Dendrites

At multiple scales

Nature likes fractals. These two are at remarkably different scales – the NASA image at left of the Lena River delta is about 80 kilometers wide, while the manganese oxide dendrite at right is about 15 millimeters wide.

Russia’s Lena Delta debouches into the Arctic Ocean. The position of the Lena River, one of the great rivers of Siberia, is controlled by the eastern margin of the East Siberian Craton, and the river follows the persistent weak zones along that margin.

The dendrite above at right is on a white substrate which is the weathering rind on a chunk of black porcellanite or fireclay in the Cretaceous Coberly Formation in the northeastern Flint Creek Range, Montana. Porcellanite is a silicified mudstone, essentially chert, often formed by mineralization of volcanic ash in silica-rich mud. The rounded spots are lichens which seem to have a preference for the manganese oxides.

Dendrites are branching, sometimes fractal-like mineral deposits that usually form on a surface or in a narrow crack in rocks (of any type). Probably the most common are black growths of manganese oxides that may look fern-like, but they have nothing whatever to do with plants.

All the examples here are from Montana, with locations indicated on the individual photos (except for the image of the Lena Delta, Siberia, for comparison).

If you are an old geologist like me, you probably learned to call those black mineral sprays pyrolusite dendrites. Pyrolusite is a simple manganese oxide, but there are no known pyrolusite dendrites in the world. Analysis shows that they are less well-known manganese minerals, such as cryptomelane (potassium manganese oxide), manjiroite (sodium manganese oxide), birnessite (hydrous sodium-calcium-potassium manganese oxide), nsutite (manganese oxide-hydroxide), todorokite (hydrated sodium-calcium manganese-magnesium-aluminum oxide), romanèchite (hydrous barium manganese oxide), and vernadite (manganese-iron-calcium oxide-hydroxide hydrated).

Despite the unfamiliarity of those names, some are probably pretty common as dendrites and crusts in rocks. Manganese is the fifth most abundant metal in the earth’s crust (after aluminum, iron, magnesium, and titanium) and twelfth overall, far more abundant than sulfur, carbon, copper, zinc, or lead.

On Precambrian Belt supergroup mudstone, deposited 1.4 billion years ago.

Dendrites form because of a low concentration of the chemicals to make the minerals. If there were more, a solid crust would form. And they branch rather than make a smooth growth front because that low concentration of chemicals deposits preferentially on particular crystallographic positions – some faces tend to grow faster than others. Disequilibrium on the crystallization front can be a factor as well, and the temperature of the fluid, irregularities in the substrate, and other factors may guide the details of dendrite growth.

The overall growth of dendrites is generally fast, a reflection of the low concentration of chemicals and their affinity for the preferred crystallographic positions. “Fast” may mean weeks to years rather than the usual geological time scales of millions of years.

Although manganese dendrites deposited on surfaces are probably most common, iron oxide and hydroxide minerals (especially goethite, iron oxide-hydroxide) do it too, both on surfaces and by diffusion into gels or colloids (often of silica) to produce mossy-looking features in agates, such as the famous Montana agates. Ice is another Montana mineral that often forms dendrites such as those you might see on the windshield of your car.

Field of view about 2 cm

There’s no official cut-off, but for me I’d use the word dendrite for the generally flat branching minerals you see on rock surfaces, and the word arborescent for thicker, stouter crystal growths such as sometimes develop in native metals like silver, copper, and gold.

The word dendrite is from Greek ‘dendron,’ a tree, and it means ‘tree-like.’ ‘Arborescent’ means essentially the same thing, but by way of Latin ‘arboresco,’ meaning ‘to become a tree.’

Although Butte with almost four billion pounds was probably the primary producer of manganese in the United States historically, the US has not mined manganese since 1970, and the US is 100% dependent on imports for manganese ore, mostly from Gabon (62%) and South Africa (24%). Manganese is used mostly as a critical element in steel alloys, where it increases strength, decreases brittleness, and helps remove oxygen and sulfur from iron ore. Some manganese is present in almost all steel, but high-manganese steel (up to 13% manganese) is used for railroad tracks, rifle barrels, prison bars, and other uses where strength and wear resistance are important. It is also used in dry-cell batteries, animal feed and fertilizer, and ceramics.