Saturday, October 15, 2016

Howlite, an evaporite mineral from southern California

This post concerns an interesting mineral which can be found not too far from where I live in southern California. The mineral is howlite (also called “white turquoise” by some collectors). It is a borate mineral (calcium borosilicate hydroxide) that is found in evaporite deposits. In my area, these kind of deposits occur in the middle Miocene Mint Canyon Formation in Tick Canyon, Soledad basin. The howlite found there occurs in its most common form, as nodules. This locality is known for its high-quality howlite, including some rarely found crystals of this mineral. To see pictures of these crystals (I do not have any), go to Wikipedia and type in the word “howlite.”

Nodule of howlite, 5 cm width.
The nodules are white with fine gray or black veins, which create an erratic weblike patterns. The nodules are used in jewelry. They are easily dyed to imitate other minerals, especially turquoise, which also commonly has a veining pattern.

This picture shows some of the variation found in nodules of howlite. Nodule is 5 cm width.

In Tick Canyon, howlite was mined for a time, along with other evaporate minerals. The mining operation has long since closed down.

Friday, September 30, 2016

Rubellite from Pala, San Diego area, California

Rubellite, a gemstone variety of the mineral tourmaline, can be transparent and possesses a beautiful reddish pink to violet or light green color. It can also be nearly colorless.  One of the best known localities for this gemstone is in the Pala mining district, at Pala, near San Diego in southern California. About 10 years ago, I was a member of a group of geologists invited to tour the historic Stewart Mine at Pala. We were allowed to search through discarded diggings, but I was able to find some nice specimens that were not too weathered. It is important to mention that this mine is on private land and getting permission to enter the land is absolutely needed.

The individual crystals of rubellite on the left side of the picture are approximately 1 cm in height. The ones that appear somewhat blackish are transparent enough to show the black color of the background material on which they were photographed. The cluster of weathered crystals on the right is 5 cm in width. 

Rubellite from the Stewart Mine is commonly in a matrix consisting of the lithium-bearing mineral lepidolite (it is purple and shines a lot like mica). The cluster of rubellite shown above is in lepidolite.

The rubellite in the Stewart Mine is found in a complex and highly mineralized granitic pegmatite-aplite dike. 

Tuesday, September 13, 2016

The world's oldest fossils (maybe).

In September of 2011, the discovery of the so-called "world's oldest fossils" was written up in   Nature Magazine, mentioned in newspapers, and discussed online.

These "fossils" are 3.7 billion years old and were found in southwestern Greenland following recent melting of some snow. They might be stromatolites made by cyano-bacteria, and, if so, they would be 220 million years older than previous substantiated previous finds of Precambrian cyano-bacteria.

I include the best picture I could find online showing these Greenland "fossils."

The Greenland "fossils," 1 to 4 cm high. Picture credit: Allen Nutman/Nature Magazine. To see more information about these "fossils" go to <>
Upon seeing the photo above, my immediate reaction was that they might be flame structures, which are sedimentary (non-organic) structures created by dewatering of "soupy sediment" when it is compacted. The sediment literally is squeezed into the overlying sediment and "flows" as it does, creating the appearance of "flame tips." Flame structures can be of any geologic age.

I include a picture of a flame structures in an outcrop that I came across several years ago while doing some geologic mapping. This example is in deep-sea turbidite deposits in the lower Pliocene Towsley Formation in northern Los Angeles County, southern California. You will have to admit, at least, that the resemblance is striking.

A flame structure (closeup shown on the right) in the Towsley Formation just north of Sunshine Canyon, Santa Susana Mountains, southern California. The staff is 1.5 m in length, and the flame structure is a few centimeters high. Note: There is a watch (for scale), without a band, on the talus slope just left of the flame structure.

Thursday, September 1, 2016

The "ins and outs" of two snail shells with tall spires

Sectioning a seashell longitudinally can reveal some interesting internal details, and these can be highly useful for the identification purposes. In the case of Telescopium, notice the presence of an indentation (fold) on the inside of the aperture (the large opening at the anterior end of the shell). This fold is also present on most of the height of the central axis (columella) of the interior of the shell.


Exterior and interior views of modern-day specimens of Telescopium telescopium (Linnaeus, 1758) from Indo-Pacific mangrove mud flats. Height of shells approximately 3 inches.

Although Turritella superficially resembles Telescopium, the columella of Turritella does not have an indentation. The aperture of Turritella is round and without a fold.


Exterior and interior views of modern-day specimens of Turritella cerebra (Linnaeus, 1758) from Indo-Pacific shallow-marine waters. Height of shells approximately 4.5 inches.

Sunday, August 14, 2016

Elimia tenera: A commonly misidentified Eocene freshwater snail

Over the years, as I have viewed various collections of fossils, I have come across specimens of a small fossil gastropod that occur in great abundance. Rocks containing these shells can be found for sale in rock shops or online, and the shells are commonly and incorrectly called “Turritella agate.” These rocks do not consist of Turritella; rather they consists of specimens of the freshwater gastropod Elimia tenera (Hall, 1845), which have been preserved in chalcedony. Actual specimens of Turritella can be much larger, possess only spiral ribs, and are known only from shallow-marine deposits. Elimia tenera has both radial and spiral ribs, and the aperture of Elimia is quite unlike that of Turritella.

