Pyrite Pseudomorphs

 This post concerns chemical weathering that can occur to the mineral pyrite [= iron sulfide FeS₂ = “fool’s gold].” The weathering transforms the pyrite molecules into another mineral, but the external shape of the pyrite crystal remains. This phenomenon, which is called a “pseudomorph after pyrite,” is not restricted to pyrite, as it can occur in other minerals, as well.

A pseudomorph is when one mineral is replaced by another mineral without any change in the external form. In the examples shown here, the original pyrite has largely not been replaced, but the surfaces of the original pyrite crystals have been altered by weathering. Thus, these altered specimens are only partial pseudomorphs. This kind of distinction is not commonly discussed in the literature.

Unaltered pyrite crystals are cubic, shiny, heavy (dense), and leave a black-color streak when scratched on a streak plate (see one of my previous posts). Also, the crystal surfaces of pyrites can be striated.

Unaltered crystals of pyrite (no striations).

The cluster is 5 cm across.

                        Unaltered crystals of pyrite with striations.

                            The cluster is 3 cm tall.

           Incipient pseudomorph of pyrite after goethite/limonite. 

                            The cluster is 2.5 cm tall.

Later stage of other partial pseudomorphs of pyrite after        goethite/limonite. The cluster is 3 cm tall.

The last image, which is of two co-joined crystal faces of pseudomorphs, is the most weathered. It has some areas of a soft-chalky coating of white, red, or brown material; its striations are still present but are not pristine. The composition of the coating material is a variable mixture of two minerals: goethite [FeO(OH)], a hydrated iron oxide-hydrous oxide and limonite [FeO(OH)•nH₂O], a hydrous iron oxide. Goethite typically is the dominant mineral. 

GYPSUM REVISITED: AN ADDENDUM TO THIS INTERESTING AND COMMON MINERAL

This post is an update (with new images and new text) of one that I did five years ago, on Sept. 30, 2017. 

There are numerous varieties of the mineral gypsum (selenite), a sulfate mineral, which has the chemical formula CaSO4.2H20. A keep clue is identifying gypsum is that it does not react with hydrochloric acid. Gypsum is normally the first salt precipitated and deposited in the evaporation of sea water or salty late water. As the salinity of the water increases, the gypsum is followed by the deposition of anhydrite and halite (sodium chloride). Gypsum can also form as vein-filling cracks in rocks.

Variety 1 is colorless selenite: it is transparent, smooth, and non-fibrous. 

(a)

lmage (a) shows a transparent crystal of selenite (43 mm long and 17 mm wide).

(b)

Image (b) shows a large transparent crystal of selenite (70 mm wide) that apparently has been artificially tapered by a machine.

                   

Variety 2 is colored selenite: image (c) shows a slightly colored crystal of selenite (30 mm long) of gypsum. This specimen has been scratched by fingernails. When I was teaching, I would pass this hand specimen around the classroom, and some students would   scratch this specimen. Gypsum is very soft, with a a Mohs Scale hardness of only 2 [see my previous post about the Mohs Scale].

                                              (c)

                                               (d)

Image (d) is of another slightly colored crystal (30 mm long) of selenite. This transparent crystal of gypsum has been scratched by fingernails. When I was teaching I would pass this hand specimen around the classroom, and some students would  convince themselves that they could scratch this mineral, which is very soft and has a Mohs Scale hardness of only 2 (see my previous post on the Mohs Scale).

Variety 3 is twinned crystals of selenite: See one of my previous posts about the subject of twinning in crystals.

                                               (e)

Image (e) is of two specimens of twinned crystals (the biggest one is 37 mm in length) of selenite. 

