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.

A clam for the ages

Venericardia is a widely distributed genus of shallow-marine bivalves (clams) in the family Carditidae. This genus was abundant during the Paleocene and Eocene epochs (a cumulative range of 66 to 34 million years ago). The highest biodiversity of Venericardia was in the Eocene. The genus needs detailed classification studies of all its various subgenera found throughout the world. Detailed studies are much needed to evaluate the likely possibly of over naming of species/subspecies found on the west coast of North America. The genus is now extinct, with the last survivors of this genus dying out apparently in the early Miocene.

Lamarck (1801) first described the genus Venericardia based on well preserved shells of this bivalve in the Paris Basin, France. The aragonite shells of this genus are commonly large and very sturdy, with wide radial ribs crossed by concentric growth lines on the exterior. The interior of the shells are characterized by long posterior teeth, much shorter anterior teeth, and two prominent elliptical-shaped muscle scars connected ventrally by a continuous line. Some species have prominent nodes along the inner margins of the shell.

These two images show the plaster replicas of the exterior and interior surfaces of a left-hand valve (6.5 cm high and 7 cm wide) of Venericardia planicosta Lamarck, 1801 from an Eocene shell bed at Grignon, Paris Basin, France. 

Venericardia (Pacificor) lutmani Turner, 1938, lower Eocene (Ypresian Stage), southwestern Oregon; plaster replicas. Left image is the exterior of a left valve, and the right image is the exterior of the right valve of a single specimen (9.25 cm high and 10 cm wide) which, upon burial, became separated from one another. 

These two images are the corresponding interior views of the same plaster replica shown immediately above.

Venericardia (Pacificor) hornii calafia Stewart, 1930, middle Eocene (Lutetian Stage), southern California. Exteriors of left-hand and corresponding right-hand valves of the same specimen (whose valves [9 cm high and 9 cm wide] are closed very tightly).

Dorsal (hinge) view of same specimen of V. (P.) h. calafia shown immediately above. Left valve is on the left side of image, and right valve is on the right side of image

Venericardia was an infaunal (burrowing) suspension feeder that lived buried just beneath the surface-water interface. Its optimum habitat was in relatively deep, shallow-marine (shelfal) environments, but its shells are commonly found as transported remains in coastal-storm beds.

Southern California’s Highest Mountain

Google Earth Pro Oct., 2019 image showing location of San Gorgonio Mountain.

 San Gorgonio Mountain is the highest peak [11,503 feet (3506 m)] in southern California. The peak is 27 mi (43 km) east of the city of San Bernardino, San Bernardino County and is a easy drive from most of Los Angeles and Orange counties. It is the peak with the greatest vertical gain (5,840 feet) in California. The mountain hosts the longest recorded line of sight in the contiguous United States; it is plainly visible from the summit of Mt. Whitney, 190 miles away.

View north-northeast from a commercial airliner. Image taken in mid July, 2010.

Closer view north from the same airliner. San Gorgonio Mountain has a few patches of snow near its summit. Under the wing, Big Bear Lake is visible. The mountain has a somewhat pyramid shape with a steep north face and a slightly shallower south face. 

An even closer view of summit of San Gorgonio Mountain. The summit plateau is large and broad (1 square mile). The summit has an Alpine climate, and snow can be present, even as late as mid-July. 

Northwest view of San Gorgonio Mountain (its summit has a few patches of snow left on it). Three major southern California rivers have their source on the mountain: the Whitewater River (shown here just right of the center of image), the San Gorgonio River, and the Santa Ana River.

San Gorgonio Mountain is part of the Transverse Ranges, an east-to-west mountain chain formed by tectonic forces between the Pacific and North American plates along the San Andreas fault, which lies just south of San Gorgonio Mountain.

