Garnet and tourmaline in granitic pegmatite

Granitic Pegmatite from Mount Mica, Maine

Tourmaline and garnet in pegmatite

Fig. 1: Pegmatite with pink garnet and black tourmaline: the host rock is largely white albite (sodic plagioclase feldspar) and greyish translucent quartz, plus (see Fig. 3) some muscovite mica. The average grain size varies by mineral, apparently increasing in the sequence albite < quartz < garnet < muscovite < tourmaline. The mineral colour alone suggests that the garnet is rich in the almandine (Al-Fe) molecule (probably plus the Al-Mn spessartine molecule: documented, see Simmons et al., 2022, p.108), while the tourmaline is dominated by the Fe-rich schorl variant. This material was collected on 08 September 2022 from the Mount Mica pegmatite in Maine (cf. the Emmons, Havey and Plumbago North bodies - see "pegmatite" in the Rock of the Month Index, or list at foot of page, for details). For a colourful overview of minerals and the history of mining at Mount Mica, see Simmons et al. (2005).

"Rock of the Month # 283, posted for January 2025" ---

Tourmaline and garnet in pegmatite

Mount Mica, in Paris township, is part of the Oxford county pegmatite field of southwest Maine. This is a significant historical locality in the annals of gemstone mining in the state, with gemmy tourmaline and other valuable minerals discovered here, with fluctuating interest and activity, since initial finds of elbaite in 1820-1821. It seems appropriate that the entrance road gleams with mica flakes! The pegmatite body is a subhorizontal sheet of coarse granite pegmatite, which is zoned (a wall zone and an inner core), common rock-forming minerals including quartz, albite, muscovite, with local black tourmaline (schorl) and garnet (almandine). The core contains masses of microcline K-feldspar, and minerals typical of more evolved compositions, such as elbaite and lepidolite. The roof to the sheet is a rusty-weathering biotite schist. Tourmalines of variable chemistry may be found at the contacts (e.g., roof zones) of pegmatite against wall rocks (Dyar et al., 1999) and also in pockets, typically located in specific horizons within zoned pegmatites (Robinson et al., 1992; Simmons, 2007; Simmons et al., 2022).

The history and mineralogy of Mount Mica and many other Maine pegmatites is documented and displayed at the Maine Mineral & Gem Museum in Bethel. A tour here (MMGM, 2022) is very instructive, and a treat for the eyes! This paragraph notes some of the many facts to be gleaned from the displays at the museum (as seen in late 2022). Beside the ticket office is a spectacular elbaite prism from Mount Mica. Elbaite was discovered at Mount Mica in 1820, but the gemstone mining began in earnest a half-century later, in the 1870s, and by 1910 it was a local cottage industry. Minerals at the Mount Mica Whispering Pines quarry include schorl, rhodochrosite and rose quartz, as well as chrysoberyl, columbite and perhamite, elbaite, hydroxylherderite, almandine, zircon, biotite and beryl. Mount Mica operated sporadically after World War I and the modern phase of gem mining began in 2004. Mount Mica mineral assemblages include elbaite- lepidolite- albite and elbaite- sheet muscovite. An early tribute to the deposit was a self-published 1895 book by the physician-author Augustus Choate Hamlin of Bangor, Maine entitled "The History of Mount Mica of Maine, USA, and its Wonderful Deposits of Matchless Tourmaline" (72pp. + 43 plates).

In modern terminology (Marchal et al., 2014), Mount Mica is a poorly zoned, sodic LCT (lithium- caesium- tantalum -enriched rare-element) pegmatite with micas as the main K minerals. Other minerals, which are restricted to the pegmatite core zone, include fluorapatite, Cs -rich beryl and pollucite. Micas may be rich in Li, and sodic plagioclase is the primary feldspar. Oxford county pegmatites such as Mount Mica, Emmons and Havey may have formed at circa 270-250 Ma. They may be anatectic pegmatites formed by decompressional melting, associated with the early stages of rifting of Pangea (Simmons et al., 2020).

Figs. 2-3: Two further views of samples from Mount Mica, with similar mineralogy to the view in Figure 1. Some cm-size flakes of silvery muscovite mica are visible at the base of Fig. 3.

elbaite in pegmatite [684 kb]

Fig. 4: A very large, circa 30-cm-long spray of bicolour, prismatic elbaite tourmaline in the rock wall adjacent to the entrance to gated underground workings in the Mount Mica pegmatite. If not a prime cabinet specimen, it is still very impressive! The tourmaline family is very complex when studied in detail, beyond the most-familiar three molecules of schorl, dravite and elbaite (in a nutshell, respectively Fe-rich and black; Mg-rich and brown; Li-rich, red to pink to green). Tourmaline specimens from Mount Mica have been featured in collector publications (e.g., Francis, 1985; Robinson et al., 1992; Wilson, 2008; Moore, 2024). Mount Mica has long been known for the elbaite and other rare or exceptional minerals, e.g., beryl and cassiterite, cleavelandite and pollucite (Rand, 1957).

