Saturday, November 11, 2017

Barnacles make interesting fossils

Barnacles are classified as cirriped crustaceans and belong to phylum Arthropoda (a large group that also includes trilobites, crabs, insects, etc.). The name "cirriped" (cirri, curl; ped, foot) refers to the six pairs of delicate appendages which are used for filter feeding.

Barnacles are not very different from other arthropods in that they hatch as an egg, have a short existence as free-swimming larval forms, and molt (shed) to grow larger. In adult life, however, barnacles do not resemble other arthropods at all. With a shelly (calcareous) covering of many plates enclosing their shrimp-like body, most barnacles grow attached to hard substrate. Genus Megabalanus Hoek, 1913one of the so-called “acorn" barnacles, is common in the fossil and modern-day record of the eastern Pacific. 

Few people know that Charles Darwin, yes, that Charles Darwin, was an expert on barnacles. He derived some of his concepts about evolution based on his detailed studies of them.

bivalve shell is 3.4 cm in diameter
A bivalve shell (the "jingle" shell Anomia) with a large Megabalanus base (circular) at the top of the shell and 15 Megabalanus shells elsewhere on the shell. These fossils are of late Pleistocene age from the vicinity of Long Beach, Los Angeles County, southern California.

large barnacle is 1.5 cm high 
Megabalanus shell encrusting a shallow-marine gastropod shell, and a small Balanus shell encrusting the larger Megabalanus shell. All of these fossils are of late Pleistocene age from the same locality as those shown in the previous image.

acorn-barnacle operculum: smaller parts are 4.5 mm length;
 larger parts are approximately 5.5 mm length
Covering the top of an acorn-barnacle shell is a calcareous structure consisting of four interlocking plates. These two pairs of plates close together, just like a tight-fitting “lid,” used when the barnacle is not feeding or when it is disturbed. The opercular plates, which show continuous growth records (like tree rings) and are not shed during molting. Upon death of the animal, these plates eventually fall apart, thus they are mostly missing on fossil barnacles. The pair of plates shown (above) on the left are called terga, and those on the right are called scuta.

cluster is 2.2 cm wide
 The image shown above is a small cluster of modern-day barnacles with their opercular plates in life position. These are specimens of Balanus amphitrite saltonensis Rogers, 1949 from the Salton "Sea," an inland lake with very salty waters in southeastern California. This lake was created by accident in the early 1900's, when an aqueduct overflowed. This subspecies of barnacle, which is closely allied to the globally widespread B. a. amphitrite Darwin, 1854, was  introduced into the lake. This introduction was most likely via  migratory water birds.

cluster is 21 cm in maximum dimension
A cluster of five large-sized Tamiosoma gregarious from Pliocene rocks in the central San Joaquin Valley of central California.

each component is 2.8 cm length
These four images above show the exterior (above) and the interior (below) of two scuta of Tamiosoma gregarious found in Pliocene beds in the central San Joaquin Valley of central California. These are unusual finds.

2.6 cm height
This is the side view of a modern-day "gooseneck" barnacle (Pollicipes polymers) from a rocky intertidal zone in the vicinity of Goleta, Santa Barbara County, California. Notice the leathery stalk (pedicle) which is used by the barnacle to attach to rock. The stalk does not get preserved, thus, in the fossil record all one can find are the individual calcareous plates that make up the upper part of the animal.

The geologic time range of barnacles is Paleozoic (Silurian) to Holocene [= modern day]. Gooseneck barnacles evolved first. "Acorn" barnacles did not appear until the early Cenozoic.

3.5 cm maximum diameter
Lastly, I show an image of the barnacle genus Chelonibia, which attaches to the carapaces of sea turtles. This modern-day specimen is from Baja California Sur, Mexico.

Saturday, October 28, 2017

Colors of fluorite

The mineral fluorite consists of calcium fluoride (CaF2). It can have variation in color, largely due to impurities in its crystal structure. Flourite can be found throughout the world. It is common in hydrothermal deposits, where it can be associated with quartz, calcite, baryte, and galena (lead sulfide). 

Uses of fluorite include jewelry making, as well as a flux for smelting. 

