"Rock of the Month # 261, posted for March 2023" ---

Locality: The Havey (Berry) pegmatite is within sight of the classic Mount Apatite (Pulsifer), just east of Poland, south of Bethel in the Oxford pegmatite field of western Maine. While K-feldspar is the premier mineral commodity mined here, there has been sporadic, local production of gem-quality mineral specimens starting in the first quarter of the 19th century. In the late 1890s, feldspar mining began at both Mount Apatite and Berry-Havey. Logging took place in between the two mines, along the valley of the Androscoggin River. A tabulation of the Maine Geological Survey (1957) provides maps and data on 184 pegmatite mines and prospects, concentrated into a broad region of southwest Maine. The Berry quarry (as it was then known: site BA-10 in the report, in Poland township) was reported to yield apatite, beryl, cassiterite, feldspar (microcline), lepidolite, muscovite and tourmaline.

The Havey pegmatite is a sill-like body, a sheet reported to be at least 2,000 feet long, 1,000 feet wide and 20-40 feet thick. (610 x 305 x 6-12 m thick). Unlike some pegmatites in the region, such as Plumbago North, lithium values are low overall, although a mass of some 8 tonnes of lepidolite was removed low in the body. But gemstones such as purple fluorapatites are recovered, and a wide range of other mineral specimens (not all of them gemmy, e.g., pseudomorphs of loellingite after garnet, a strange superposition, are very interesting, if actually quite ugly!). Rich zones have garnets with rims of blue tourmaline. Pockets, which may exceed 1.5 metres in diameter, may host fine tourmaline crystals. Larger blocky feldspars may be an indicator of the proximity of larger pockets. Loose material seen on the September 2022 visit included fine-grained lepidolite with coarse muscovite and the cleavelandite habit of albite feldspar. Random wanderings of the pegmatite reveal coarse schorl (some boudinaged by a late deformation), cleavelandite and graphic granite.

The modern Havey specimen mining operation opened almost two decades ago, a newcomer in a region with a 200-year history of quarrying and specimen mining. Havey is located east of Poland in Androscoggin county, and is noted for mineral specimens: green elbaite tourmaline, fluorapatite, calcite and other minerals (see, e.g., Simmons, 2012). A display, and other reference material from the quarry, can be seen at the Maine Mineral & Gem Museum (MMGM) in nearby Bethel. As the museum displays illustrate so nicely, in the early 1900s, over 150 granite quarries operated in Maine, some of them also yielding large crystals of quartz, beryl, tourmaline and other minerals.

The pegmatite is located northeast of the Sebago batholith, in a terrane now termed the Migmatite Granite Complex. Immediate host rock is a gneissic amphibolite. The pegmatite sheet is zoned. As described by Encarnacion Roda-Robles at the Pegmatite Workshop in Bethel, September 2022, below the homogeneous wall zone there is an intermediate zone, the upper part of which displays much graphic granite, while the lower part contains quartz, K-feldspar and schorl. The margin of the core contains masses of cleavelandite, tourmaline and garnet. The core may yield morganite and aquamarine (beryl) and the phosphates apatite (some of it purple and gemmy) and amblygonite. The finest crystals in these pegmatites are typically found in open pockets. The textures and chemistry of tourmaline and micas, garnet and feldspars vary from wall to core.

Dating of some of the regional pegmatites such as Mount Mica, Emmons and Havey suggests they are late Permian, circa 270-250 Ma old. They may be anatectic pegmatites formed by decompressional melting in the crust, associated with the early stages of rifting of Pangea (Simmons et al., 2020, 2022, pp.238-254). This mode of formation (anatexis, crustal melting) is in contrast to the generation of pegmatites as the final stage of crystallization of a magma, often at the apex of a granitic body (e.g., the roof of a granite stock in Cornwall, England).

Sparse basalt / diabase dykes cut the pegmatite: a sampled example on the entrance road strikes N37-40E, and is subvertical, dipping steeply west. In this region, perhaps the dykes are related to the voluminous Culpeper basins to the south, in Virginia and adjacent states, which are of Triassic (252-201 Ma) age (?).

