New Mexico Mineral Symposium — Abstracts

The word pseudomorph itself means 'false form." The Manual of Mineralogy by Hurlbut and Klein defines them thus: The existence of a mineral with the outward crystal form of another mineral species is known as pseudomorphism. If a crystal of a mineral is altered so that the internal structure or chemical composition is changed but the external form is preserved, it is called a pseudomorph, or false form. They are often downright ugly!

Pseudomorphs are found in most mineral collections and are probably the least understood of any class of minerals. Though specific occurrences have been studied quite extensively, little scientific information of them as a class is available. Yet the formation of pseudomorphs implies the original mineral becomes unstable under changed physical or chemical conditions. They can often provide valuable evidence in unblocking the geological history of the rocks containing them. They may indicate the nature and composition of circulation fluids. If stability fields of the original and replacing mineral are known, it may be possible to estimate temperature and pressure at the time of alteration.

The authors of this paper are collectors, and though in no sense professional authorities on any mineralogy subject, have tried to classify pseudomorphs into various groups according to their chemical constituents. The authors have drawn upon three well-known textbooks for collectors in developing this classification, using photos of their own specimens in the digital presentation. Those books are Mineralogy: Concepts, Descriptions, Determinations, Berry and Mason, 1959; Dana's Manual of Mineralogy, Hurlbut and Klein, 19th edition, 1977; and Textbook of Mineralogy by Dana, 4th edition.

The classification in general follows Berry and Mason's scheme, but with the addition of one type listed in the Manual of Mineralogy. There are two principal types, namely paramorphs in which no changes in chemistry occur, and the other in which there is addition of some element or elements and/or removal of others.

The second type is further subdivided as follows:
1. Loss of a constituent such as copper (Cu) after cuprite (Cu₂O).
2. Gain of a constituent or constituents such as malachite (Cu₂(CO₃)(OH)₂ after cuprite (Cu₂O).
3. Partial change of constituents such as goethite (Fe⁺³O(OH)) after pyrite (FeS₂).
4. Complete change of constituents such as quartz (SiO₂) after fluorite (CaF₂).
5. Encrustation pseudomorphs.

Berry and Mason do not list encrustation as a pseudomorph, but Hurlbut and Klein do. We have included the encrustation type because most collectors who exhibit in the Mineral Federation shows follow the display rules that allow encrustation as pseudomorphs.

A quick look at the chemical formula of both original and replacement minerals places a specimen in one of the above categories. In the case of polymorphs where there is no change in chemistry, there is a change in crystal structure. Many of the pseudomorphs in the first three subdivisions also change crystal system.

Since it is possible for remnants of the original mineral to remain in the pseudomorph, it follows that just as a change in environment causes the pseudomorphing to begin, a further change may stop the process. These changes in environment may be of several types, including changes in pressure and/or temperature, changes in availability of elements for growth or replacement, and ground water acidity changes. Oxidation chemistry plays a large part in forming many pseudomorphs. Particularly when there is a complete change of constituents, acidity is often the culprit responsible for removing the original mineral.

For the authors placing their specimens in specific categories helps them to learn and understand the mineralogical forces that caused the pseudomorphism. We hope it may also help others.

Acknowledgments
Dr. Peter Modreski has been most helpful in answering many questions about the mineralogy of pseudomorphs and searching for helpful articles about pseudomorphs. Thanks are also due him for his insightful suggestions in preparing this presentation.

References:

1. Berry, L. G., and Mason, B., 1959, Mineralogy: concepts, descriptions, determinations: W. H. Freeman and Company, San Francisco, pp. 179-180.
2. Dana, E. S., and Ford, W. E., 1932, A textbook of mineralogy, 4th edition: John Wiley and Sons, New York, pp. 205,357-358.
3. Hurlbut, C., and Klein, C., 1977, Manual of Mineralogy, 19th edition: John Wiley and Sons, New York, pp. 154-155.