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).

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.

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.