Variety 4 of selenite is satin spar: 

                                                (f)
Image (f) is of a rectangular hand specimen (4 inches at its widest and 1.5 inches thick) is milky white, silky, and  has fibrous with a characteristic parallel structure. shown here. You can clearly see the distinctive fibrous parallel, satiny structure of this soft mineral. Surprisingly, this piece of satin spar is quite heavy. Unfortunately, the name “satin spar” has also been applied to a fibrous variety of the mineral calcite (CaCO3), but that mineral which reacts with hydrochloric acid.

                                                 (g)

Image (g) is of a machine-cut (in a spiral (corkscrew) shape) of a piece of satin spar (6.5 inches tall). This sculpture demonstrates how the softness of selenite, which allows for it to be easily manipulated into complex shapes. 

Variety 5 of selenite is "desert rose": These are rosette-shaped crystals that can resemble a flower shape because of the curved gypsum crystals. These crystals are twinned (see comment mentioned above). Desert rose crystals can be gradational with so-called, "desert-flower" gypsum crystal.

(h)

Image h is a "desert rose," (2.5 inches across) consisting of a cluster of intergrown gypsum crystals.                                          

Variety 6 of selenite is alabaster. This variety of the mineral gypsum is massive material. It looks just like white limestone or marble. Alabaster is softer than those rocks and, therefore, is commonly used for carving statues, vases, and ornaments.

                                                (i)

Image i is a closeup of the surface of a slab of alabaster. The crystal boundaries are somewhat indistinct because of weathering of the surface of the slab.

“Cystoids”: A Paleozoic Group of Echinoderms with a Twist

“Cystoids” are Middle Ordovician (about 460 million years old) to Late Devonian (about 560 million years old) echinoderms that lived attached to the ocean floor via a column (stem). Modern echinoderms are starfish, brittle stars, sand dollars, etc. As indicated by the use of quote marks around the name “cystoids,” their classification  in a state of flux and has been for many years. They are currently regarded as consisting of two classes: the rhomboiferan “cystoids,” and the much less common diploporites “cystoids” (Prothero, 2004).

The rhomboiferan “cystoids” were never very abundant, but they are common in some Middle Ordovician rocks.  

Both classes of “cystoids” have an ovoid shape exoskeleton. Their food-catching ambulacral arms (also called brachioles) extend into the water column rather than being fused to the sides of their calyxes (bodies) as found on blastoid echinoderms (e.g., Pentrimites--see my previous post).  The calyx plates of cystoids are characterized by being perforated by tiny tubes or canals (thecal pores). The arrangements of these thecal pores (they can be concealed or raised) is very important in the classification of cystoids.

This image shows the main morphological parts of a cystoid.

The image above shows a partial “cystoid” exoskeleton with the basal part of the exoskeleton and its accompanying main-body section (slightly crushed, diameter 23 mm), including its minute thecal pores. But, only the lower parts of some of its segmented ambulacral arms are present.

An even more incomplete “cystoid” shows only the lower part of the basal area (diameter 12 mm); (note: the minute thecal pores present) and a small part of the column. 

The diploporites “cystoids” are characterized by having a very flexible prehensile column that could wrap around a foreign stationary object. 

Sketch of a diploporite “cystoid” with a prehensile (flexible) column.

Reference:

Prothero, D.R. 2004. Bringing fossils to life: an introduction to paleobiology. McGraw    Hill. 503 pp. 

The Blastoid Pentremites

Genus Pentremites, of Carboniferous age, belongs to phylum Echinodermata, which includes two main types of echinoderms: those attached (e.g., blastoids, crinoids, cystoid) to the ocean bottom and those that are free-to-move around (e.g., starfish, brittle stars, sea urchins, and sand dollars). All echinoderms have a complex water-vascular system, which is an internal apparatus containing watery fluids. This apparatus resembles nothing else in the animal kingdom. Its primary function is to operate the tube feet, which serve food gathering, locomotion (in mobile echinoderms), and sensory functions, as well as respiratory requirements and burrow building.

The skeleton of echinoderms consists of porous calcite (calcium carbonate) plates, which are spiny and covered by a thin skin. The echinoderm skeleton is a product of internal secretion, like the bony skeleton of vertebrates, and the individual hard parts increase in size during the life of the echinoderm.

