to the mile (Geological: unpublished) Sheet 61
Six-inches to one mile: Inverness, Sheet 136 S.E.
Locality
On high between Loch Eilt and Loch Shiel, about midway between Diollaid Mhor and Diollaid Bheag, 5½ miles W. of Glenfinnan and 2⅛ miles W. 25º S. of eastern end of Loch Eilt. Altitude: 2,000 O.D.
The deposit is situated on the col between Diollaid Mhor and Diollaid Bheag approximately 1⅛ miles S. of the Mallaig-Fort William Railway along the south side of Loch Eilt. It lies about 1,800 ft. above the level of the railway, and the intervening ground is both rough and steep. A rude track (right of way) leads to the head of Loch Eilt from the flat (1,200 O.D.) below the outcrop. An alternative but longer route from the deposit is southward through Glenaladale to Loch Shiel. In either case transport would require to be by pack horse.
Nature of the deposit
The mica occurs in a dyke-like or vein-like body of white, quartz-rich albite pegmatite about 12 ft. in width and concordant with the strike of the adjacent schists. The mass can only be traced continuously for a few yards, but outcrops of similar pegmatite are found at intervals for a few hundred yards towards the east and probably mark a continuous of the same mica-bearing zone. The mica books vary much in size with the largest some 8 inches in diameter and 4 inches in thickness. The other pegmatites in the vicinity also carry much mica but the maximum size of books noted in these latter bodies does not exceed 2 inches in diameter and about ½ inch in thickness. No development work or exploration of this body has been carried out and little can therefore be said as yet regarding its potential value as a source of commercial mica.
Loch Nevis Mica Prospect, Knoydart, Inverness-shire
Locality 5 P528073
Maps: One-inch to the mile (Geological: unpublished) Sheet 61
Six inches to one mile: Inverness, Sheet 107 N.W.
Locality
North side of Loch Nevis, Knoydart District, 1,100 yds. S. 12° E. of summit (2454 ft.) of Sgurr Coire nan Gobhar and near end of ridge 300 yards S. 30° E. of small loch in Coir’ an Lochain. About 7½ miles due E. of Mallaig. Altitude: ca. 1 1800 O.D.
The deposit is situated near the southern termination of the semi-circular ridge which forms Coir’ an Lochain and lies approximately one mile, or slightly less, in a direct line from the shore of Loch Nevis. The approach from the loch is steep with an average gradient of 1 in 3.
From the nearest point on the shore of Loch Nevis to Mallaig Harbour the distance is, by boat, nine miles, and Mallaig forms the terminal point of the L.N.E. Railway to Fort Willian and the south. There is a small crafting community situated on the stretch of flat ground at Kylesknoydart.
No roads exist in the district and transport offers considerable difficulties.
Nature of the deposit
This deposit is the most extensive and potentially the most valuable source of mica at present known in the Western Highlands. The nature of the occurrence is, in general, similar tot hose described above, but the amount of mica in sight and the size of the individual books are greater.
The actual deposit occur in coarsely crystalline mica gneisses or injected mica schists and consists of a zone of country rock heavily impregnated with veins and lenticles of extremely quartz-rich pegmatite containing books of white mica up to 18 inches or 2 ft. in diameter.
The mica-bearing body as a whole is concordant with the enclosing rocks, which dip to the south-east at 45° – 50°, and is essentially a broad lenticular mass at its western end but tails out eastward into a more normal sheet-like zone. The main western lens is itself rather irregular in form and has an overall length of 100 ft. and a maximum breadth of 50 ft. It appears to be a highly inclined shoot rather than a true lens, and from the internal structure exhibited may be assumed to pitch in a direction approximately E. 40° S. at an angle of 45°.
Although the deposit as a whole is concordant, the internal structure of the western lens is complicated, and much folding, obviously contemporaneous with the emplacement of the pegmatite, is apparent. This folding does not affect the mica which appears to have grown under stable conditions and has not subsequently been distributed. The type of internal structure developed resembles rodding on a very large scale, the mica-bearing lens thus forming a cylindrical mass with the pitch indicated above.
That this interpretation regarding the cylindrical nature of the body is probably correct finds confirmation in a somewhat similar although small-scale example in the general area. An exposure of mica-bearing gneisen of the same type, but with small books, was located a few hundred feet to the south of the original prospect and was clearly seen to form a cylindrical mass with determinable pitch.
