POPULAR PSEUDOMORPH MINERALS (Part I)
More Than What They Seem To Be
The term pseudomorph, abbreviated ps or pseudo, means “false form.” This two-part article will describe a wide variety of minerals and fossils that have changed from their original form to something else and how that happens. Every good collection should have examples of common pseudomorphs.
The keyword, when discussing “pseudos,” is change. Minerals crystallize and exhibit a particular shape or crystal form. Such things as solution Ph, temperature, pressure, the richness of the solution, even location all play a role in crystallization. Some minerals, once formed, are unstable, so changes also occur as conditions change.
One typical example of a pseudomorph is iron rusting to iron oxide. The original shape remains but not the properties. Iron sulfide pyrite can also change, altering to hydrous iron oxide goethite. When pyrite changes to goethite, it retains the original crystal form of a cube or octahedron but is no longer brassy pyrite but dull dark brown goethite. Goethite after pyrite is a common form of a pseudomorph process called replacement.
UNDERSTANDING REPLACEMENTS
Replacements are substances in which the original entity is slowly replaced moleculeby-molecule with a new substance while preserving the original form. Other forms of pseudomorphs include casts, paramorphs, and epimorphs. Plus, pseudomorph changes can happen in fossils as well as minerals.
A common example of a replacement pseudomorph in fossils found all over the world is petrified wood. This replacement pseudomorph forms when silica-rich water slowly invades and replaces wood that is quickly buried to avoid rotting. The wood is best preserved or petrified when buried in silica-rich volcanic ash. Arizona has the world’s finest example of this process in Petrified Forest National Park. The huge colorful stone logs present in this park are replacement pseudos as their wood cells are perfectly preserved in their original structure. These solutions carry trace amounts of iron oxide, manganese oxide, and even colorful uranium compounds, which create Arizona’s lovely gem-quality petrified wood.
The specific petrified wood deposit in Arizona formed eons ago when giant trees were felled in some distant place then carried and deposited by fast-flowing waters losing their limbs in the process. Once in place, they were quickly buried by nearby volcanism and the silica-rich volcanic ash, given time, slowly replaced the wood cells with silica minerals. Now Arizona’s petrified wood is the state’s official gem.
Fossilized wood is not the only replacement type of fossil. We find perfectly preserved stone or pyrite ammonites and shells in many limestone deposits, perfect images of the original hard parts. Dinosaur bones we unearth, reassemble and display are replacement fossils. The most unusual dinosaur bone I’ve seen is
a massive leg bone, which had been cut in half to reveal the original soft tissue replaced by lovely banded agate.
IDENTIFYING CASTS FROM PARAMORPHS
Another widespread type of pseudomorph is casts. These are precisely what the name implies, perfect hollow replicas of a mineral crystal. The casting material is often quartz that forms on a previously formed crystal, often calcite or barite. The original crystal is dissolved away, leaving the perfect quartz cast. These are quite brittle, usually snow-white in color. If only partial casts form, they reveal their hollow interior. Technically these are deposition pseudomorphs that form when a mineral, like quartz or calcite, is deposited atop an existing crystal creating a cast of the original. In some cases, when it coats the original mineral, the second mineral is perfectly aligned internally with the axes of the original crystal. Scientists call this epitaxial growth often seen in sulfides.
The third type of pseudomorph that fools us is a paramorph sometimes called an epimorph. There is no doubt what the original mineral was and what it becomes with cast and replacement pseudos. Paramorphs, on the other hand, look unchanged but internally have gone through a complete structural and chemical change.
When a mineral forms, it does so in a given environment of pressure, temperature, and other factors. Some minerals are stable only within a specific range of conditions, especially temperature. As their environment changes, they are unstable enough for the molecular structure to shift as energy is lost. The original crystal form of the mineral may change to a different crystal structure. This change happens internally and is not visible. The external form we see remains the same, so change is not evident. Only testing with x-ray or other means will reveal the modification. The atoms or molecules have rearranged to a new mineral, sometimes
even changing the crystal system. Such minerals were given the name paramorph meaning “other form.”
A classic example of a paramorph is the silver sulfosalt mineral acanthite. During formation from hydrothermal solutions whose temperature is above 173 degrees Celsius, the silver sulfosalt mineral that forms is not acanthite but argentite with a crystal lattice structure that is isometric or cubic. At and above 173 degrees Celsius, argentite remains stable. But once formed and the deposit environment tends to cool, the argentite crystals will slowly begin to change, and the argentite’s internal structure, being unstable, causes the molecules to start moving around to achieve stability. They settle into a monoclinic form, and the mineral is no longer isometric but the monoclinic silver sulfosalt acanthite, which is stable. The original cubic form of argentite remains externally and is what you see, though internally, the mineral now has a monoclinic crystal lattice.
