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Home>Learn and Explore!>Rocks, Minerals and Prospecting>Pseudomorphs by Paul Hlava

Pseudomorphs by Paul Hlava

(from Trivial Pursuits, Albuquerque Gem and Mineral Club)

The name pseudomorph obviously means "false form" and it is applied to material that retains the external form of a previous crystal. The original material usually changes chemically, but sometimes it can change internal structure or merely be coated by later material. In fact, there are four ways in which pseudomorphs are formed, and they are described below.

l) Alteration occurs when some new material is added to the old, or when some of the old material is removed, or both Ė as long as some original material is retained but some of the chemistry is changed. Last month I mentioned the very common example of pyrite cubes (in dinosaur bone in that cave) being altered to limonite. The correct description of this would be "Pseudomorphs of limonite after pyrite."

Limonite after Pyrite

Limonite after Pyrite

This is probably the most common type of pseudomorphism and I can think of dozens of examples of this type Ė such as copper after azurite, malachite after azurite, malachite after linarite, both limonite or goethite after pyrite, calcite after glauberite, malachite after cuprite, and even turquoise after apatite. One of the most memorable samples was a sample of coarse, garnet schist from New England where each of the one-inch diameter garnets had been perfectly mimicked by chlorite. One commonly finds remnants of the original material in the cores of such pseudomorphs.

2) Substitution occurs when all of the formed material is replaced by new elements. This probably involves the gradual removal of the old with simultaneous addition of the new, with no chemical reaction between them. This often happens to fossil material, as is the case with some silicified, opalized, or agatized wood. (In a lot of petrified wood, part of the organic cell walls still remain.) But Iíve also seen opal after clam shells, Glauconite and pyrite after coprolites, chalcocite after wood, and sphalerite after crinoid stems and brachiopod shells, as well as silica after barite and fluorite. In the chert on top of the Sandias, you can find silicified crinoid stems. The most elegant example occurs in the Glass Mountains of Texas where brachiopod shells, in a limestone, have been silicified. If you take large blocks of the limestone and dissolve them in acid, you can recover small brachiopods with thin, fragile spines four or more inches long.

3) Incrustation occurs when one mineral coats another. This occurs to some extent at the Blanchard Mines when quartz coats both barite and fluorite. One of my favorites in this category is the chalcocite coated pyrite from such places as the Santa Rita pit in New Mexico. The pyritohedra there are perfectly formed and steel gray.

4) Paramorphism can only occur when a mineral changes its internal structure to that of a polymorph. The aragonite crystals from Santa Rosa that are not calcite constitute a prime example. Others include marcasite going to pyrite, argentite to acanthite, hawleyite to greenochite, olivine to ringwoodite, and wurtzite to sphalerite. One that I havenít seen yet is graphite going to diamond. You know, Superman used to do that.

Republished by permission of Albuquerque Gem and Mineral Club
Paul Hlava, author