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

Manganese oxides

Pyrolusite, tetragonal Mn^IVO₂), 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 possible mode of 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 in the rich pegmatite fields of New England (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).

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