The cylindrical rather than true lens-shaped nature of the body is emphasized as a guide to the probable extension in depth. In the case of a lens it is usually assumed in mining practice that the depth will be approximately equal to the length. In the case of a cylindrical shoot this empirical law does not necessarily hold and the depth may, in such a case, considerably exceed the length.
This western end of the mica-bearing pegmatite zone outcrop on top of a small cliff and dips into the hillside as shown in Section A-B (Fig. 2).
The eastern part of the body is more normal in form and consists of a concordant sheet-like zone which outcrops near the top of a 60-ft. cliff falling to the north. This part of the zone or vein can be traced for approximately 150 ft. and it seen to vary from 2 to 7 ft. in thickness. The main lead is more or less continuous throughout and consists of a vein of quartz or very quartz-rich pegmatite which contains large books of excellent mica and swells out locally giving a succession of lenticular masses. The mica forms rather thick books and is well spaced, without, at the outcrop, any crowding or mutual interference.
The relationship of the vein in this part of its course is indicated by the Sections C-D and E-F (Fig. 2). As regards continuation in depth little can be said, although the body might reasonably be expected to behave as a vein. It must be pointed out, however, that, in the option of the writer, there is little prospect of the vein continuing eastward beyond the point indicated on the plan.
Some exploration work has been carried out at this locality and several hundred pounds of mica have been extracted during these operations. The largest book so far obtained was about 2 ft. in diameter, and crystals a foot or more in size are fairly common. The mica itself is of excellent quality and is a good fair-stained ruby.
A trial consignment of 211 lbs., which consisted of uncobbed books with some adhering rock, gave 53 lbs. of sheet mica of the following sizes:
| Size | lb. | Oz. |
| 1 | 1 | 1 |
| 2 | 0 | 14 |
| 3 | 3 | 15¼ |
| 4 | 4 | 1¾ |
| 5 | 5 | 11 |
| 5½ | 8 | 11¼ |
| 6 | 27 | 11½ |
| Total | 52 | 10¾ |
In addition, about 2 oz. of No. 1 size clear ruby were obtained. This represents a yield of 25 per cent sheet from run of mine mica and compares favourably with worked deposits in other countries.
The deposit is also rich in beryl and contains crystals of that mineral up to 12 inches in length.
Strathgarve district, Eastern Ross-shire
Within this area, which includes a tract of country extending north-eastwards from Loch Garve to the slopes of Ben Wyvis, the geological conditions are favourable for the development of mica, and a recent reconnaissance has shown that scattered deposition do occur. It must be emphasized, however, that no systematic prospecting has yet been undertaken and that the following occurrences are merely those which were encountered and examined during the preliminary investigation.
Càrn Feàrna, near Garve, Ross-shire
Locality 1, P528074
Maps: One inch to the mile (Geological) Sheet 93
Six inches to one mile: Ross and Cromarty, Sheet 75 S.W.
Locality
Approximately 3 miles due E. of Garve Railway Station, at southern outlet of small lochan, 350 yards E. 15° N. of trigonometric station 1408 ft. On Càrn Feàrna. Altitude: 1250 O.D.
The deposit is situated on a comparatively flat part of the hillside about 1¼ miles due north of the nearest point on the Dingwall – Kyle of Lochalsh railway and about 2 miles due east of Garve Railway Station. The intervening ground between the railway and the deposit is rough but not excessively steep and would office no obstacle to transport by pack horses.
Nature of the deposit
The mica occurs in a coarse, dyke-like mass of white muscovite pegmatite, 25 ft. in width, which cuts the highly injected pelitic Moine Schists of the district. The pegmatite extends in a direction E. of 15°N and runs parallel to the strike of the enclosing rocks. The body contains large masses of barren white quartz which, particularly along the northern margin, are accompanied by books of mica up to 8 inches in length and 2 inches or so in thickness. The mica belong to the ruby type and most of the books are perfectly clear and of excellent quality. Some, however, show slight surface staining and a few are slightly spotted with magnetite. The pegmatite itself can be traced for a distance of at least 80 yds. and may extend further below the superficial deposits. At several points it stands up well above the enclosing rocks and would lend itself to the opening of quarry faces five or six feet in height.
