Pyrolusite - the culprit of apparent plant fossils

Sometimes minerals appear in forms that deceptively resemble fossils of mosses and ferns. Unlike true fossils, these are so-called dendrites, or clusters of tiny crystals formed by crystallization of minerals dissolved in solutions penetrating tiny cracks in rocks. Such pseudofossils are usually formed by oxides or hydroxides of manganese and iron, and pyrolusite is among them.

Powdered mineral as a pigment for painting on rocks was already used by cave artists thousands of years ago. Manganese black dye was readily used in oil painting because it dried best of all black pigments due to its manganese content.

The name pyrolusite was given to the mineral in 1828 by Austrian mineralogist Wilhelm Karl Ritter von Haidinger (1795-1871). In a study titled "Mineralogical Description of Manganese Ore," in the chapter "Prismatic Manganese Ore, Pyrolusite," the scientist used a compound of two Greek words pyr = "fire" and luo = "wash." The name refers to the mineral's past use in glassmaking to remove harmful tints. The iron oxides contained in quartz for glassmaking, which changed the color of the product, were removed at high temperatures using pyrolusite. As Haidinger wrote, "the green and brown color of the glass, formed by carbonaceous substances and iron oxide, was got rid of."

Properties of pyrolusite

Pyrolusite is a mineral of the oxide cluster and consists of manganese dioxide (_MnO2). The color of the mineral varies from gray to black, often with a metallic, silvery sheen. It leaves a blue-black mark on the porcelain plate (scratch). It crystallizes in a tetragonal system, forming prismatic-needle crystals. Pyrolusite concentrations, on the other hand, are sometimes earthy, powdery, clustered, fibrous, sometimes massive.

The mineral has an uneven to shell-like break and excellent flakiness. It is opaque, the luster can be dull to metallic. The hardness of pyrolusite is also highly variable: while massive clusters show a Mohs hardness of only 2 to 4, crystals range from 6 to 6.5 on a 10-point scale. Density ranges from 4.5 to 7.9 g/cm³.

Formation and occurrence of pyrolusite

The mineral is mostly formed in oxidizing, manganese-containing hydrothermal environments. It can also form from manganite or precipitate as a chemical precipitate in water bodies and swamps. Dendritic formations are also encountered in limestone and sandstone rocks. Several forms of pyrolusite origin can be mentioned:

sedimentary - in lakes, marshes, lagoons, under conditions of high oxygen content and low water circulation;
hydrothermal - precipitates under oxidizing conditions in hydrothermal solutions containing manganese;
secondary - due to oxidation of manganese minerals;
synthetic - electrolytic manganese dioxide.

Significant deposits of pyrolusite are located in Georgia, Ukraine, India, Brazil, Canada, the USA and South Africa, among others.

Pyrolusite applications

The use of pyrolusite is due to its chemical composition and properties:

The mineral is often used as an ore to extract metallic manganese.
Manganese oxide is used to make dry batteries.
In glassmaking, it is used to decolorize glass containing iron or to dye it purple.
Also today it is used as a black pigment in oil painting.
Because of its good resistance to alkaline substances, manganese oxide is suitable for coloring cement.
In metallurgy, it is used to refine steel and harden aluminum and magnesium alloys.
Pyrolusite crystals are sought after by collectors, and dendrites formed with the mineral are used to make ornaments.

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Pyrolusite - the culprit of apparent plant fossils

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