This rock (77 mm width) is fully packed with specimens of E. tenera.
This polished slab (37 mm width) shows only the
cross section of shells of E. tenera.
These three specimens of E. tenera are internal casts (i.e., each one shows
only the interior of a shell, which was filled with chalcedony).
The largest specimen is 14 mm height. 
There has been considerable disagreement in blogs and websites as to whether or not the E. tenera specimens, found in rock shops, have been replaced by chalcedony or agate. Technically speaking, chalcedony is the “culprit.” It is a microcrystalline form of silica, and chalcedony has many varieties, including agate, which commonly has multi-colored curved or angular banding. The specimens of E. tenera that I have seen were replaced by a fairly uniform brown or gray color of “ordinary” chalcedony and not replaced by the more eye-catching, beautiful colors typically associated with agate.
Elimia tenera: Specimen on the left (19 mm height) shows the spiral ribs, and the
specimen on the right (14 mm height) shows both spiral and radial ribs.
Elimia tenera, which used to be (and incorrectly) called Goniobasis tenera, is a freshwater snail that lived in shallow subtropical lakes with intermittent volcanic eruptions nearby. This gastropod is of Eocene age and is commonly found in the Laney Member of the Green River Formation in Utah. This is the same formation that famously has many very well preserved fish, insect, leaf, and other fossils.

Genus Elimia belongs to family Pleuroceridae, and, as currently defined, this family  today is confined entirely to North American fresh waters: The eastern United States and into Texas and from southern Canada to Florida. Pleurocerids might be relicts (“living fossils”) from earlier geologic times (Paleozoic?) in eastern North America.

Monday, August 1, 2016

The Nautilus shell

The chambered pearly shell of Nautilus pompilius, named by C. Linnaeus in 1758, is not only pretty but its spiral growth is an excellent example of logarithmic spiral growth, similar to the spiral bands of clouds in a hurricane or the arms of a spiral galaxy (i.e., Google "logarithmic spiral" for more information on this subject).

The Nautilus is a cephalopod, and this group of animals also includes the squid, cuttlefish, and octopus. Living specimens of N. pompilius can only be viewed in their natural state at a few locales in tropical waters in the southwest Pacific Ocean, or in controlled environments in public or private aquariums. Nautilus shells can be found as beach drift on some beaches. 

Adult shell of Nautilus pompilius (swimming mode orientation);
 maximum diameter 5.5 inches (14 cm).
Juvenile shell of Nautilus pompilius shell, 
maximum diameter 2.9 inches (7.3 cm) 
Notice that the juvenile Nautilus is fully covered with stripes, whereas the adult shell only has the stripes on its early part. The stripes provide camouflage for the juvenile because it spends its time on or near the ocean floor. The stripes allow it to blend in. The adult spends most of its time swimming or floating in the water column, and stripes are not needed, at least, on the ventral part of its shell. If a predator looks at the adult Nautilus shell from below, the shell looks like the sun-lit waters near the surface of the ocean.

Cut-away (median-longitudinal) section of adult Nautilus pompilius shell
 showing interior structures; diameter 6.3 inches (16 cm).
As shown in the above picture, the early part of the shell has numerous, closely spaced chambers called camera (single chamber = camerum), which provide great strength to the shell when the animal sinks into the depths (several hundred feet deep) of the ocean during the day. If the shell did not have this added strength, it would implode.

The camera are filled with nitrogen gas, which gives buoyancy to the shell. The siphuncle is a fleshy tube that connects all the camera and serves as a conduit for the transfer of the gaseous contents. The buoyancy also affects the shells after death of the animal. The empty shells can drift long distances. If you submerge an empty Nautilus pompilius shell in a bucket of water, the shell will bop up, rather than sink.

The interior of the N. pompilius shell consists of "mother-of-pearl" shelly material, which is the biomineral aragonite. This mineral  was secreted by the animal as the shell grew, and that is why the term "biomineral" is used here. 

Nautilus is one of only two genera of extant (living) cephalopods known as nautiloids. Fossil nautiloids have a geologic record that goes back to the Cambrian Period, 550 million years ago, although shells did not become common until the subsequent Ordovician Period. These early nautiloids had a straight shell and are called orthocone nautilioids, as opposed to the more modern, coiled nautiloids, like N. pompilius.

Example of Eutrephoceras, shell incomplete (dorsal margin partially missing).
 Maximum diameter 1 inch (2.54 cm).

Eutrephoceras, an extinct coiled nautiloid whose geologic range is Late Jurassic to Miocenediffers in its morphology from Nautilus by having straighter septa (also called sutures).

Thursday, July 21, 2016

Pyrgulifera humerosa, a Late Cretaceous freshwater snail from Wyoming

For many years, I have been intrigued by a fossil freshwater gastropod (snail) belonging to the genus Pyrgulifera, whose geologic range is Late Cretaceous to Eocene (i.e., from about 95 to 40 million years). In addition to its occurrence in North America (Wyoming, Idaho, and Utah), it is also found in Europe (e.g., Austria and France), Asia, and India. The morphology (shape) of Pyrgulifera reminds me of some shallow-marine gastropod genera although the resemblance is coincidental, rather than evolutionary.

One of the best examples of this genus is Pyrgulifera humerosa (Meek, 1860), of Late Cretaceous age (Cenomanian Stage). It is an extremely common species, especially in the Bear River Formation near Evanston, Wyoming, in the extreme southwest corner of this state. Many varieties of this species are known.  
Back-side view of same specimen.

height 25 mm; incomplete (tip is missing)

Specimens of P. humerosa are generally well preserved although the margins of the aperture are usually crushed or broken off. The shell has a short-fusiform shape with several well defined whorls, concave ramp, angulate shoulder bearing several short spinose projections, and rather wide spiral ribs, especially on the last whorl. 

Species of Pyrgulifera lived in freshwater or fresh-brackish (<5% salinity) waters in coastal plain estuaries or lower courses of rivers.