User:Rendallina/Buckhornite

Buckhornite is very rare and relatively new mineral from abandoned Buckhorn mine in Colorado near Jamestown. It is closely associated with tetradymite, and aikinite. The chemical formula of Buckhornite is AuPb2BiTe2S3. Common impurity is Cu. Essential elements are:Au,Bi,Pb,S, and Te. In reflected light it is brownish gray, bireflectant and opaque. The symmetry orthorhombic, and space group is Pmmm. Have hardeness of 2.5 (Mohs scale) and specific gravity of  8.5 g/cm3

Composition
Buckhornite is lead subjugated sulfide-telluride, Bi and Au are essential elements. However,Sb also found in natural Buckhornite ( not senthisezed ), which contest may exid up to 3.4 wt%  there are also trace of Cu could occure which found less rarely than Sb. Empirical formular Au1.02Pb1.97Bi1.04Te2.06 derived from averaged composition and distinguished 9 atoms in formula unit gives chemical formula of Buckhornite, that  is AuPb2BiTe2S3. It is considered as a member of homologous series and belongs to sulfides group.

Structure
Crystal system of Buckhornite is orthorhombic. Chemical investigation was made by electron-microprobe on synthetic buckhornite. The crystal structure was determinide by X-ray refraction, shows that buckhotnite’s space group is Pmmm. The atomic arrangement shows a distinct layer structure formed by two different sheets. Fist planar AuTe4 configurations are edge-connected in [100]; they are linked by Te contacts and parallel to (001). Te and Au atoms are in a distorted arrangement in four fold rotation. Second sheet of (Pb,Bi)S are located between these AuTe2 layers. Cell parameters are: a = 4.09Å, b = 12.24Å, c = 9.32Å. Ratio a:b:c = 0.334 : 1 : 0.761Article text. .

Physical Properties
Buckhornite’s streak is gray, it is sectile and flexible. Fracture is spatially proposed on elongated blades and it has  well-develop cleavage. Color predominantly is black. Luster is metallic due to high percentage of Lead (Pb), for the same reason it has very high specific density, that is 8.25 gm/cc. Because crystal is euhedral it shows good external form. The mineral could be scratch by finger nail which tells that hardiness of Buckhornite is 2-2.5 on Mohs scale.

Geologic occurrence
Most Buckhornite were synthesized in laboratories in hope of finding high-temperature superconductive materials. Buckhornite is closely associated with Nagyagite Pb5Au(Te,Sb)4S4-8 which is more available in nature than Buckhornite, they both have same physical properties, X-ray data,   and  close chemical formula that assign them to parts of homologous serious. Due to unavailability of Buckhornite in nature it  crystal structure were  assumed that of Nagyana, that have been proved to be correct after  comparing crystal structure of synthesized Buckhornite. However, order in Au and Te atoms was confirmed in Buckhornite only. In Buckhornite form slice from two sheets of (Pb,Bi)S whereas in Nagyagite slice consist of four (Pb,Sb)S sheets. Buckhornite is extremely rare mineral in nature. Little specimens were found in Colorado, mount Buckhorn ( the name originated from)  in 1992, and few years later some were discovered in Czech republic. Buckhorn mount minded primary for silver and lead. The first collaborative authors of this mineral were Francis, Crindle, Stanley, Lange, Shieh. They describe occurrence, optical behavior, morphology, cell metrics, X-ray data. Hoever the first person who encounter this mineral, in 1940, was Professor of Harvard Cornelius Hurlbut. He made partial analysis of crystal where he reviled presence of Au and Pb and after that the mineral was forgotten for almost 50 years until current, 1990s investigation.