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.

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.

Linarite, a beautiful blue mineral

Linarite is somewhat rare mineral with an intense deep blue color. It is a combined copper lead sulfate hydroxide mineral, which is made of up flat (monoclinic) crystals that are soft (hardness of only 2).

The specimen shown here is one that I collected back in 1964, when I was an undergraduate geology student. The specimen is from the world famous Blanchard Claims in the Hansonburg Mining District, Sierra Oscura Mountains, south of Bingham, New Mexico. When I visited the site, Ora Blanchard was the caretaker. I remember her as a very colorful character. She did not take kindly to thieves trying to sneak onto her property. She wore a pistol, and she also had a flock of geese to serve as "watch dogs."

Mrs. Blanchard allowed me to collect in the famous Royal Flush mine. The linearite crystals occurred with galena, aquamarine-colored fluorite, bladed barite, and druzy quartz, among with many other minerals. The rock matrix is the Pennsylvanian-age Madera Limestone, which was invaded by hydrothermal fluids (about 200°C) emanating from the nearby Rio Grande Rift. Supersaturated fluids moved along any open space and deposited beautiful crystals of the minerals, including linarite.

linarite hand specimen, length 7 cm (2.75 in.)

Mariposite from California

Mariposite is not an officially classified mineral, rather it is a chromium-rich variety of the green mineral phengite. The green color is imparted by the element chromium. Mariposite/phengite occurs in a quartz-rich metamorphic rock also called mariposite. This rock, which is streaked with thin bands of green color alternating with bands of grayish quartz, is named for its occurrence in the southern-most part of the “Mode Lode” (i.e., gold) region northeast of Merced and southwest of Yosemite Valley in Mariposa County, Northern California. Mariposite is associated with gold-bearing quartz veins and has been reported as occurring with visible gold.  

Mariposite rock, length 7.5 cm (3 in.)

Mariposite formed when the rock serpentinite, which was derived from the Earth's mantle, became altered under pressure by mineral-laden hot (650°F) water. These hydothermal fluids flowed upward along fractures, faults, and fissures, and where these fluids reacted with serpentine, they formed deposits of quartz, chromium-rich mica, sulfides, and gold.

Mariposite is a popular landscaping stone. It is also used as a building stone (veneer on walls), as well as for jewelry (as the trade name "Emerald Quartz).

A cut and polished mariposite stone (length 3 cm, 1.2 in.)

The large shallow-marine gastropod Forreria belcheri, past and present history

Forreria belcheri (Hinds, 1843) is a rather large gastropod found today along the west coast of Southern California and Baja California, Mexico. Forreria belongs to the family Muricidae, commonly referred to as the "rock shells." Forreria belcheri is the type species of genus Forreria.

This gastropod has a fossil record that extends back about 8 million years to the late Miocene. In the past, it ranged farther north (to central California) than it does today. It lives today mainly offshore on sandy bottoms in relatively shallow water of 60 to 100 feet deep, but it can also be found in bays, lagoons, and mudflats. This gastropod is carnivorous and uses its file-like radular teeth to drill holes through shells of oysters and mussels, as well as other mollusks. 

Forreria belcheri is characterized by a large, heavy shell that can be as much as 6 inches (15 cm) long. The shell is heavy and adult specimens have about 12 spiny nodes on its whorls. Its opening (aperture) is large and the anterior end of its shell is twisted and upturned. 

The first two views in the below series of photographs show the front (apertural) side of a modern specimen from Baja California, Mexico versus a Pleistocene specimen from Newport Beach Mesa, Southern California. The last two views of this series show the back side (abapertural) of the same modern and Pleistocene specimens, respectively.

Forreria belcheri: A modern specimen (13 cm height) vs. a Pleistocene specimen (11.6 cm height).

The next two views show the spatial arrangement of the spiny nodes on the spire whorls of the same two specimens: Modern specimen (on left) vs. Pleistocene specimen (on right). The Pleistocene specimen is somewhat worn and has its lower right-hand spire coated by bryozoans.

This last view, which is of the side of the modern specimen, shows how the twisted anterior end also turns upward (a characteristic feature).

Ancient wave-formed ripple marks

I have always enjoyed finding ancient examples of sedimentary structures that help a geologist determine depositional environments. The example I show here are ripple marks made by waves in a middle Miocene (about 13 million years old) lacustrine (lake) deposit in the Mint Canyon Formation, northern Los Angeles County, southern California.

The above picture is a vertical look at the ripple marks. The rock is siltstone, and these ripple marks were found in a section of muddy rocks containing very small freshwater gastropods.

This picture is a side view of the rock slab shown above. The up-current side of each ripple mark is low angle, whereas the down-current side is high angle. So, as viewed in this photograph, the wave current moved from right to left.

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.

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.