This body has only been examined in a superficial manner and further exploratory work is necessary in order to assess its potential value.
Glensgaich, near Garve, Ross-Shire
Location 2, P528074
Maps: One inch to the mile (Geological) Sheet 93
Six inches to one mile: Ross and Cromarty, Sheet 75 .E.
Locality
About 3 miles due E. of Garve Railway Station, on hillside 1 mile W. of 25° N. of Glensgaich Cottage and 1,300 yds. S. 20° E. of summit (1,705 ft.) of Càrn Gorm. Altitude: 1,200 O.D.
The deposit is situated within ¾ mile of the Dingwall – Kyle of Lochalsh railway, about 3 miles east of Garve Station and an equal distance west of Auchterneed. The hillslope is not excessively steep and the locality is not more than 600 ft. above the level of the railway.
Nature of the deposit
Mica of commercial size is found in two distinct types of pegmatite at this locality. These may represent independent bodies, but they lie on the same line of strike and their mutual relations have not been properly established.
Deposit No. 1. The more southerly of the two occurrences is essentially a feldspar deposit with subordinate mica. It consists in the main of an extremely coarse, pink, feldspathic pegmatite composed almost entirely of feldspar (microfline perthite) and quartz with little or no mica. The mass is dyke-like in form with a width of some 75 ft. and can be traced along the strike in a direction N. 30° E. for at least50 yds. Good mica (muscovite) in books up to at least 8 inches in diameter is found along a narrow marginal zone on the north-west edge of the mass and appears to be confined more or less entirely to this zone. The possibilities of this deposit as a source of mica have not however been explored.
Deposit No. 2. Approximately 150 yds. N.N.E. of the southern locality a second, probably independent dyke-like mass of white quartz-rich pegmatite, some 20 ft. in width, is exposed. This pegmatite is garnetiferous and contains large masses of barren quartz and books of white mica, up to 6 or 8 inches in diameter. The mica is a ruby muscovite and appears to be of good quality despite slight surface staining. Flattened garnets are occasionally associated with the mica but there appears to be no staining with magnetite.
Along its eastern margin the mica-bearing pegmatite forms a low scarp some 6 ft. in height, and initial quarrying could be carried out with ease.
Further investigation is necessary in order to determine the possible value of the deposit and more particularly the extension of the mica-bearing zone towards the north-east.
Càrn Gorm (No. 1), near Garve, Ross-shire
Locality 3, P528074
Maps: One inch to the mile (Geological) Sheet 93
Six inches to one mile: Ross and Cromarty, Sheet 75 N.E.
Locality
Approximately 700 yds. N.N.E. of summit (1705 ft.) of Càrn Gorm, and nearly 3½ miles E.N.E. of Garve. Altitude: about 1650 O.D.
The deposit is situated on the hillside about 1¾ miles from the nearest point on the Dingwall – Kyle of Lochalsh railway, and about 3½ miles N.W. by N. Auchterneed Station. The best means of approach is from Auchterneed whence a good road extends to a point on the railway about ¼ mile E. of Raven Rock. From there a path runs for almost 1¾ miles up the Allt Glean Sgathaich valley to within ½ mile of the deposit. The route therefore is over rough and peaty ground but is nowhere too steep for transport by pack horses.
Nature of the deposit
The mica occurs in a dyke-like mass of coarse pegmatite, which cuts across the strike of the highly injected pelitic Moine Schists of the district. The outcrop, which is lenticular, extends for about 100 yds. in a direction roughly E. – W. , carrying in width from 20 ft. at its western extremity to a maximum of about 50 ft. some 70 yds. to the E.S.E.; from there it tapers rapidly to the east, and disappears under a cover of heather and peat. Apart from a thin cover of peat in the hollows, the mass as a whole is well exposed, particularly along its northern margin, where it forms a low cliff varying in height up to about 10 ft. Where exposed, the contact between pegmatite and country rock is often sharply defined, and it either vertical or highly inclined; along the western end of the mass it conforms to the dip of the schists, which here have an inclination of about 55° towards the S.E. Where, however, the pegmatite contains numerous large inclusions of country rock, and where the enclosing schists are saturated with pegmatite material, there is no sharp line of demarcation, but a gradual fading of the one into the other.