A sketch of the side view of a blastoid echinoderm Pentremites, which is characterized by having three main body sections: calyx, column (stem), and roots.

Sketch of the dorsal view of a blastoid showing the position of the mouth and the 5-ray symmetry characteristic of the calyx, with five ambulacral areas and five spiracles. The ambulacral areas are covered by feeding grooves, used for transporting food particles upward to the top of the calyx, where the mouth was positioned. The spiracles were used regulating the pressure of seawater that travelled thorough the porous calyx.

Sketch of the side view of two of the ambulacral areas of a calyx.

Side view of a specimen showing two of the ambulacral areas of a blastoid. Actual specimen is diameter 13 mm. 

Dorsal view of a specimen showing the five actual ambulacral areas and spiracles. Specimen slightly crushed, diameter 14 mm.

A Very Distinctive Fossil: the Bryozoan Archimedes

The subject of this post, the fossil genus Archimedes, belongs to phylum Bryozoa (bryo, moss; zoon, animal), which has an extensive fossil record. These animals are always colonial, and the colonies range in size from microscopic to, rarely, more than 1 meter across. Bryozoans live (and lived in the past) mainly in marine environments from shallow seas to abyssal depths and from tropical to polar regions.

The fenestrate bryozoa (Ordovician–Triassic) comprise a distinct group within the Bryozoa. They have delicate lattice-like morphology that grew in upright fan-shaped or funnel-like colonies (expansions) cemented to some foreign object by means of a root-like structure, near the base of the colony. Fenestrate bryozoans ate microscopic organic matter that they strained out of the water.

One of the most spectacular fenestrate bryozoan fossils is named Archimedes, which is a Greek name for a screw used to elevate water. Archimedes has a calcified corkscrew-shaped central spiral axis, and branching off from the axis are lattice-like fenestrate fronds. Finding a spiral axis with its branches intact is very rare.

A complete colony of an Archimedes is shown on the left. Also, an enlarged view of some of the fenestrate fronds are shown to the right.

An actual specimen of just the fenestrate fan part (36 mm wide and 40 mm high) of an Archimedes fossil. 

The spiral axes (or screws) of Archimedes can have either sinistral (anticlockwise) or dextral (clockwise) coiling direction, and this phenomenon is referred to as “chirality.” Two “screws” shown above are both 33 mm in length. The one on the left is sinistral coiled, whereas the one on the right is dextral coiled.

To the uninitiated, large specimens of just the central spiral axis (like those shown above) of Archimedes can resemble the backbone of an animal. I know this first-hand because one of the first fossils I ever saw was of a large Archimedes collected by one of my relatives, on his farm near St. Louis, Missouri. I thought that it was a backbone, but many years later when I was an undergraduate in college, my historical geology instructor quickly identified it as an Archimedes. Per his request, I donated that specimen to the “Mineralogy and Paleontology Museum” of the University of New Mexico in Albuquerque. As far as I know it is still there and still on display.

Archimedes is common in the Paleozoic fossil record, with a geologic range from Carboniferous (Mississippian and Pennsylvanian) to Permian. The Carboniferous age specimens are 350 to 325 million years old and are found in the mid-western United States (especially Missouri, Arkansas, and Kentucky). A few Permian species have been found in Russia. During the Carboniferous, the mid-western United States was part of the supercontinent  called Pangea and was located just north of the equator. This area was covered by shallow, warm seas.  

Some references:

Condra, G.E. and M.K. Elias. 1944. Study and revision of Archimedes (Hall). Geological Society of America Special Paper 53:1–243. This is a classic study. NOT FREE.

Taylor, P.D. and C. Sendio. 2013. Chirality in the Late Paleozoic fenestrate bryozoan Archimedes. Bastalleria 19:41–46. Online for FREE!