The mountain is a massive block of quartz monzonite igneous rock (please see my earlier post--March 3, 2017, San Andreas Fault Displacement of a Distintive Granite), which sits on an ancient platform of Precambrian gneissic metamorphic rocks. Glacial and fluvial deposits dominate the surface of the lowest part of the mountain. During the Pleistocene “Ice Age,” there were two separate episodes of glaciation (both Wisconsin age, which was 75,000 to 11,000 years ago) on San Gorgonio Mountain, as evidenced by cirques and huge terminal embankments of coarse angular debris, up to 700 feet thick.

Xenophora, the "Carrier"-Shell Gastropod, Recent and Past

This post returns to the subject of family Xenophoridae Philippi, 1853. Its most common genus is Xenophora (usually pronounced as "Zen-off-fer-a"), one of the more unusual gastropods found today in warm-water oceans (for example, Indo-Pacific and Caribbean). Xenophora, which is a Greek word, meaning "foreign bearing," was named by Fischer von Waldheim, in 1807. 

Xenophora is unusual among gastropods because it can attach shells, rock fragments, and other debris to its shell, in order to provide protection and/or camouflage. 

                                                                                        

Xenophora pallidula (Reeve, 1842) dorsal view, one of the more common Xenophora found today in offshore to deep water in the Indo-Pacific Ocean; maximum diameter of entire shell complex (including extended attachments) is 8 cm. This single shell has amassed ("collected") quite a collection of other gastropod shells. 

Same specimen as above, ventral view, showing the smooth aperture, which is also flat. If you look closely on the right-hand side of the image, you will see how the tips of the attached shells have been cemented into the margin of the shell of Xenophora. 

The animal within the shell of Xenophora reaches out of its apertural opening and selects what it wants to attach. It cleans the particles with bites of its mouth and then holds (with the aid of its head) the particles to the margin of the aperture until they are attached firmly by shell secretions from the soft tissue of the mantle (the organ which secretes the shell).

Same specimen as above, side view. Notice how the flat bottom of the actual shell is elevated relative to whatever it is resting upon. This elevation creates a "feeding cage," which enables Xenophora to "hang down" from its shell and feed on the ocean floor. The animal also leaves no trace (smell) of its presence on the floor, thus it helps deter predators.

This is a dorsal view of another specimen of X. pallidula, maximum diameter of entire shell complex 9 cm (including rock attachments), which attached small stones to its shell, rather than other shells. It was a matter of what material was available. A few encrusting annelid-worm tubes attached subsequently to the stones.  This shell is from 30 m depth in waters off of Japan.

Ventral view of same specimen shown immediately above.

The three specimens shown above are of the middle Eocene Xenophora stocki Dickerson, all about the same size in diameter (15 mm), from southern California. The shells are not preserved, but impressions of where shells used to be attached are visible as small depressions (e.g., lower left side).

Like today, fossil Xenophora, ares indicative of warm waters (subtropical to tropical). The geologic range of Xenophora is Late Cretaceous to Recent.

In my posting on October 28, 2018, I mentioned the above-pictured Late Cretaceous species of Xenophora (Endoptygma) hermax Squires and Saul, 2001 from southern California. These specimens could be mistaken for small piles of stones.

For comparison, the above three views of the same specimen (7 cm diameter) show a deep-water, modern-day Stellaria solaris (Linnaeus, 1767) from Taiwan. Stellaria Möller, 132, belongs to the same family as Xenophora, but Stellaria has no attachments. Its species do not need to "carry" shells in order to elevate their shell above the ocean floor. Instead, Stellaria has evolved in such a way as to grow its own "stilts."

Miocene spring-deposited tufa in southern California Pt 2

Part 1 concerned an occurrence of Miocene spring-deposited tufa in the Barstow Formation, Calico Mountains, southern California. Part 2 is about another Miocene occurrence of spring-deposited tufa, 118 miles to the southeast of the Barstow locality, in the Diligencia Formation in the Orocopia Mountains, northeast of Salton Sea, Riverside County, California.

