Mineral Two on Moh's Scale (or) some really big Selenite Crystals

Monday, December 24, 2007
I trust that most people noticed the spectacular image on the inside cover of this issue of Wat On Earth. I must admit that when I received this photograph together with several others about a year ago from a friend in the United States my initial reaction was … “Well, someone has been busy with Photoshop!” However, a little checking on the web revealed that these are genuine photographs and they tell a very unusual tale of an interesting locality in Mexico. 
 
Now I (like many of you) have found individual selenite crystals before. In fact selenite and its more massive and shiny siblings (gypsum and satin spar) were some of my favourite mineral finds as a child growing up in the vicinity of sequences of Triassic marls in South Wales. On my Saturday afternoon walks back from the National Museum of Wales in Cardiff, my coastal route took me along the cliffs at Penarth where nodules of gypsum were falling out of the face, and, if I was really lucky, I could find veins of Satin Spar. Later in my geological career I found even larger gypsum crystals developed in mudflows derived from Jurassic and Tertiary shales in the Midlands  and Eastern England (Figures 1 and 2) and finally saw some really large crystals exhibited in Tuscon (Figure 3). However, all of them pale in comparison to the specimens found at the Naica Mine in Chihuahua, Mexico. 
 A Selenite (CaSO4 .2H2O) crystal.
 

Figure 1: A Selenite (CaSO4 .2H2O) crystal.

The Naica mine is situated at 27° 50' N, 105° 29' W; approximately 470 km south of El Paso, Texas. Located in carbonate rocks the mine has been exploited for galena, sphalerite and silver within skarns in the Cretaceous limestones of the region. High temperature (55ºC) calcium carbonate and calcium sulphate-charged groundwater have been brought into the limestone along two large fault systems known as the Gibraltar and Naica Faults. The hydrothermal waters, likely emanating from, or passing near, regional magma bodies are responsible for the deposition of the calcite and gypsum minerals — selenite and anhydrite — as well as the economic ore bodies. 
 Selenite crystals from central England
 

Figure 2: Selenite crystals from central England.

The Naica Mine was first exploited in 1794 by prospectors who discovered silver. By the late 1800s it was being examined as a source of lead and zinc ores and the mine was formally opened in 1900. The Mine was closed in 1922, reopened in 1935 and was taken over by the Penoles Group, the current owners, in 1961. The Naica Mine’s large gypsum and calcite crystals (The Cave of the Swords) have been known since 1910 (Foshag 1927) and another similar occurrence at the Potosi Mine in Santa Eulalia, near Chihuahua, about 100 km north of Naica, was discovered in 1912 .  
 
In 1910 selenite crystals between one and two metres long were recorded from the Maravilla property (now within the Naica Mine) and the adjacent Lepanto property at depths of about 120m. It was assumed by Foshag (1927) that the cavities that housed the crystals were created by surface water dissolution and also owe their origin to the oxidation of sulphide ores. He was not able to resolve how the crystals that are now found in air-filled cavities were formed. 
 
 A group of Selenite “swords” about 1 m long in the Tucson Desert Museum
 

Figure 3: A group of Selenite “swords” about 1 m long in the Tucson Desert Museum

More recent and far deeper mining exploitation set the stage for the latest discoveries within the mine at the 300m (1,000 feet) level. In 1999 the Penoles Group calculated that regional water levels had been sufficiently lowered to allow penetration of the Naica Fault. On December 4th 1999 Juan and Pedro Sanchez were drilling a new tunnel that broke through into a steam-filled cavity that houses the giant selenite blades in the Cave of the Crystals (illustrated on the inside front cover). The crystals were truly spectacular with grey white and translucent forms more that 10m long and up to 2m in diameter. These are amongst the largest crystals ever recorded. Thanks to the forethought of the mining engineer, Roberto Gonzales, action was swiftly taken to minimize any damage to this natural wonder. 
 
 A person exploring the Naica Mine with giant crystals in foreground.
 

Figure 4: A person exploring the Naica Mine with giant crystals in foreground.

The “Cave of the Crystals” is now protected by an iron gate to prevent vandalism and looting of the crystals although the ambient conditions inside the cavern (60°C and 100% humidity) almost provide a sufficient natural deterrent!  The cave has been visited by a number of photographers and cave exploration groups and attempts are reportedly being made by the Penoles Group to open the cave to the public through installation of electrical lighting and air conditioning. Perhaps more on this later. 
 

Reference: 

Foshag, W.F., 1927. The Selenite Caves of Naica, Mexico. Amer. Min, V. 12, pp. 252 – 256. 
 
Alan V. Morgan. 
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