New Mexico Mineral Symposium — Abstracts

The term pseudomorph, literally meaning false shape, refers to a mineral that exhibits the external form of a different mineral species. There is some disagreement about what actually constitutes pseudomorphism in minerals, so for the sake of simplicity, the definitions of Edward S. Dana will be followed in this presentation. Dana (1907) described three classes of pseudomorphism with six subdivisions:

1) pseudomorphism by substitution
2) pseudomorphism by deposition

• A) pseudomorphism by incrustation
• B) pseudomorphism by infiltration

3) pseudomorphism by alteration

• A) paramorphism
• B) with loss of ingredients
• C) with gain of ingredients
• D) with exchange of ingredients

Pseudomorphism by substitution involves the gradual replacement of original material by an unrelated mineral without chemical reaction between the two. Petrified wood is a common example of this class of pseudomorph. New Mexico examples include opal or the agate variety of quartz after wood from various localities, chalcocite after wood from Sandoval County, and carnotite after wood from the Grants district, Cibola and Valencia Counties.

Pseudomorphs by incrustation form when one mineral coats another with or without the removal of that other. Quartz very commonly forms crusts on, or casts of, other mineral species and New Mexico examples include pseudomorphs of fluorite, barite, and galena from the Hansonburg district, Socorro County, and quartz and fluorite after calcite from the Cuchillo Negro district, Sierra County. Other examples include pseudomorphs of limonite¬aurichalcite-calcite after calcite from the Magdalena district, Socorro County; turgite coatings on quartz from the Lordsburg district, Hidalgo County; and pyrite after sphalerite from the Hansonburg district, Socorro County.

Pseudomorphs by infiltration form when a cavity made by the removal of a mineral is filled by another mineral. This type of pseudomorphism is more important in paleontology than in mineralogy, and examples such as quartz or pyrite pseudomorphs of brachiopods, pelecypods, trilobites, etc., abound in New Mexico.

Paramorphism results when a mineral undergoes a change in internal structure without a change in chemical composition. Perhaps the best known example is the phase inversion of argentite (cubic) to acanthite (monoclinic) at temperatures below 177°C. New Mexico examples include calcite (hexagonal) after aragonite (ortho-rhombic) from numerous localities and pyrolusite (tetragonal) after ramsdellite (orthorhombic) from Lake Valley, Sierra County.

New Mexico examples of pseudomorphism by alteration with loss of ingredients include copper (Cu) after cuprite (Cu₂O) from Santa Rita, Grant County and malachite (Cu₂(CO₃)(OH)₂) after azurite (Cu₃(CO₃)₂(OH)₂) from the Magdalena and Hansonburg districts in Socorro County, the Lordsburg district in Hidalgo County, and the Organ district in Dona Ana County.

A New Mexico example of pseudomorphism by alteration with addition of ingredients is cuprite (Cu₂O) after copper (Cu) from the Santa Rita district in Grant County.

Pseudomorphism by alteration with exchange of ingredients is probably the most important type of pseudomorphism because most specimens commonly observed belong to this category. New Mexico examples abound and include rose muscovite after microcline from the Harding mine, Taos County; cerussite and anglesite after galena from many localities; malachite and/or limonite after chalcopyrite from the Orogrande district, Otero County and from the Hansonburg district, Socorro County; linarite, brochantite, cerussite, anglesite, and/or wulfenite after galena from the Hansonburg district; malachite and cerussite after linarite, also from the Hansonburg district; rosasite and/or malachite after cuprite from the Magdalena district, Socorro County; ramsdellite after groutite from Lake Valley, Sierra County; hematite and/or chalcocite after pyrite from Santa Rita, Grant County; and limonite after pyrite from numerous localities.

Often, pseudomorphs exhibit characteristics of more than one of these classes presented. Chrysocolla may pseudomorph any copper-bearing species by alteration and incrust adjacent minerals as well. Specimens of groutite from Lake Valley, Sierra County, are pseudomorphed first to ramsdellite by alteration and then to pyrolusite by paramorphism. Finally, azurite from the Rose mine, Grant County, has been pseudomorphed by a combination of copper and calcite. The copper forms microscopic arborescent crystal groupings that are partly to completely altered to cuprite. The cuprite is often further altered to malachite. They are thus copper, cuprite, and calcite, cuprite and calcite, cuprite and malachite, or malachite pseudomorphs of azurite. Classification is further complicated because the pseudomorphing materials filled azurite molds in the surrounding clays giving them aspects of both alteration and infiltration pseudomorphs.

References:

1. Dana, E. S., 1907, A text-book of mineralogy with an expanded treatise on crystallography and physical mineralogy: John Wiley & Sons, New York, pp. 144, 252-253.