The largest crystal (green color one) is 4 cm long.

Flourite can crystallize in several forms, including the octahedral form, which is my favorite.
Octathedral crystals: the clear one is 3 cm tall.

Flourite has a hardness of four on the Mohs Scale. Optically clear crystals, like the one shown above, has low aberration, thereby making them valuable in the construction of microscopes and telescopes.

Saturday, October 14, 2017

Copper and Molybdenite

COPPER (Cu) is a naturally occurring pure element with a reddish-orange color.

This is the element copper in its pure natural form. The specimen, which is 5 cm long, shows the dendritic habit of copper. The gray material is gangue (wall rock) material consisting of calcite.

Copper has with very high electrical and thermal conductivity, and the main uses for this soft element are for electrical wire, plumbing parts, and industrial machinery. In order to make copper harder, it is purposely combined (alloyed) with other metals:

brass = an alloy of copper and zinc

bronze = an alloy of copper and  tin

cupronickel = alloy of copper and nickel (used in making coins, like the U.S. nickel = 75% copper and 25% nickel).

Copper is one of the few metals that occurs naturally as a directly usable form. This led to its being used by early humans, as far back as 9000–8000 BC. The discoveries of making alloys out of copper happened later (e.g., the Bronze Age about 3700–100 BC).

Like aluminum, copper is readily recyclable without any loss in quality. It has been estimated that 80% of all copper ever mined is still in use today.

MOLYBDENITE (MoS2is the mineral molybdenum sulfide, the principal source of the metallic element molybdenum. This high-temperature hydrothermal mineral is silvery/gray in appearance (similar to graphite = pencil "lead"), is greasy to the touch, and peels apart (in one direction) in somewhat heavy but flexible sheets.

This specimen of molybdenite is 4 cm long (from upper left to lower right).
Molybdenite can withstand extreme temperatures without significantly expanding or softening. The element molybdenum  readily combines with other elements and forms hard, stable alloy materials, which are used for making high-strength steel. In particular, these alloys are used in military armor, aircraft parts, electrical contacts, industrial motors, etc. 

This specimen came from a quartz veinlet in granite at a commercial mine in Climax, Colorado.

Saturday, September 30, 2017

Some varieties of the mineral gypsum

I decided to do a post on the mineral gypsum just after my previous post on calcite because the two minerals can resemble each other. 

Gypsum is a very common mineral and is the most common sulphate mineral. It forms as an evaporite mineral and is commonly found in dry-lake beds. Gypsum consists of calcium sulphate dehydrate (CaSO4•2H20). If it becomes dehydrated it forms plaster of paris. 

Gypsum is very soft and can be easily scratched by a fingernail. On the Mohs Scale of Hardness, gypsum has a hardness of 2. Diamond has a hardness of 10 on this same scale. By the way, it does not fizz in acid.

This mineral is used in making fertilizers, plaster, wallboard (drywall), blackboard chalk, and some cements.

There are several varieties of gypsum, and some are shown below.
The scale in each of the three images is the same: increments of centimeters.

This is the clear (transparent) variety called selenite. It contains no significant amount of the element selenium; rather "selenite" refers to the ancient Greek word for the Moon.

This is the tabular, massive (fibrous/silky) variety called satin spar.

These five crystals show "fishtail" twins or "swallowtail" twins of gypsum. I discussed the topic of twinning in crystals in one of recent posts.

Crystals of gypsum can be extremely large in size and are the largest crystals (39 ft. = 12 m long) of any mineral on Earth. An image of these largest crystals is shown above. Note the human for scale. This image is from Wikipedia (accessed Sept. 2017) and shows the "Cave of the Crystals in Naica, Mexico" ["Cristales Cueva de Naica, México], where these enormous crystals are found.

White Sands National Monument in southern New Mexico (USA) consists of a 270 sq. mile expanse of white gypsum sand/dunes. The gypsum was eroded by way from nearby gypsum beds and deposited in the adjacent valley. As a teenager, I visited White Sands and enjoyed sliding down the dunes. The experience is much better than sliding down normal sand dunes made of quartz sand because the gypsum crystals are soft and not abasive. This image is from Wikipedia (accessed Sept. 2017).