Fig. 2a,b: A smaller sample, 14x10x3.5 cm, 612 grams. Similar texture and mode as seen in Fig. 1. At right (2b): close-up of texture.

Graphic granite:

The term graphic granite refers to an intergrowth of quartz and perthitic microcline feldspar, a lithology on a cm to m scale, characteristic of pegmatites (see, e.g., London, 2008; Clanin, 2012; Simmons et al., 2022). In fact, the term pegmatite referred originally to the graphic granite intergrowth (London, 2008, p.4). The intergrowths, rendered bold by the darker grey quartz wedges, resemble the strokes that compose ancient Egyptian hieroglyphs (fancy a nice digression from rocks (?): read Adkins and Adkins (2000) entertaining biography of Jean-Francois Champollion, and the story of the decipherment of the hieroglyphic script). Introduced in the 19th century, the usage of both pegmatite and graphic granite spread rapidly, appearing in classic topographical mineralogies such as that of Heddle (1901).

It is also familiar in many granitic rocks under the microscope (Wahlstrom, 1955; Hatch et al., 1972; MacKenzie et al., 1982). The graphic granite intergrowth appears to form by simultaneous crystallization of quartz and feldspar (Fenn, 1986). The feldspar is a perthitic microcline. Microcline is an alkali feldspar, of ideal formula KAlSi₃O₈. Perthitic texture develops when sodium-rich plagioclase feldspar exsolves from the cooling, solid K-feldspar host, forming characteristic blebs and lamellae (see perthite). It may be microscopic, but is often visible to the naked eye, or under a hand lens.

We might note that mineral intergrowths are quite common and arise in diverse ways. In terms of granitic rocks, three quartz-feldspar intergrowths are of particular interest: granophyre, graphic granite and myrmekite. Graphic granite is the only one of these that is generally readily apparent in hand specimen, the others generally appearing at low magnification in the microscope. Granophyre appears as a rather patchy, intimate mixture of quartz and K-feldspar (the term has also been employed for some rocks with abundant granophyric patches), whereas myrmekite is a mixture of vermicular quartz and sodic plagioclase feldspar (see MacKenzie et al., 1982, pp.46-52, for fine photographs of these textures). For further notes on the subsolidus exsolutions displayed as perthite, myrmekite and granophyric intergrowth, see also Cox et al. (1979, pp.296,301-302).

Graphic granite in pegmatitic rocks can be very attractive, and be used in ornamental stones, as for table and counter tops, stairwells and facia (see this example in a hotel). Graphic granite features in many popular guides to rocks and minerals (e.g., Kirkaldy, 1968). Graphic granite is attractive, and features on many regional guides to rock collecting (e.g., in the Grenville province of southwest Quebec and southeast Ontario: Sabina, 1971, 1987: see also Storey and Vos, 1981). Masses of graphic granite are widespread in the Grenville, as in the old feldspar quarries of the Hybla valley, north of the town of Bancroft. Granitoid rocks with graphic granite textures are widespread throughout the Americas and beyond and, confusingly, some refer to the rocks themselves as “graphic granites”.

Fig. 3: Pegmatite sample composed largely of graphic granite, 16x13x6 cm, 1521 grams. The host pegmatite is largely coarse K-feldspar and albite (slightly greenish against white K-feldspar, polysynthetic twin planes visible), quartz and muscovite. Again, quartz apparently nucleates on first-crystallized, skeletal feldspar, the intergrowth developing at a high angle to coarse feldspar crystals which predominate in the host pegmatite.

The evolution of granites as they cool from a melt (“below the liquidus") is quite complex. There is a sequence of crystallizations of one mineral, then another, each mineral taking its essential elements from the melt. Additions of elements may result from the assimilation of wall rocks. The temperature at which the melt entirely crystallizes (the solidus) can be lowered by the content of volatiles in the magma. Water is the most universal of these, but others are the halogens F and Cl, and the less-common but locally important light elements Li and B which (with Be) are common in the Oxford pegmatite field of Maine.

