Wakabayashilite is a rare arsenic

Arsenic

Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

, antimony

Antimony

Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...

 sulfide mineral

Sulfide mineral

The sulfide minerals are a class of minerals containing sulfide as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenides, the tellurides, the arsenides, the antimonides, the bismuthinides, the sulfarsenides and the sulfosalts...

 with formula [(As,Sb)₆S₉][As₄S₅].

Discovery and occurrence

Wakabayashilite is a rare mineral first discovered in Nevada prior to 1930 but it was mistaken for a hair orpiment. In the 1970s, wakabayashilite was found in Japan and scientifically identified as a new mineral. It was named after Yaichiro Wakabayashi (1874–1943), a Japanese mineralogist of the Mitsubishi Mining Company in Japan. Wakabayashilite has been discovered later in several places around the world such as in Saha Republic (Russia), Zhuang region (China) and Hautes-Alpes (France). Originally, Wakabayashiltie was found in druses of quartz associated with orpiment

Orpiment

Orpiment, As2S3, is a common monoclinic arsenic sulfide mineral. It has a Mohs hardness of 1.5 to 2 and a specific gravity of 3.46. It melts at 300 °C to 325 °C...

 and realgar

Realgar

Realgar, α-As4S4, is an arsenic sulfide mineral, also known as "ruby sulphur" or "ruby of arsenic". It is a soft, sectile mineral occurring in monoclinic crystals, or in granular, compact, or powdery form, often in association with the related mineral, orpiment . It is orange-red in colour, melts...

, however, it was later found embedded in calcite

Calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

 as well. Generally, it mostly occurs with other arsenic minerals in high-arsenic gold deposits.

Structure and optical properties

Even though Wakabayashilite possesses the hexagonal pseudosymmetry, it actually belongs to the orthorhombic crystal system

Crystal system

In crystallography, the terms crystal system, crystal family, and lattice system each refer to one of several classes of space groups, lattices, point groups, or crystals...

 meaning it contains three axes of different length but are mutually perpendicular to one another. It also belongs to the space group

Space group

In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...

 Pna2₁. For its optical properties, Wakabayashilite is a biaxial mineral and its color in plane polarized light is golden-yellow to lemon-yellow and it also has strong pleochroism.

For now, since wakabayashilite is rare, it is of no obvious economical interest. However, there have been some recent research conducted on its interesting structure, pseudosymmetry and twinning.