Macdonaldite
- IMA 1964-010
Macdonaldit is a very rarely occurring mineral from the mineral class of " silicates and Germanates ". It crystallizes in the orthorhombic crystal system with the composition BaCa4 [ Si8O18OH ] 2 • 10H2O, that is chemically a hydrous barium-calcium - silicate.
Macdonaldit usually develops needle-shaped and perpendicular to the a axis elongated crystals to about six millimeters in length, which are usually colorless, transparent and on their surfaces a glass-like shine. The mineral can also be found in the form of fibrous, granular or radialstrahliger mineral aggregates, training appear due to their multi-crystalline by multiple refraction know the transparency decreases accordingly and especially in fibrous aggregates, an undulating glow similar to that produced from silk.
With a Mohs hardness of 3.5 to 4 Macdonaldit belongs to the more medium-hard minerals that are similar to the reference mineral fluorite ( 4) can be easily scratched with a pocket knife.
Etymology and history
Was first discovered Macdonaldit on the rivers Rush Creek and Big Creek in Fresno County of the U.S. state of California. Described it in 1965 by John T. Alfors, Melvin C. Stinson, Robert A. Matthews and Adolf Pabst, the named the mineral after the American volcanologist Gordon Andrew MacDonald ( 1911-1978 ). In addition to the Macdonaldit their first description included six more Bariumminerale from the excavation site in eastern Fresno County: Krauskopfit, Walstromit, Fresnoit, Verplanckit, Muirit and Traskit.
Type material of the mineral is stored among others in the "California Division of Mines & Geology " in San Francisco ( California, USA).
Classification
In the now outdated but still in use 8th edition of the mineral classification by Strunz the Macdonaldit belonged to the mineral class of " silicates and Germanates " and then to the Department of " layer silicates ( phyllosilicates ) ," where he, together with known Cymrite, Delhayelith, Hydrodelhayelith, Kampfit Lourenswalsit, Monteregianit - (Y), Rhodesit, Tienshanit and Wickenburgit formed a distinct group.
The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the Macdonaldit also included in the category of " silicates and Germanates " and there in the department of " layer silicates ( phyllosilicates ) ". This division, however, is further divided according to the structure of the layers, so that the mineral is "with 4 and 6 rings involved double networks " to find according to its construction in the subdivision where it only together with Rhodesit the unnamed group 9.EB.05 forms.
The mainly common in English-speaking classification of minerals according to Dana assigns the Macdonaldit in the class of " silicates and Germanates " and there in the department of " layer silicates: Two-dimensional infinite layers with other than six -membered rings " one. Here he is with Rhodesit, Monteregianit - (Y), Delhayelith, Hydrodelhayelith and Seidit - (Ce ) in the " Rhodesitgruppe " with the system no. 72.05.01 within the subdivision " layer silicates: in other than six -membered rings Two-dimensional infinite layers: corroded and complex situations" to find.
Education and Locations
Macdonaldit forms in the form of veins, brittle crusts or eingesprenkt in sanbornite and quartz -containing metamorphic rocks.
As a very rare mineral formation Macdonaldit previously could (as of 2011) only detected in a few samples or few localities. Apart from its type locality, the rivers Rush Creek and Big Creek in Fresno County, the mineral found in the United States still in the sanbornite deposit at Trumbull Peak west-northwest of Incline in Mariposa County and the " Baumann prospect " in the Chicken Coop Canyon near the town of Exeter in Tulare County.
The only other previously known locality is Italy, more specifically, the pit Vispi in San Venanzo in the province of Terni ( Umbria ).
Crystal structure
Macdonaldit crystallizes in the orthorhombic space group Cmcm ( Raumgruppen-Nr. 63) with the lattice parameters a = 14.08 Å; b = 13.11 Å and c = 23.56 Å and four formula units per unit cell.