Google Earth (2018) image,

The Diligencia Formation spring-tufa deposits resemble fossil "logs," as shown above. These "logs" occur mainly as branching-horizontal tubes, in contrast to the mostly vertical buildups of the Barstow Formation spring-tufa deposits. The Diligencia tufa deposits are confined to a single thin bed that crops out for a least 6 km. This bed represents a lake-shoreline environment, which interfingers with basalt flows and river-delta deposits. The hammer, which is the same one shown in all the subsequent images, is 27 cm in length.

The Diligencia tufa tubes, are commonly parallel to bedding, can be up to 65 cm in length and 25 cm in diameter.  Hammer is 27 cm length.

This is a horizontal tube, which has been extensively weathered.

In some places, the Diligencia tufa deposits are transported accumulations of closely packed tubules concentrated into a thin bed of limestone.

In other places, the Diligencia tufa deposits occur as scattered, weathered remains.

All the Diligencia tufa deposits consist predominantly of wavy microcrystalline calcite laminations surrounding by small core that is filled with darker sediment and/or partly with coarser grained calcite. The "dime" coin is 18 mm in diameter.

Polka-dot granite

"Polka-dot granite" is a distinctive rock, which has been found at several localities in Southern California. It has been used to indicate offset along the San Andreas Fault system, but some geologists have reported that this distinctive granite might not all have been derived from the same magmatic source. More geochemical research is needed.

“Polka-dot granite” is a granite with clots of cordierite (a mineral containing magnesium), biotite, garnet, and other minerals surrounded by an irregular halo (absorption sphere) consisting of white granite with little or no mafic (dark) minerals. The inner and outer margins of the halo are irregular but distinct. The halo is surrounded by the same granite that occurs in the center of the halo. The result looks like large polka-dots, which range in diameter from less than a centimeter to 8 cm.

"Polka-dot granite" collected from Southern California by Dave Liggett.



Same specimen shown above but photographed in the shade.
"Polka dot" is 4.4 cm in diameter.



Close-up of previous photograph.



Another "polka dot" (1.5 cm diameter) from the same locality.

By the way, cordierite is known for its ability to withstand extreme temperatures. It is used for making "pizza stones" because you can take the stone from a low temperature to a very high temperature without the risk of breaking it.

Preferred orientation of Turritella

Fossil shells belonging to the shallow-marine gastropod Turritella are prone to have been preferentially aligned by waves and currents because their shells are long an straight. I used the first slide below in one of my earlier posts (July 24, 2014) on the subject of "Taphonomy of Mollusks Shells." Taphonomy is the study of post-mortem processes (waves, currents, bored by other organisms, etc.) that affect shells.

Eocene Turritella andersoni lawsoni shells in the Llajas Formation
of Simi Valley, Ventura County, Southern California. The
longest shell is 6 cm long. These shells occur in situ, in
a bed of silty fine-grained sandstone.

More Eocene T. andersoni lawsoni shells from the Llajas Formation
of Simi Valley. The longest shell is 6 cm long. This slab is a piece of
loose rock ("float") from the formation.

Green fluorite

One of my more recent posts deals with a locality where corundum (sapphire, ruby) can be found in Southern California. This new post deals with another Southern California mineral locality, and it is where green fluorite can be found.

Flourite consists of calcium fluoride. It is a common mineral and used as an indicator of a hardness of 4 on the Moh’s Scale of hardness from 1 to 10. Flourite can come in a wide variety of colors (especially purple), but green fluorite is a relatively less common color.

The green-flourite locality is called the “Felix Mine” locality, which is just north of the city of Azusa, California in the foothills of the San Gabriel Mountains. The mine, established in 1892, is no longer accessible because of urban sprawl, and the vein which yielded the green fluorite has long been mined out. The specimen shown below was recently kindly donated to me by a collector.

Green fluorite (maximum dimension 2.3 cm) from the Felix Mine, Southern California.
The black material is the mineral galena (iron sulfide).

The largest crystals ever found of green fluorite at the Felix Mine were reportedly about 8 cm long. Most of the crystals, however, were very small to small size. The fluorite occurred in numerous subparallel veins cutting through decomposed granite. The mineral galena is commonly associated with the green fluorite.