Septarian Nodules

 The name “septarian nodules” is derived from the Latin word “septum,” meaning seven, in reference to partitions dividing cavities. More specifically, the name refers to carbonate-rich nodules having an internal structure comprised of a series of carbonate-filled cracks or cavities, which separate polygonal blocks of hardened sedimentary material, such as mudstone.

The exterior of a septarian nodule, 13 cm tall and 11 cm wide. Utah.

Septarian nodules on the outside are not exactly eye-catching.

The interior of the same nodule, shown above. The interior of a septarian nodule can be, however, very interesting and eye-catching.

The nodules usually form early in the burial history of the muddy sediment before the rest of the sediment hardens into rock. Afterward, compaction causes cracks/cavities to form, and groundwater with dissolved minerals infills the cracks/cavities. Upon evaporation, the minerals crystallize out of the water. 

Septarian nodules, more accurately referred to as septarian concretions, have a smooth rounded exterior, which is normally gray. The mineral-filled cracks in the interior are typically yellow (calcite), but some also can be partially brown (aragonite = another carbonate mineral).

This septarian nodule (12 cm wide and 15 cm tall) was cut (slabbed) in order to show the intricacy of the infilled cavities.

Lastly, I included this slabbed (cut) septarian nodule because it is a nice specimen and one that was used in one of my previous posts (July 17, 2014) entitled "Pseudofossils."The rounded and smooth exterior of most of the septarian nodules have been mistakenly by some collectors as “dinosaur eggs,” which they are NOT! Pseudofossils are inorganic objects.

Rock hounds commonly cut the septarian nodules in half and polish them. They can sell for low to high prices, depending on the complexity of the radiating crack pattern and the variety of the coloration.

Blister Pearls

A pearl is a calcareous object produced within the soft tissue (mantle) of an oyster (family Pteriidae). The sheen or luster (iridescence) of a pearl, referred to as nacre or “mother-of-pearl,” is caused by the intergrowth of microscopic crystals of aragonite (calcium carbonate) and conchiolin (a protein). Both are formed by the mantle tissue (soft part) of an oyster. As I have mentioned on some of my previous posts, nautiloids and some gastropods can make nacre. Nacre is relatively soft and it ranges from 2.5 to 4.5 on the Mohs scale. A copper penny has a hardness of 3.5 and a knife blade has a hardness of about 5.5. 

As a “side note”: aragonite can form two ways: inorganically or organically. If it is inorganically formed, then it is a mineral. If it is organically formed, then it is a biomineral. Only biogenic aragonite has the nacreous sheen.

The purpose of this blog post is to focus on the blister pearls or “half pearls,” which are the result of part of the growth of the pearl against the inside of the shell, rather than totally within mantle tissue. Blister pearls can be as big as 10 mm diameter. Blister pearls are not valuable, but they can still have beautiful iridescence.

This image shows several blister pearls (each one about 7 mm diameter) on a cut-out portion (6 cm diameter) of a pearl-oyster shell (genus, species, and provenance unknown). 

Pearls can be formed by freshwater (river) oysters or by marine oysters. In some cases, other types of bivalves, both fossil and modern-day, are known to produce pearls. These fossil pearls are usually not preserved very well and have been recrystallized to calcite, with a loss of the sheen or luster.

All pearls, whether they occur in oysters living in shallow-marine waters or in freshwater rivers, are formed in response to an irritant (e.g., a grain of sand becomes embedded, by chance, into the mantle). The mantle then secretes nacre around the irritant. Pearls that are cultured (note: they are not simulants) in aqua-labs have a tiny piece (machine rounded) of another oyster shell artificially introduced (“seeded”) into the mantle of the host oyster. Several species of the saltwater oyster Pinctada and, to a much lesser extent Pteria species, are the main pearl-producing oysters.

Two views (exterior and interior) of a right-valve (7 cm height) of the saltwater oyster Pinctada sterna (provenance unknown), with an unusually large (2 cm diameter) blister pearl. This species ranges from southern California to Peru and can be as much as 10 cm in height. The posterior wing on at the top of the left side of the first image is missing.

Fossil Bony Fish Ear Bones

Otoliths ("ear bones") occur in all vertebrates, including humans, whales, and dolphins [see my previous posts: Fossil whale ear bone, August 15, 2014; Fossil dolphin ear bones, Oct. 4, 2015] for these latter two examples) and allow for perception of linear acceleration, both horizontally and vertically. Otoliths are not skeletal components; rather they form as three separate bones (in each ear) within the auditory capsules. Their composition is calcium carbonate (mainly the mineral aragonite). They can also be fossilized and many are microscopic in size. Some, however, can be macroscopic in size.

Otoliths of modern-bony fish (teleosts) can be common in Tertiary freshwater and marine environments. The shapes and proportions of otoliths vary with fish species. Fish from reef or rocky bottom habitats (snappers, groupers) have larger otoliths than open-clean fish (tuna, mackeral). The growth rings in otoliths are similar to tree rings, in that they can be used to estimate the age of fish, as well to determine early life transitions from planktic larvae to benthic juveniles. The stable isotopes of oxygen and carbon found in the otoliths can be used to determine ancient temperatures of their environments. 

Front, side, and back views (in vertical order) of a matched pair of macroscopic left-and-right otoliths (24 mm long and 7 mm thick) from a 1-meter long, 40-pound White Sea Bass from the northern Gulf of California, Mexico.

Crystals That Show Twinning

In my March 31, 2017 post concerning a distinctive granite, I showed an orthoclase crystal with twinning, which occurs when two separate crystals of the same substance share some of the same crystal lattice. Instead of a normal single crystal, the crystalline structure appears doubled. 

In this new post, I show some other common examples from my personal collection of twinned crystals. They are aragonite, pyrite, gypsum, quartz, and staurolite. 

aragonite (40 mm length)

gypsum (37 mm length)

pyrite cubes (47 mm length)

quartz (47 mm length)

staurolite (18 mm length)

pseudomorph of staurolite (40 mm)

pseudomorph of staurolite (40 mm length)

       
















pseudomorphs are formed
when a mineral is replaced
by a foreign substance