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Pseudomorphs

Pseudomorphs are very interesting specimens, each by itself telling an interesting story with one mineral appearing in the habit of another. The specimens are always described as 'A pseudomorph after B,' meaning that the specimen contains the mineral A, appearing in the habit of mineral B, and 'pseudomorph' indeed means 'false form'.
Previously, before x-ray diffraction became the standard method of identifying minerals, pseudomorphs received considerable attention. It was obvious, some well- known minerals suddenly appeared in the shape of another - for example Copper in the habit of Aragonite or Azurite. Something was obviously wrong, and it received a great deal of attention simply because chemical analysis and measurement of angles between crystal faces were the established methods of identification. Consequently, a number of pseudomorphs of something fine-grained after something with sharp crystal faces got their own name - Achtaragdite is an example. Achtaragdite is a very fine-grained mixture of Grossular, clay minerals, and something else pseudomorph after what were probably very sharp Helvite crystals. Because it was fine-grained, it was not possible to distinguish individual components, and though the chemistry was complex and varied (as in amphiboles, pyroxenes, and other well-known minerals), the habit was distinct and it was described as a species.
Every now and then the old fashioned mineralogists had the upper hand on us. Many mineral collectors believe they have pretty crystals of Argentite. They don't. Sure, Argentite was described as a proper mineral by Haidinger in 1845, cubic silver sulfide, and indeed in 1855 Kennegot described Acanthite as monoclinic silver sulfide. The two minerals are dimorph, have the same chemical composition but different crystal structure. Later we have learned, that cubic silver sulfide is only stable at temperatures above 177 °C, and if the temperature falls below that it spontaneously recrystallises into the monoclinic habit. At the molecular level this is not a tremendous change, but it means that silver sulfide deposited at a higher temperature will appear in the habit of die shaped, cubic crystals, but the internal organisation (that you examine by x-ray diffraction) is monoclinic. That is, the 'Argentite crystals' are not Argentite but Acanthite (internal structure) in the shape of Argentite (external morphology). Sorry to disappoint you, but this is not Acanthite pseudomorph after Argentite. Because they have the same chemical composition, the specimens are called Acanthite paramorph after Argentite.
Paramorphs are not that common. Acanthite paramorph after Argentite is the classic, Microcline paramorph after Orthoclase the most common (a considerable number of Orthoclase specimens are probably Microcline paramorph after Orthoclase), Pyrite after Marcasite (rather than Marcasite) and Sulfur after Rosickyite (rather Rosickyite) are probably the rule, a couple of fun things may happen in the Quartz group, but by then we have pretty much covered most possible paramorphs. Paramorphs form by an internal reorganisation of molecules, because a different phase is stable at a lower temperature, but the reorganisation does not affect external morphology.
Pseudomorphs can form in several different ways. The conceptually simplest is basically an internal cast: Clay, marl, gypsum or some other firm but porous matrix envelops the original mineral, that is dissolved to leave a void, that fills with something else. By that approach almost everything is possible and you get pseudomorphs like Copper after Aragonite, where there is no chemical similarity between the minerals. Virtually all pseudomorphs show a chemical relationship between the original and the final mineral. Bismutite pseudomorph after Bismuthinite is a bismuth oxy-carbonate after a bismuth sulfide, the bismuth stays and the Bismutite forms by oxidation of the Bismuthinite - that is not conceptually too difficult. Similarly Malachite pseudomorph after Azurite is one copper hydroxy-carbonate after another, chemically very similar, and probably the result of a slight change in chemical conditions in the local environment.
Every now and then some people get carried away and overextend the concept of pseudomorph. Particularly 'Quartz pseudomorph after Calcite' and 'Pyrite pseudomorph after Calcite' seem popular. The specimens I refer to are really external casts of Quartz or Pyrite, that was deposited as a thin layer on top of Calcite crystals, that were subsequently dissolved by Nature or by acid. Such specimens are obviously not pseudomorphs, or anything sitting on top of anything else would be a pseudomorph, making the term superfluous. If such specimens are indeed natural and not produced by etching away the Calcite, they should be called 'cast' or even better 'external cast' (personally I find the term 'perimorph' cute, albeit a tad posh). They are unusual, but not desperately rare - Tsumeb (Namibia) has produced a number of Dolomite cast after Calcite, some of which carry tufts of Malachite, guaranteed no acid on those!
Our own records identify the following 113 pseudomorphs, some of which occur at several localities, and many others are known. Note, all these are proper pseudomorphs, not paramorphs or casts.

Specimen Handling

Pseudomorphs are mostly stable in a normal household environment, depending on the mineral replacing the original. Pseudomorph specimens can be fragile and should be handled with care like any other mineral specimen.

Bibliography

Most general references discuss pseudomorfs at least briefly. These are some of the few special references:
Blum, J.R. 1843. Die Pseudomorphosen des Mineralreiches. Stuttgart.
Blum, J.R. 1847. Die Pseudomorphosen des Mineralreiches, Nachtrag 1. Stuttgart.
Blum, J.R. 1852. Die Pseudomorphosen des Mineralreiches, Nachtrag 2. Heidelberg.
Blum, J.R. 1863. Die Pseudomorphosen des Mineralreiches, Nachtrag 3. Erlangen.
Blum, J.R. 1879. Die Pseudomorphosen des Mineralreiches, Nachtrag 4. Heidelberg.
Landgrebe, Georg. 1841. Ueber die Pseudomorphosen im Mineralreiche und verwandte Erscheinungen.

This page is authored by Claus Hedegaard.