Pyrolusite dendrites from the Emmons quarry

Oxford county pegmatite field, western Maine, U.S.A.

Mn dendrites [318 kb]

Fig. 1: Pyrolusite dendrites growing across a feldspar-dominant (leucogranitic aplite-pegmatite) protolith. Distinctive fern-like two-dimensional shapes on the white quartzofeldspathic host rock represent a weathering phenomenon, in which manganese from one or more nearby minerals has leached out and redeposited as a black manganese oxide (analogous to the orangey ferric iron oxide species such as goethite / limonite). One wonders if a difference in solubility at normal earthly temperatures of 0-50°C (where H2O occurs in liquid form!) can account for the different modes of occurrence of Mn and Fe oxides. A quick glance at published solubilities of Fe and Mn hydroxides suggests not. Whereas Mn oxide may form dendrites or occur as thin films, Fe oxide ("rust") occurs in the latter form, as a uniform sheet-like wash of colour on the host rocks.

Although granitic pegmatites are famed for a variety of coarse-grained silicates, some of them potential gemstones, they may have a diverse mineralogy, with possible sources of the Mn in the dendrite films including carbonates (e.g., rhodochrosite), oxides and assorted species of Fe-Mn-(etc) phosphates (such as triphylite-lithiophilite, graftonite-beusite, triplite-zwieselite, sarcopside, sicklerite and more).

"Rock of the Month # 275, posted for May 2024" ---

Manganese oxides

Pyrolusite, tetragonal MnIVO2), is perhaps the most common of the many black Mn oxides, usually seen as coatings on rocks. Pyrolusite dendrites feature in one earlier Rock of the Month, from a very different climate than that of moist, cool-temperature maritime Maine, in the Atacama desert of northern Chile. The subaerial formation of these dendrites is touched upon in that essay. We may note, in passing, that Fe-Mn oxide coats can also be deposited in submarine settings, as in the formation of deep-sea ferromanganese nodules (Pattan et al., 2016).

The host pegmatite

The Emmons pegmatite quarry has exploited a complex zoned pegmatite. The paragenesis (sequence of minerals) is complex, with two generations of muscovite and two of rhodochrosite. Common minerals include quartz and microcline K-feldspar, schorl tourmaline, muscovite and red garnet. Sprays of black schorl radiate downward from the roof toward the core. The apices of unusual ball-shaped masses of muscovite also point toward the pegmatite core, from both above and below. The Emmons is featured in a display at the Maine Mineral & Gem Museum in nearby Bethel, where the common minerals are seen, together with many of the rarer, in some cases more gemmy constituents of the pegmatite, such as elbaite tourmaline, fluorapatite, beryl and lepidolite mica. Emmons displays the most mineral species of any pegmatite researched to date (Falster et al., 2019). Pegmatite mineralogy, including examples from Maine, is featured in a detailed and colourful review by Simmons et al. (2022).

Maine is endowed with an abundance of pegmatites (Maine Geological Survey, 1957), including famous mineral localities such as Dunton, Mount Apatite, Mount Mica, the Perham mine, and others. Work in the past 30 years suggests that the Maine pegmatites form not by igneous fractionation processes so much as by anatexis (partial remelting of rocks in a hot metamorphic terrain at depth: see, e.g., Simmons et al., 1995, 2022).

Mn dendrites [320 kb]

Fig. 2: Another close-up view of pyrolusite dendrites.


Falster,AU, Simmons,W, Webber,KL, Dallaire,DA, Nizamoff,JW and Sprague,RA (2019) The Emmons pegmatite, Greenwood, Oxford county, Maine. Rocks & Minerals 94 no.6, 498-519.

Maine Geological Survey (1957) Maine Pegmatite Mines and Prospects and Associated Minerals. Maine Geol.Surv. Mineral Resources Index No.1, 43pp.

Pattan,JN, Parthiban,G, Moraes,C, Rajalakshmi,R, Lekshmi,S, Athira,S and Jai Sankar,S (2016) A note on chemical composition and origin of ferromanganese oxide coated and uncoated pumice samples from the central Indian Ocean basin. J.Geol.Soc.India 87, 62-68.

Simmons,WB, Foord,EE, Falster,AU and King,VT (1995) Evidence for an anatectic origin of granitic pegmatites, western Maine, USA. GSA Abs.w.Progs. 27 no.6, 411, New Orleans.

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.

Graham Wilson, DRAFT, 19-22,25 February 2024

For further information, see:

Rock of the Month Thematic Index

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