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Exhibit of the Month 2 / 2018
Franklinite
The geological collections of the Regional Museum in Most primarily contain samples of minerals, rocks, and fossils from the immediate vicinity of the city. The vast majority of finds are related to the extraction of raw materials, especially lignite.
Samples of minerals that do not occur in the Most area also come from older museum collections. One of the most significant specimens is a sample of the mineral franklinite.
Chemical formula (Zn Mn 2+Fe2+)(Fe3+Mn3+)2O4. Franklinite crystallizes in the cubic system, is relatively hard - 6 - 6.5 on the Mohs scale. It is usually black, opaque, only thin fragments are translucent, and it has a metallic or sub-metallic luster. The mineral consists of 16.59% zinc, 18.58% manganese, 37.78% iron, with possible impurities of aluminum, titanium, or calcium.
Franklinite is not a very common mineral on a global scale. In the Czech Republic, it has only been found in Hraničná near Žulová, Jeseník district, as a microscopic component of hematite-magnetite ores. In other few localities around the world, this mineral occurs in a similar microscopic form or as amorphous masses.
The only area in the world where franklinite occurs in enormous quantities is the state of New Jersey in the USA. In Sussex County, there are two huge deposits, Franklin and Sterling Hill, where approximately 33 million tons of zinc and other ores were mined from the end of the 18th century until 1986. Only in this area is franklinite found in crystals.
From a mineralogical perspective, these two deposits are among the most species-rich in the world. A total of 361 mineral species have been described here (Bernard-Hyršl, 2004), and according to the mindat.org website (2018), the number has already increased to 376, of which 73 minerals were first described from these two localities. Most of the newly discovered minerals here do not occur elsewhere in the world.
The sample of franklinite from the collection of the Regional Museum in Most comes from old museum collections. The sample was probably donated to the museum by the Most mineral dealer Franz Thuma. The exhibit measures 2.5x4x5 centimeters. On a whitish calcite of fibrous structure, there is a cluster of 7 black, metallic-lustered crystals, the largest of which is 1 centimeter long. Our exhibit is therefore far from the record holder from the Franklin locality, which is a crystal with an edge length of 17 centimeters and is stored in the collections of the Smithsonian Institute in New York.
The mineralogical collection of the Regional Museum in Most is one of the few collections of regional museums in the Czech Republic that owns a sample of franklinite.