Thursday, February 23, 2017

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.

Thursday, February 9, 2017

Trigonarca californica

This post concerns a common Late Cretaceous bivalve (clam) that lived in California approximately 92 million years ago (Turonian time). It is Trigonarca californica Packard, 1922, which is known from northern California (Siskiyou County) to southern California.

The specimens shown below are from the Santa Ana Mountains of Orange County, and they were collected from the Baker Canyon Member of the Ladd Formation. As this locale, where specimens can be abundant, this species lived in sandy, warm, shallow-marine waters. A collector recently kindly donated these specimens.

Right-hand valve of Trigonarca californcia Packard. Length 4.4 cm.

This unusual specimen shows the somewhat separated valves of a formerly closed-valved specimen
of Trigonarca californica Packard. The hinge with its distinctive teeth are nicely preserved. Length  of the left-hand valve (at the front of the photograph) is 4.3 cm.

The sturdy shell of this species has the shape of a rounded triangle. Its teeth (dentition) are distinctive and consist of numerous, relatively heavy, short, straight teeth along its hinge.

Genus Trigonarca, which belongs to family Glycymerididae, was widespread, with occurrences in North America, Europe, South Africa, and India. Trigonaraca is of Late Cretaceous age.

Saturday, January 28, 2017

Corundum crystals from Southern California

The mineral corundum, which is second only to diamond in terms of hardness, consists of aluminum oxide (Al2O3). Corundum comes in a variety of colors, depending on the trace amounts of other minerals (e.g., rutile = titanium oxide) it contains.

The color can be red, blue, yellow, brown, green, or purple to violet, and some crystals contain color zones. Pure corundum is white. If the color of corundum is red, it is called rubyIf the color is blue, it is called sapphire.

A friend recently gave me some corundum crystals from Cascade Canyon, San Gabriel Mountains, about 2 miles southwest of Mount Baldy, which is near the town of Upland in Los Angeles County, Southern California

A hand specimen (4 cm wide) containing small, scattered
 crystals of corundum. The color is between ruby and sapphire.
Most collectors would most likely refer to these crystals as ruby.
Close-up of the left-middle side of the hand specimen shown above.
The lenticular crystal in the lower right side is 4 mm long.

The corundum at the Cascade Canyon locality formed when complexly deformed sedimentary rock (of Paleozoic age) was contact metamorphosed (heated up) by small granitic intrusions (of Cretaceous age). 

If you want to see outcrop pictures and more information about this locality, just Google the phrase:  Cascade Canyon ruby

Sunday, January 15, 2017

A middle Eocene heart urchin

Heart urchins, also called spatangoids, are echinoderms (sand dollars, sea stars, etc.), which are generally characterized by having 5-rayed (pentameral) symmetry. This post focuses on a middle Eocene heart urchin known as Schizaster diabloensis Kew, 1920. It was named for its occurrence in sedimentary layers near Mount Diablo, just east of San Francisco.

A hand specimen of siltstone rock from the Llajas Formation has three specimens of
S. diabloensis on the same bedding plane. The hand specimen is 5 cm (2 in.) wide.
This species of heart urchin was common in northern and southern California during the middle Eocene (approx. 47 million years ago). The specimens shown here are from the Llajas Formation in Simi Valley, California. This formation was deposited in shallow-marine, warm-water conditions. The entire geologic time range for this species is late Paleocene through middle Eocene.

Five specimens of S. diabloensis from the Llajas Formation. The largest specimens are
  2 cm (0.8 in.) wide. All are top-side up.
Echinoderms, past and present, are strongly gregarious and can occur in great numbers on the ocean floor. Spatangoids have a fossil record extending back to the Cretaceous. They are burrowers and living below the surface provides protection against predators. During the Cretaceous, many new forms of predators evolved, which, which gave the force for some echinoderms (like spatangoids) to adapt to these adverse conditions by becoming infaunal (i.e., burrowers), mainly in fine-grained deposits, like siltstone.

You can readily see the five-rayed symmetry of the feeding grooves on the dorsal (top) surface of each specimen. The central groove, called ambulacrum III, is the longest and is sunken on most spatangoids, whereas the two posterior grooves are smaller. 

Monday, January 2, 2017

A Late Cretaceous stalked crinoid stem

Crinoid remains are extremely rare in the Late Cretaceous fossil record of California. A friend recently donated a stalked crinoid-stem fossil collected from Upper Cretaceous rocks in the Santa Ana Mountains, Orange County, Southern California. I have seen many fossils from these rocks but never a crinoid. Its geologic age is Turonian (about 90 million years old). The genus of this fossil is unknown.

