WHITE SANDS, NEW MEXICO

White Sands is in southern New Mexico, 16 mi west of the town of Alamogordo. White Sands, which covers 275 square miles, was established as a National Monument in 1933. It recently became a National Park. The elevation of the park is 4,235 feet.

White Sands is geologically situated between the San Andres Mountains to the west and the Sacramento Mountains, just east of Alamogordo, to the east. [Google Earth photo, 2018].

Aerial view, looking northwest. White Sands is in the distance, below a cloud layer on the horizon and "between" the two 
pointed-metal objects on the airplane wing.

White Sands consists of white gypsum sand dunes, the largest of their kind on Earth. 

During the Late Permian Period, about 250 million years ago, shallow seas covered the area. Evaporating seas left behind deposits of white gypsum crystals, consisting of hydrous calcium sulfate. Gypsum is very soft: 2 on the Moh's Hardness Scale. Fingernails can scratch gypsum. See my previous post on "Some Varieties of Gypsum" (Sept. 30, 2017).

Subsequent tectonism uplifted the San Andreas and Sacramento Mountains, and, over time, rain dissolved the gypsum deposits, and rivers transported the dissolved material to the nearby Tularosa Basin, which had no outlet the sea. The trapped water evaporated and gypsum was deposited (once again). Over time, weathering and erosion broke down the gypsum crystals into sand-size grains. In the last million years or so, prevailing winds from the southwest transported these crystals and dunes formed. There are transverse, parabolic, and barchan dunes present. 

It is quite a wonderful experience to visit White Sands. You can walk barefoot and not get burned by the sand, and you can roll around in the gypsum sands without getting abraded, like you would if the dunes were made of quartz grains (hardness 7).

Photography is a challenge because of the glare. It is like photographing a snow field.

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.)

Sandia Mountains orbicular granite

The Sandia Mountains, which are immediately northeast of Albuquerque, New Mexico, consist mostly of Precambrian "granite" [technically speaking it is a granodiorite/quartz monzonite], as well as some adjacent metamorphic rocks. Overyling the granite is the Madera Formation, a fossiliferous limestone of Pennsylvanian age. There is a profound erosion surface (unconformity) between the granite and the basal part of this limestone.

West face of Sandia Mountains, northeast of Albuquerque, New Mexico. Nearly 95 % of the face is granite.
 At the top of picture you can see the well-bedded Pennsylvanian fossiliferous limestone.
 The highest elevation area is called the Sandia Crest and is 10,678 feet (3,255 m). 

A very unusual and interesting rock, called a orbicular granite, is locally well known from about mid-way up the west face of the Sandia Mountains. It occurs in a somewhat dike-like (parallel sides) exposure, about 30 feet wide and perhaps several hundred feet long. The orbicular granite has very distinctive black-and-white orbicules. Most of them are about 2 inches in length and 1 inch wide. The longest ones are up to 5.5 inches, and the widest ones are 2.75 inches (note: not the same orbicule). The typical orbicule consists of a core rock of granite or feldspar, surrounded by a shell of black biotite, which is, in turn, surrounded by a shell of white oligoclase. The latter consists of radiating crystals. Some of the core rock appears to be biotite, but this is most likely just a function of the angle of the exposed surface.

A large piece of orbicular granite from the Sandia Mountains. Some of the centers
 of the orbicules are granite, others are feldspar, and many others appear to be biotite.
The scale (in centimeters) is 15 cm (6 inches) in length.

A hand specimen of the orbicular granite. The oligoclase crystals in the white shell are
 arranged perpendicular to the crystals in the underlying biotite shell.

The Sandia granite is one of the most age-dated granites in North America. Based on K/Ar and Rb/Sr age-dating, the orbicules and surrounding granite are approximately 1,300 to 1,350 million years old. Many researchers have done work on the orbicular rock, and the overwhelming majority of them suggested an igneous (magma) origin. Only a very few researchers have suggested a metamorphic (metasomatic origin) that involved reactions between xenoliths (hand-sized inclusions) and water-rich or vapor-rich magmatic fluids. The conditions of the orbicule formation have not been determined completely.