If you have a specimen in your collection labeled argentite, note that it started that way; but internally, it is now acanthite unless your mineral cabinet temperature is 173 degrees Celsius.
In the iron sulfide family, pyrite can form a paramorph after another iron sulfide pyrrhotite. These two brassy minerals are about the same except for changes in their electron arrangement and crystal structure. Pyrite forms in the isometric or cubic system, and a molecule consists of one atom of iron and two of sulfur. Pyrrhotite can have a varied ion and sulfur composition, with the most significant difference in their electrons. Oddly, pyrrhotite can be either isometric or hexagonal. Some of the finest pyrrhotites are superb hexagonal plates stacked like poker chips.
Be that as it may, pyrrhotite is slightly unstable, and pyrite is not, so it is not unusual for pyrrhotite to form in hexagonal plates but can be partially or
entirely replaced by the paramorph pyrite. During one trip to Mexico with Bill Pnczner, we visited a miner in Santa Eulalia, and he showed me a flat of wonderful pyrite after pyrrhotite. The original crystal form, hexagonal, was still evident in part, revealing pyrite’s presence as well.
Additionally, many of us are very familiar with the carbonates aragonite and calcite. Chemically they are both calcium carbonate, but they are paramorphs—aragonite forms in orthorhombic crystals and calcite forms in hexagonal or trigon crystals. But, aragonite forms twins that are six sides, which we call pseudohexagonal twins. Calcite and aragonite are easily confused, one with the other when orthorhombic aragonite forms six-sided twins. Luckily, six-sided aragonite twins are very easily recognized as twins because of flat terminations, which exhibit the twinning crystal boundaries on the termination distinguishing the six crystals from each other.
Aragonite can form pseudomorphs of calcite, but the reverse process, calcite after aragonite, does not happen. Why is that? Scientists found that aragonite crystals are slightly unstable while calcite is stable. This means when aragonite forms, it will remain stable as long as conditions do not change significantly. Any energy loss or other disturbance can cause aragonite’s internal atomic structure to slowly shift from orthorhombic to the more stable hexagonal calcite form. The problem is it is tough to distinguish these paramorphs visually.
DRAWN TO CASTS
As earlier described, casts are pseudomorphs that form when a crystal develops then is later overlaid by a second different mineral. If nothing else happens, the coated specimen is just that, a coated mineral. Suppose conditions change again, and a later solution slowly dissolves away the original crystal but has no effect on the second mineral. In that case, you end up with a hollow mineral that is a replica or cast of that original mineral.
The rarest of the casts I’ve enjoyed seeing and is my favorite is also beyond my pocketbook. The Virtuous Lady mine in England has produced what might be called a classic cast. The mine is reportedly named for Queen Victoria and is near Tavistock, Devon, England. This mine is credited as the source of two delightful pseudo casts, both very attractive and interesting.
One cast is called “lady slippers” and consists of tiny inter-grown siderite crystals formed over a bladed mineral with curving sides and a pointed termination like a sword blade or slipper. The original mineral is thought to be fluorite and was entirely covered by tiny brown siderite crystals. After the original cubic mineral formed and was coated with crystallized siderite, the original fluorite dissolved away, leaving a perfect siderite cast of the fluorite cube. But Mother Nature was not finished. Inside the hollow siderite cube, an iron-rich solution developed a small cluster of brassy chalcopyrite crystals on the siderite cast floor. As a final touch, a small spray of white quartz crystals developed atop the chalcopyrite. Gratefully, often a hole in the side of the siderite cube allows for a view inside. The best of these is in the Museum of Natural History, London.
Stibnite presents as another fine example. I’ve always enjoyed stibnite, probably because the first mineral collection I ever saw — at the age of 10 — was at Yale Peabody Museum and included an amazing collection of Japanese stibnite crystals. Stibnite is antimony sulfide in orthorhombic crystals, which are lustrous, black. Crystals can range from needle size to fence posts. In Japan, such big stibnite crystals were once used as such by farmers. Stibnite also forms a pseudomorph stibiconite that looks like stibnite except for color. It is a hydrous antimony oxide found in a pale to rich orange tan mineral, a hydrous antimony oxide in cubic form. These lovely crystals are chemical pseudomorphs and mined in Zacatecas, Mexico, and China.
In Part Two of this series, appearing in the January 2021 issue, we will discuss more common pseudomorphs commonly seen, including malachite ps azurite, copper ps cuprite, quartz ps zeolites, pyrite ps fossils, and others.