Saturday, September 16, 2017

Calcite and some of its interesting properties

The mineral calcite consists of calcium carbonate CaCO3. It is a very common mineral that makes up limestone. Beginning students in geology labs quickly learn that calcite can be easily identified because it fizzes in weak hydrochloric acid.

Also, if the crystal of calcite is clear (transparent), any writing underneath of it is "doubled" when viewed. This effect is shown in the first two images below. The first image is of  a small, clear crystal, about 3 cm across. It is sitting on a white piece of paper, with the letter "X" written on it. Light is refracted through the crystal and produces the "doubling" effect (called double refraction). Single perfect crystals like this one, have what is known as the rhombohedron shape (a six-sided prism).  

The second image is of a longer (7.5 cm) clear rhombohedron crystal, sitting on a white piece of paper with the word "calcite" written on it. 

There are also other colored varieties of calcite, as shown in the following "color wheel." For scale, the green crystal is 4 cm length.

These varieties, which I had available, show a range in color of calcite, from orange, red-orange, red, yellow-brown, clear, light yellow, blue, green, and (in the center) honey color. There are other intermediate colors. Impurities in the crystals cause the different colors.

The "black looking" crystal at the bottom of this "color wheel" is actually a clear crystal, and the black background used in making the photo shows through this clear crystal.

A last picture, shown below, is of a stalactite of calcite from the ceiling of a cave. The calcite crystals formed by dissolving out of underground water and dripping into an open space (a cave). The structure is 5 cm in height.

Saturday, September 2, 2017

Eocene oyster Cubitostrea sellaeformis

If you have been a reading my posts for the last three years, you will know that I have a strong interest in fossil oysters. I return to them with this post, which concerns the middle Eocene (about 45 million years old) oyster Cubitostrea sellaeformis (Conrad, 1832), known from Alabama, Georgia, Louisiana, North Carolina, South Carolina, Texas, and Mexico. This oyster has a strongly arched, large-size shell, and, if you look at the digital image below, you can see the U-shaped "line" that separates the two valves shown in side view.

I collected this complete specimen in 1989, when I visited southern Alabama. The specimen is from the upper part of the Lisbon Formation. The lower valve (left valve), which is thick and heavy,  sat on the bottom of the shallow ocean. The upper valve (right valve), which is lid-like is smaller and comparatively lighter.

This is the exterior of the lower valve, which is 14 cm long.

This is the exterior of the upper valve, which is 13 cm long. The "ears" (auricles) are part of the other valve.

These are the interiors of both valves, with the lower valve on the left side of the picture, and the right valve on the right side of the picture. If you look closely, you can see the muscle scar, especially on the upper middle part of the lower valve.

If you are wondering what caused the peculiar shape of this species, "join the crowd." No one has determined the answer with any degree of certainty. Its large size allowed it to live in shallow-marine waters in front of barrier beaches (unlike brackish-water, lagoon-living smaller oysters). It is possible that individuals crowded together and took on unusual shapes so as to withstand agitated-water conditions. The "ears" possibly served as stabilizing "anchors" for the lower valve.

Saturday, August 19, 2017

Two Late Cretaceous species of the bivalve Pterotrigonia

In some cases, recognition of two species of the same genus can be problematic. In the examples shown here, however, the recognition  is straightforward. The species are of a shallow-marine bivalve (clam) of Late Cretaceous age from the west coast of North America. These species lived as burrowers in subtidal shelfal depths.

Pterotrigonia klamathonia, 4.5 cm length, Santa Ana Mountains,
Turonian age, Orange County, southern California.

Pterotrigonia klamathonia (Anderson, 1958), which is of Turonian age, is characterized by its closely spaced radial ribs, 20 to 25 in number.

Pterotrigonia evansana, 4.5 cm length, Campanian age,
Simi Hills, Ventura County, southern California.
Pterotrigonia evansana (Meek, 1858) is geologically younger and is of Coniacian through Campanian age (see time table below). This species is characterized by its widely spaced radial ribs, commonly 10 or so in number.

