Melanophlogite

It’s not black

Melanophlogite, Fortullino, Rosignano Marittimo, Livorno Province, Tuscany, Italy. Botryoidal intergrown crystal masses with one cubic crystal, on serpentine. Specimen 16 mm wide.

If you know any Latin or Greek etymology, you might expect melanophlogite to be dark, even black. But it’s typically colorless, even water-clear, or white. It often looks like beads of water on a rock.  

Melanophlogite is a strange mineral. It’s essentially silica, SiO₂, but its structure is not that of quartz, tridymite, cristobalite, or opal, some of the most common forms of silica. Melanophlogite is a clathrate.  

Clathrates are compounds whose crystal arrangements form lattice works, cages, which can trap other molecules. Perhaps the most familiar are methane clathrates, “the ice that burns,” where natural gases (especially methane, CH₄) are bound within the crystal structure of ice to form a solid. They are well known and abundant, interbedded in deep-sea sediments, where pressures are high and temperatures are low. The word is from Latin clathratus meaning 'with bars, latticed.'

Apparently the incorporation of methane (and/or carbon dioxide and nitrogen) in this particular silica structure is regular enough that it produces an accepted mineral with the formula 46SiO₂ · 6(N₂,CO₂) · 2(CH₄,N₂). Minerals must have a definite, regular chemical composition, so you can’t just randomly stick anything into the silica cage.

Obviously this mineral has to occur in places where there’s methane or other gas to get into the crystal lattice. One such place (the type locality) is at a sulfur mine in Sicily where petroleum was present and presumably generated methane. Others include the source of my specimen, Fortullino in Livorno Province, central Italy. There, the melanophlogite is in serpentine breccia associated with magnesite, magnesium carbonate. That’s the likely source of carbon dioxide, which dominates the clathrate inclusions in melanophlogite from Fortullino, more so than methane, but I haven’t uncovered the whole geologic story.

Close up of the one cube on my specimen (lower right), 0.7 mm on an edge

Even though it often looks like drops of water or clear ice, melanophlogite is almost as hard as quartz, at about 6½ to 7 on the Mohs hardness scale. It does form crystals – you can see one in my specimen that looks like a cube, and it was originally described as belonging to the isometric (cubic) crystal system. But more recent work suggests that low-temperature melanophlogite is actually tetragonal in its crystallography rather than cubic; the length of the c axis is slightly shorter than the other axes, so these crystals are properly called pseudocubic, or they may be pseudomorphs of low-temperature tetragonal melanophlogite after high-temperature cubic melanophlogite. The nature of the transition from high- to low-temperature forms is not understood (D’Alessio, 2018, Synthesis, phase transitions, degassing behaviour of melanophlogite (Type I clathrate): Doctoral Dissertation, University of Parma, Italy; see also Tribaudino and others, 2008, Single-crystal X-ray and Raman investigation on melanophlogite from Varano Marchesi (Parma, Italy): American Mineralogist 93:88-94).

Arnold von Lasaulx, c. 1886

So what’s the story of the “melano-“ in the name, which means “black” in Greek? Melanophlogite was named in 1876 by von Lasaulx because the specimens he studied turned black when he heated them. The “-phlogite” part of the name is from a Greek word referring to burning (von Lasaulx, 1876, Mineralogisch-kristallographische Notizen, VII. Melanophlogit, ein neues Mineral: Neues Jahrbuch für Mineralogie, Geologie und Palaeontologie: 250-257). German mineralogist Arnold von Lasaulx (1839-1886) was one of the earlier workers in microscopic petrography, also remembered as an important student of earthquakes in Germany in the 1870s. He invented a “seismochronograph” that dropped a ball when an earthquake struck, stopping a clock and recording the time of the quake.

Until 2009 melanophlogite was the only known natural silica clathrate, but in that year and in 2014, two new clathrasils were described, chibaite and bosoite, both known only from one locality in Japan. They contain ethane, propane, and other natural gases in addition to methane. Understanding mineral structures like these may be of importance in sequestering greenhouse gases in synthetic materials.

Learning things like this is why I love having weird minerals, even tiny 16-mm specimens like this melanophlogite.