Graphic granite is an indicator of rapid crystallization, in which undercooling of a water-undersaturated melt, and growth of K-feldspar that is rapid relative to the diffusion of silica in the melt, leads quartz to deposit on skeletal faces of feldspar (Simmons et al., p.33). In the Dunka River region of Minnesota, felsic rocks that intrude gabbros along the base of the huge Duluth mafic intrusive complex in the area have coarse graphic textures, and are chemically unlike granophyre complexes of the rift, possessing especially high silica contents (70.8- 78.7% at one location). These graphic granites (the intrusions, with the texture) may be the products of partial melting of country rocks along the base of the Duluth complex (Harris and Wirth, 2000).

Acknowledgements: Thanks to Havey mine owner and guide Jeffrey W. Morrison, and the organizers and presenters of the Pegmatite Workshop, Bethel, Maine, September 2022: I learned a lot from you all, and from the displays at MMGM. Thanks also to Walter Mroch for transport, and pegmatite enthusiasm! Any errors on this page are no doubt attributable to the author, and his hastily, horribly scribbled notes!

References

Adkins,L and Adkins,R (2000) The Keys of Egypt: The Obsession to Decipher Egyptian Hieroglyphs. HarperCollins Publishers, 335pp.

Clanin,J (2012) The Fundamentals of Mining for Gemstones and Mineral Specimens. New England Historical Publications, Massachusetts, 403pp.

Cox,KG, Bell,JD and Pankhurst,RJ (1979) The Interpretation of Igneous Rocks. George Allen & Unwin, London, 450pp.

Fenn,PM (1986) On the origin of graphic granite. Amer.Mineral. 71 nos. 3- 4 (R.H. Jahns Memorial Issue), 325-330.

Harris,ECA and Wirth,KR (2000) Petrogenesis of granitic rocks of the Dunka River region, eastern Mesabi Range, Minnesota. Abs. 46th Annual Meeting, Institute on Lake Superior Geology, vol. 46 part 1, 73pp., 15- 16, Thunder Bay.

Hatch,FH, Wells,AK and Wells,MK (1972) Petrology of the Igneous Rocks. 13th edition, Thomas Murby (George Allen & Unwin Ltd), London, 551pp.

Heddle,MF (1901) The Mineralogy of Scotland. Volume I, lviii+148pp., plates 1-51, and Vol.II, viii+247pp., plates 52-103.

Kirkaldy,JF (1968) Minerals and Rocks in Colour. Blandford Press, London, 2nd edition, 184pp.

London,D (2008) Pegmatites. Canadian Mineralogist Spec.Publ. 10, 347pp. + CD ROM.

MacKenzie,WS, Donaldson,CH and Guilford,C (1982) Atlas of Igneous Rocks and their Textures. Longman, 148pp.

Maine Geological Survey (1957) Maine pegmatite mines and prospects and associated minerals. Maine Geol.Surv., 43pp., reprinted with permission in 1995 by Rochester's Eclectic Emporium, Oxford, ME.

Sabina,AP (1971) Rocks and Minerals for the Collector: Ottawa to North Bay, Ontario: Hull to Waltham, Quebec. GSC Pap. 70-50, 130pp.

Sabina,AP (1987) Rocks and Minerals for the Collector: Hull Maniwaki, Quebec, Ottawa Peterborough, Ontario. GSC Misc.Rep. 41 (revised version of GSC Pap. 69-50), 141pp.

Simmons,WB (2012) Two centuries of gem production from Maine pegmatites. Elements 8 no.4, 318-319, August.

Simmons,WB, Falster,AU and Freeman,G (2020) The Plumbago North pegmatite, Maine, USA: a new potential lithium resource. Mineralium Deposita 55 no.7, 1505-1510.

Simmons,W, Webber,KL, Falster,AU, Roda Robles,E and Dallaire,DA (2022) Pegmatology. Pegmatite Mineralogy, Petrology and Petrogenesis. 2nd edition. Rubellite Press, Cana, VA, 287pp.

Storey,CC and Vos,MA (1981) Industrial Minerals of the Pembroke Renfrew area, Part 2. OGS MDC 22, 214pp. plus OGS map P2209, 1:126,720 scale.

Wahlstrom,EE (1955) Petrographic Mineralogy. John Wiley and Sons, Inc., New York, 408pp.

Graham Wilson, 05,11,13,14,25 February 2023

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