Footnotes:

1. Cleavelandite is a varietal name for a typically chalk-white, coarse sodic feldspar (albite) with a platy crystal habit, formed relatively late in the paragenesis of a pegmatite. It is found mostly in granitic pegmatites, but is also known from nepheline syenites. In the granitic bodies it is often associated with gemmy developments of minerals such as tourmaline. It has been described in pegmatites worldwide. A selection of early descriptions might include Derry (1931),Quensel and Gabrielson (1939) and Quensel (1952). For modern descriptions of cleavelandite-bearing gemstone pegmatites, see Laurs et al. (1998) and Menezes (2009). In the much-studied Varutrask Li pegmatite in Sweden, Quensel and Gabrielson (1939) noted that schorl occurs exclusively in the early marginal pegmatite, developed prior to invasion of fluids rich in Na and Li which caused mineralization in the central parts. Thus schorl concentrates near the upper and lower contacts, near the amphibolitic greenstone country rock. In contrast, elbaite occurs especially in the intermediate zone between the microcline pegmatite rim and the lepidolite-rich core of the pegmatite. Elbaite occurs with the invading cleavelandite, and with quartz, lepidolite, and local amblygonite, manganapatite, spodumene and petalite.

2. Graphic granite is a term used for a intimate intergrowth of perthitic microcline feldspar and quartz, crystallizing simultaneously. This characteristic texture occurs on a range of scales, and is described elsewhere in these pages, e.g., in the entry on the Havey pegmatite.

Acknowledgements - It gives me pleasure to acknowledge the talented coterie of pegmatite enthusiasts (museum staff, collectors, researchers and gemstone miners) who call Bethel home. Some of them assembled the wonderful "Pegmatology" volume that is a colourful record of their passion for these rocks and minerals. The Mount Mica deposit is owned and operated by Gary and Mary Freeman, Coromoto Minerals.

REFERENCES

Derry,DR (1931) The Genetic Relationships of Pegmatites, Aplites and Tin Veins. PhD Thesis, University of Toronto, 145pp.

Dyar,MD, Guidotti,CV, Core,DP, Wearn,KM, Wise,MA, Francis,CA, Johnson,K, Brady,JB, Robertson,JD and Cross,LR (1999) Stable isotope and crystal chemistry of tourmaline across pegmatite - country rock boundaries at Black Mountain and Mount Mica, southwestern Maine, U.S.A. Eur.J.Mineral. 11, 281-294.

Francis,CA (1985) Maine tourmaline. Mineralogical Record 16 no.5, 365-388.

Laurs,BM, Dilles,JH, Wairrach,Y, Kausar,AB and Snee,LW (1998) Geological setting and petrogenesis of symmetrically zoned, miarolitic granitic pegmatites at Stak Nala, Nanga Parbat-Haramosh massif, northern Pakistan. Can.Mineral. 36, 1-47.

Marchal,KL, Simmons,WB, Falster,AU, Webber,KL and Roda-Robles,E (2014) Geochemistry, mineralogy, and evolution of Li-Al micas and feldspars from the Mount Mica pegmatite, Maine, USA. Can.Mineral. 52, 221-233.

Menezes,L (2009) The Sapo mine, Ferruginha district, Conselheiro Pena, Minas Gerais, Brazil. Mineralogical Record 40, 273-292.

MMGM (2022) Maine Mineral & Gem Museum. MMGM, 99 Main Street, Bethel, ME, tour, 07 September.

Moore,TP (2024) Tucson show 2024. Mineralogical Record 55 no.3, 347-387.

Quensel,P (1952) The paragenesis of the Varutrask pegmatite. Geol.Mag. 89, 49-60.

Quensel,P and Gabrielson,O (1939) Minerals of the Varutrask Pegmatite 14: the tourmaline group. Geologiska Foreningens Forhandlingar 61, 63-90.

Rand,JR (1957) Maine pegmatite mines and prospects and associated minerals. Maine Geological Survey Mineral Resources Index 1, reprinted 1973, 43pp.

Robinson,GW, King,VT, Asselborn,E, Cureton,F, Tschernich,R and Sielecki,R (1992) What's new in minerals? Annual world summary of mineral discoveries, covering April 1991 through April 1992. Mineralogical Record 23 no.5, 423-437.

Simmons,WB (2007) Gem-bearing pegmatites. In `Geology of Gem Deposits' (Groat,LA editor), MAC Short Course 37, 270pp., 169-206.

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.

Simmons,W, Laurs,BM, Falster,AU, Koivula,JI and Webber,KL (2005) Mt. Mica: a renaissance in Maine's gem tourmaline production. Gems & Gemology 41, 150-163.

Wilson,WE (2008) The American mineral treasures exhibition, Tucson Gem & Mineral Show 2008. Mineralogical Record 39 no.3, 171-222.

Graham Wilson, 09-12 January 2025

For further information, see:

Rock of the Month Thematic Index
-- try "borosilicates" or "pegmatites" (?)

or, visit the Turnstone "Rock of the Month" Chronological Archives!

Maine pegmatite "Rocks of the Month": Classification of Sample Descriptions available on this Site

Class/Group/Family	Topics
The "Rock of the Month"
Pegmatites - graphic granite	--- #261 --- Graphic granite, Havey pegmatite, Maine, U.S.A.
Pegmatites - granite (tourmaline)	--- #283 --- Tourmaline and garnet, Mount Mica pegmatite, Maine, U.S.A.
Pegmatites - weathering - Mn	--- #275 --- Manganese oxide (pyrolusite) dendrites, Emmons pegmatite, Maine, U.S.A.
Pegmatites - granite (spodumene) - Li	--- #257 --- Spodumene, Plumbago North pegmatite, Maine, U.S.A.