The pegmatite consists mainly of quartz and mica, with subordinate white feldspar; small euhedral garnets and prisms of black tourmaline are locally abundant, with beryl as a rather rare accessory. Masses of barren quartz and of white feldspar commonly occur throughout the body, which also contains large inclusions of pelitic schist, more or less saturated with pegmatitic material. The mica, both muscovite and biotite, is most strongly developed along the margins of the pegmatite body, round the eyes of barren quartz, and in close association with the included blocks of schist. It occurs to some extent also as porphyroblasts in the country rock in the immediate vicinity of the pegmatite. Biotite occurs mainly in books up to 15 inches in diameter, but is also found intergrown with muscovite. Books of muscovite, up to 8 inches in diameter and about an inch in thickness, are numerous throughout the body. In the western part, where a small test face was opened up some years ago, books 12 inches in diameter and 3 to 4 inches in thickness are fairly common, and a few up to 18 inches were seen. The books are in general flat and well spaced.
The muscovite belongs to the ruby type, and is clear and of good quality, especially in the western part of the deposit. While the books so far collected show signed of surface weathering and cross-fracturing, preliminary examination has shown that a fair yield of mica, in flawless sheets up to 12 square inches (sizes 3 to 7) can be obtained from them. Ruling and A-structure are sometimes developed, but not to any series extent, and the number of books spoiled by intergrowth with biotite is relatively small. In depth, where the mica is free from surface staining and weathering, a good yield of high grade sheet mica should be obtainable.
Further exploration and systematic work are, however, necessary in order to ascertain the extent of the deposit, both laterally and in depth, and to assess its potential value as a source of commercial mica.
Càrn Gorm (No. 2), near Garve, Ross-shire
Locality 4, P528074
Maps: One inch to the mile (Geological) Sheet 93
Six inches to one mile: Ross and Cromarty, Sheer 75 N.E.
Locality
About 300 yds. west of occurrence (B3), and about 500 yds. N. by W. of the summit of Càrn Gorm. Altitude: 1750 to 1800 O.D. The deposit is situated on the hillside about 1¾ miles from the nearest point on the Dingwall – Kyle of Lochalsh railway, and about 3½ miles N.W. by N. Auchterneed Station. The best means of approach is from Auchterneed whence a good road extends to a point on the railway about ¼ mile E. of Raven Rock. From there a path runs for almost 1¾ miles up the Allt Glean Sgathaich valley to within ½ mile of the deposit. The route therefore is over rough and peaty ground but is nowhere too steep for transport by pack horses.
Nature of the deposit
The mica occurs in a broad dyke-like mass of white quartz – pegmatite, about 70 yds. in width, which cuts across the strike of the injected pelitic schists of the area. The pegmatite, which extends in a direction approximately E. – W., can be traced for almost 400 yds. along the top of a broad ridge. It is poorly exposed except near the eastern and western ends of the ridge, where it forms a number of low crags.
The pegmatite, which contains large inclusions of schist, consists of quartz and muscovite, with subordinate white feldspar; pink garnets are locally very abundant, and a few small crystals of pale green beryl have been detected. Muscovite occurs throughout in flakes and books which are generally too small to be of commercial value. Near the eastern and western ends of the ridge, however, pegmatite carrying numerous large books of muscovite occupies a central zone, 20 to 30 ft. in width, with a broad band of relatively barren rock on each side. This central zone appears to strike in a direction parallel, or almost parallel, to the strike of the main body, but its exact relation to the mass as a whole has not been ascertained. Mica in good-sized books occurs along the southern margin of the main body, and at other points on the ridge, but apparently with a pockety distribution.
Near the eastern end of the ridge muscovite occurs in numerous fairly thick books up to a foot in diameter, and commonly 6 to 8 inches. The mica exposed at the surface is badly weathered, being heavily stained with iron oxide. Some of it is dark and almost opaque, due to the present of innumerable dusk-like inclusions between the laminae. In addition some of the books are spoiled by inclusions of garnet. Until exploratory work has exposed unweathered material it is impossible to estimate the potential value of the deposit at this point. At the western end of the ridge the mica exposed is for the most part clear and of good quality, being free from garnets and other inclusions. The number of flat and well-spaces books in sight here is sufficient to indicate that appreciable quantities of mica, of commercial grade and size, can be obtained.
Further investigation is required in order to determine the precise extent of the mica-rich zones, and to ascertain their relation to the pegmatite body as a whole.