Genus Pterotrigonia, which is the state fossil of Tennessee, is extinct. It belongs to the family Megatrigoniidae, whose geologic time range is latest Jurassic through the end of the Cretaceous. The family was very widespread in the world during the Cretaceous.

Saturday, August 5, 2017

"Desert Roses"

“Desert roses” are clusters of crystals of baryte or gypsum containing inclusions of sand. “Baryte” is the new official spelling of the mineral barite. “Desert roses” are also called “sand roses,” “rose rock,” “gypsum rose,” and “gypsum rosettes.” The clusters can be spherical shaped, irregular, or columnar shaped. They are commonly red or reddish brown.

These three pictures shown baryte "desert roses," and the largest one shown above is 2 inches in diameter. They feel heavy to the touch because of the element barium.

"Desert roses" form as evaporites when shallow waters (marine lagoons or lakes) rich in sulfates containing the elements barium or calcite precipitate out of solution. 

“Desert roses” are not that common although it seems that every collector I know has one or two. They are found in Kansas, Oklahoma, Arizona, California, Egypt, and a few other places in the world where lake beds have dried up and become evaporites. They can be of any geologic age (e.g., Permian [250 million years old] or Plio-Pleistocene [less than about 3 million years old]).

Some of the most famous and largest (up to 39 inches tall and weighing as much as 1,000 pounds) specimens of “desert roses” are from the Permian-age (about 275 million years old) Garber Sandstone in Oklahoma. In fact, in 1968, “desert roses” became known as the official state rock of Oklahoma.

Like the one shown above (1.5 inches long), not all clusters of baryte contain inclusions of sand nor do they necessarily have to form as evaporites. 

Saturday, July 22, 2017

Sodalite and lapis lazuli

Sodalite can be confused with the rarer and more expensive lapis lazuli (shortened or casual version of this word is lapis), which is also blue. This post deals with how to tell them apart.

Sodalite is a mineral. It is named for its sodium content, consists of the elements sodium, aluminum, silicon, oxygen, and chlorine. It belongs to a group of minerals called the feldspathoids, which resemble feldspars but have a different crystalline structure, a much lower silica content (i.e., feldspathoids are never found in rocks congaing primary quartz), and contain sulfur or chlorine. Sodalite is an ornamental gemstone and is commonly used in jewelry or in making bookends, etc. It is best known for its blue color, but it can also be gray, yellow, green, and commonly mottled in color. It commonly has white veining. It rarely has inclusions of pyrite, and it is not opaque (thus light can transmit through its edges).
Bookends made of sodalite. They are 13 cm hight.

Other side of the bookends shown above.
A small piece of sodalite (5 cm maximum dimension) with a polished surface.
Sodalite has poor cleavage, therefore, it is useful for making carvings of animals. This mineral is commonly found as vein fillings in plutonic igneous rocks (such as nepheline syenites). Associated minerals are microcline, albite, calcite, fluorite, and baryte (barite). It is found in Canada (Ontario, Quebec, and British Columbia), as well as in Maine and Arkansas. 

Sodalite is a "poor man's lapis lazuli."

Lapis lazuli is a metamorphic rock. The most obvious and important  component of this rock is the mineral lazurite, a feldspathoid silicate mineral consisting of sodium, calcium, aluminum, silicon, oxygen, chlorine, and sulfur. It is the presence of sulfur that gives lazurite its intense deep blue color. Most lazurite also contains the minerals calcite (white), sodalite (blue) and sparkling pyrite, as well as small amounts of mica, hornblende, etc.  

The gem form of Lapis lazuli has been prized since antiquity for its deep-blue color. This rock has been mined for thousands of years in Afghanistan and Pakistan (note: "lapis" is an Arabic word). It is opaque, thus light does not transmit through its edges. Pyrite is commonly present, but in minor amounts.
A small piece of polished gem-quality lapis lazuli (3 cm maximum
 dimension). Notice the flecks of pyrite.

Flip side of the lapis lazuli shown above. Notice the vein of calcite
with some pyrite veinlets.
Lapis lazuli takes an excellent polish and can be made into jewelry, carvings, mosaics, ornaments, small statues, and vases.