COLORADO PSEUDOMORPH: HEMATITE AFTER SIDERITE

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  Here the perfect cubes of pyrite crystals are preserved except the original iron sulfide has been replaced by a hydrated iron oxide.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.

 

Substitution pseudomorph: hematite after siderite.  Park County, Colorado.  Width specimen ~2.2 cm.

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.