For many years I taught courses in paleontology and
one of the basic types of fossilization students learned about was
“replacement.” It was easy to understand
as the class used petrified wood as an example.
Here the original wood is replaced by microcrystalline quartz (usually
chalcedony) or opal (or other minerals).
The gross structures of the wood such as “bark” and tree rings are often
preserved. Another common example was
looking at Cretaceous snails replaced by pyrite and/or marcasite. These large snails are common in the black
shales of the Kiowa Formation exposed in central Kansas and were an interesting
target in field trips. Today these replacement
examples are more commonly termed pseudomorphs---false forms.
In the last few years I have muddled around in the
world of minerals and have come to realize that there are a substantial number
of mineral pseudomorphs, especially substitution
pseudomorphs where there is a mineral to mineral replacement. Some are fairly easy to recognize such as the
goethite after pyrite crystals found on the shores of Utah Lake at Pelican
Point. Here the perfect cubes of pyrite
crystals are preserved except the original iron sulfide has been replaced by a
hydrated iron oxide. Others pseudomorphs
are much more difficult to recognize, at least for me: acanthite (silver
sulfide Monoclinic Crystal System) after argentite (silver sulfide Cubic
Mineral System). The cubic crystals of
argentite are present but the internal molecular structure changed as solution temperatures
decreased. The problem is that acanthite
“looks like” argentite” and it seems to take some nifty instrumentation to
recognize each. For other examples check
Blog postings April 16, 2013 cervantite after stibnite; September 2, 2012 calcite
after glauberite; and August 23, 2012 goethite after pyrite.
Pseudomorphs that are much more difficult to
understand are
paramorphs. Aragonite is a calcium carbonate [CaCo3] polymorph (Orthorhombic Crystal System) that is unstable in
the current earth’s atmosphere and alters to calcite over time. Calcite [CaCo3] is the stable calcium
carbonate polymorph and belongs to the Trigonal Crystal System. Most aragonite seems younger than about 350
Ma. However, there seems to be some evidence
that very old algal mats called stomatolites found in Precambrian (~2.7 Ga)
rocks of Australia may contain tiny aragonite crystals that have been protected
by microbes (Lepot and others, 2008).
A perimorph is a type of pseudomorph where a mineral
(#2) encrusts or encases mineral #1 and then mineral #1 dissolves and mineral
#2 forms a cast or shell indicating the original crystal form of #1. Quartz, since it crystallizes from solution
late in the mineralization process, is a common encrusting mineral (#2).
Fluorite and calcite are often minerals encrusted (#1) and seem easily removed
through dissolution. I cannot locate
perimorphs in my modest collection.
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Substitution pseudomorph: hematite after
siderite. Park County, Colorado. Width specimen ~2.2 cm.
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The reason behind this entire pseudomorph banner is
that Colorado has produced some unique pseudomorphs of hematite (iron oxide Fe2O3)
after siderite (iron carbonate FeCO3). Some of these pseudomorphs also are found with
quartz crystals and microcline. As best
that I can tell the specimen in my collection came from Park County and was
collected by the well-known mineralogist Clarence Coil. See Blog posting on March 5, 2014 describing
other Coil finds.
REFERENCES
CITED
Lepot, K., K. Benzerara, G. E. Brown, Jr, and P.
Philippot, 2008, Microbially Influenced
Formation of 2,724-million-year-old Stromatolites: Nature Geoscience, v. 1.