Dendrites--crystalline, black, fern-like patterns that resemble forest scenes--have captured the fancy of collectors for centuries. Dendrites, normally deposited on fracture surfaces in rocks, are formed from manganese that has been leached from surrounding rocks and soil by water.
For many years, scientists presumed that dendrites were composed of pyrolusite, a manganese oxide common in ore deposits. However, this mineral identification could not be confirmed because dendrites are formed from crystals so small that they cannot be characterized by X-ray diffraction, the standard diagnostic tool for analyzing minerals.
Recently, California Institute of Technology geochemists George R. Rossman and Russell M. Potter applied infrared spectroscopy, an analytical technique that illuminates mineral samples with infrared radiation, to identify the mineralogy of dendrites. Because specific minerals absorb specific patterns of infrared wavelengths, infrared spectroscopy has become a valuable tool for analysis of very fine-grained minerals.
Results of infrared spectroscopy analyses demonstrate that dendrites are formed by any one of several manganese oxides -- none of them pyrolusite. Manganese oxides are differentiated on the basis of the internal arrangement of their atoms and the content of some minor elements. For example, the manganese oxide romanechite forms dendrites in pegmatites of the Black Hills region of South Dakota; hollandite dendrites are from Afton Canyon, California; todorokite is found in the gem mines of Pala, California; and cryptomelane is from the southwestern United States. Each dendrite is formed from a specific manganese oxide. No mixing of manganese oxides within the dendrite was observed in the samples tested.
Infrared spectroscopy has also been applied to the analysis of
desert varnish [see Origin of Desert
Varnish]. It was found that desert varnish is about 70% fine
clay and 30% manganese and iron oxides. The manganese mineral
in desert varnish is the oxide birnessite. In comparison, dendrites
contain virtually no clay, and none of those analyzed thus far has
The preceding article was published in the May 1993 issue of Lithosphere, the official bulletin of the Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch (Editor).
The material is in the public domain, and may be republished freely.
Last updated: 18 September 2002