Rhodochrosite with Pyrite and Agate (#62)

Polished slice showing the banding.
South America Catamarca Province, Argentina 6" x 2 ¼" x ¾"


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Before we get to rhodochrosite, let's let agate take center stage. The huge centerpiece of this display is quartz along with many of the numbered specimens. None of them look like the brown, banded agate in this specimen, but they are all SiO2. Agate is one of the many forms of chalcedony, which is cryptocrystalline quartz. Crystalline means that the atoms are arranged in regular three-dimensionally-repeating arrangements (crystal structures), rather than randomly or semi-randomly arranged. The prefix "crypto" means hidden - th ink "cryptographers," people who try to determine "hidden meanings" in messages. So cryptocrystalline minerals are those whose crystalline character is masked by extremely small crystal size - so small that crystals are not individually visible in an ordinary light microscope. These exceedingly tiny crystals form fibers. If the fibers of chalcedony are arranged perpendicular to the banding, then the mineral is called agate. Otherwise, depending on other properties, it may be called jasper, chert, or other names. Agate is used in jewelry and as ornamental stone. The banding may be caused by alternating fiber orientation, crystallite-size variation, or other factors, but it is just a type of quartz.

Rhodochrosite, MnCO3, is a carbonate mineral associated with hydrothermal (hot water) sulfide ore deposits. Banding is common in rhodochrosite, as in the agate that rims it in this specimen. Note that many of the banded structures seem to originate as sub-spherical "marbles" that grow concentrically and finally merge. The bands vary in color, and that may be due to variable crystal size, crystal orientation, chemical impurities, crystallization conditions, or other influences.