This specimen is 8 cm long and 3 mm wide. I also put a modern-day crinoid "stem" (from Cuba) alongside, for comparison; it is 6.5 cm long and nearly 3 mm wide. You can definitely see that the fossil is, indeed, a crinoid.

Crinoids are echinoderms. Some other examples are sea stars (starfish), brittle stars, sea urchins, and sand dollars. Crinoids were very common in Paleozoic faunas, and their remains have contributed substantially to Paleozoic limestones. Crinoids today are less abundant than they once were, but at the present time there are approximately 25 stalked genera (all attached to the ocean floor and restricted to depths greater than 100 m). There are also about 90 or so unstalked genera, and these are able to swim about when they are adults.

This drawing shows the main morphologic parts of a stalked crinoid (i.e., having a column or "stem"). The "stem" was originally somewhat flexible during life and could sway slightly with the prevailing water currents.

Both the fossil and modern-day columns shown above in the photo are missing their calyx (where the stomach was located) and their arms. 

Tuesday, December 20, 2016

Labradorite, a beautiful, dark-colored mineral with iridescence

Labradorite, which is derived from "red-hot" molten material called magma, is one of several phases (varieties) of the mineral plagioclase. All these phases have the same general formula (Ca, Na)(Al,Si)3O8. As the magma cools, a solid-solution series of different phases/varieties of plagioclase crystallize out sequentially, with varying amounts of Ca (calcium) and Na (sodium). The calcium and sodium ions mix in a continuous series with their ratio varying from 100% calcium and 0% of sodium, to the extreme opposite.

Labradorite consists of 50 to 70% calcium and occurs as blocky to lath-shaped crystals in calcium-rich igneous (magma-derived) rocks, such as basalt, gabbro, and anorthosites. Labradorite is relatively uncommon, but some rocks consist almost entirely of this mineral.
A 3 foot-high slab of labradorite used as a pedestal for a monument.
You can see how some of the crystals are large and lath-shaped.
 The bluish-iridescence of this mineral is especially evident  in the upper part of the picture.
One of the most memorable features of labradorite is its iridescent play of colors, which results from this mineral’s peculiar reflection of light. The reflection is caused by internal fractures that reflect light back and forth.

Polished piece (width 5.7 cm, 2.5 in.) of labradorite showing its iridescence. To see it, one must tilt the specimen at just the right angle to the prevailing light; a few degrees too much or too little tilt, and the iridescence disappears.
Labradorite-bearing rocks occur worldwide, especially in Labrador, Canada (where this mineral inherited its name) and in Norway.

Labradorite is used for making floor tiles, kitchen counter-tops, tables, and benches. It is also a popular gemstone.

For more information about the solid-solution series that is associated with the formation of plagioclase, please Google the term "Bowens Reaction Series." 

Note: I used to include links to topics covered in my posts, but recent changes in Google Posts now deactivate these links when posts go to the "Archive" file.

Saturday, December 10, 2016

Pyrite cubes

Pyrite is a mineral that most people have either heard about or seen. It superficially resembles gold, yet the chemical and physical properties of pyrite make it easy to distinguish it from gold. The main differences are listed below:

Pyrite is an iron sulfide, with the chemical formula FeS2. Gold's chemical formula is simply Au.

Pyrite crystal system is isometric (cubic), and crystals formed under perfect conditions will be cubes (as shown below). Gold is rarely found as crystals; rather, it occurs in nuggets, irregular blobs, or small flakes. It cannot occur in cubes.

Single cube of pyrite, width 2.9 cm (from Spain).
Cluster of intergrown pyrite cubes, total width 5 cm long (from Spain).
Pyrite is harder with a value of 6.5 on the Moh's Hardness Scale [i.e., a scale with talc and graphite the softest minerals (value of 1), and with diamond the hardest mineral (value of 10). Gold has a value of 2.5. Gold is very soft; so much so that other elements (e.g., copper, nickel, or platinum) have to be added to it (in the form of an alloy) in order to make jewelry.

Note: An ordinary steel knife (hardness value of 4.5) cannot scratch pyrite but can easily scar gold. The superior hardness and brittleness of pyrite also cause it to smash into bits if struck with the tip of a high-quality knife or shatter into small pieces, if hit with a hammer.

Pyrite's streak (its powdered from when scratched across an unglazed porcelain plate, called a streak plate), is black. Gold's streak is brassy yellow.

Small irregular piece of pyrite with its characteristic black streak on a "streak plate.
Pyrite is less dense, and small flakes normally wash away when placed under running water. Gold flakes are very dense and will sink. This is why "gold panning" works so well for finding gold.