Wiluite
A variety of Minerals
What is Wiluite?
Wiluite is a dark green, brownish, or black blocky silicate mineral with formula: Ca19(Al,Mg,Fe,Ti)13(B,Al,[ ])5Si18O68(O,OH)10. It has a Mohs hardness of 6 and a specific gravity of 3.36. It has a vitreous lustre, poor cleavage and an irregular brittle fracture. It crystallizes in the tetragonal system and occurs as well-formed crystals with good external form. It is isostructural with the vesuvianite group and is associated with wollastonite and olive-green grossulars (viluites) in a serpentinized skarn. The minerals that wiluite and viluite refer to have often been confused, and may refer to grossular, or wiluite. It was discovered in the 1990s and named for the Wilui River region, Sakha Republic (Yakutia), Russia. Viluite was introduced as a mineral name twice. Von Leonhard used it for a mineral that was considered the same as vesuvianite. However, that material was recently shown to be rich in boron and thus different from vesuvianite. In 1998 that material was named Wiluite. The other author to introduce viluite was Severgin, who used it in reference to what is widely known as grossular, a member of the garnet group.
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Quick Facts
Physical Properties
- Color
- Dark green
- Hardness (Mohs)
- 6
- Streak
- Colourless
Chemical Properties
- Chemical Formula
- Ca19MgAl4(Al,Mg)8(B,☐)4☐[Si2O7]4[(SiO4)10]O(O,OH)9 The formula is a partial simplification of the full structural formula. The large (VII-IX)-coordinated (X4)2(X3)8(X2)8(X1) sites are here combined (e.g. Ca19) and are typically filled with Ca, although other large cations such as the REE may be present. The square-pyramidal Y1 site can host a variety of M2+ and M3+ ions and is the basis for the distinction of several species. The VI-coordinated Y2 site typically is filled with Al, whereas the also VI-coordinated Y3 site may contain Al, Mg, and other cations of similar charge and size. The tetrahedral T1 site is typically vacant but may contain B (less commonly Al); the trigonal T2 site is also typically vacant but may also contain B. Some of the (SiO4) may be replaced by (H4O4), akin to the Si4+ ↔︎ 4H+ hydrogarnet substitution. Among the oxygen that are not part of the silica tetrahedra, there are eight "O11" that typically occur as OH, two "O10" that are typically O & OH or OH & OH (the latter arrangement notably when Y1 is an M2+ cation). There may also be up to three "O12" that in most vesuvianite-group minerals are absent (and are not included here), but may be present particularly when T1 is occupied. With increased filling of the T2 site by B, various charge balance mechanisms involving either increased O for OH in the "O11" position and/or increased M2+ for M3+ substitution in the Y3 site are necessary (see idealized formula for full T2 occupancy by B). In lieu of an explicitly defined "end-member" wiluite, more generic representations of site fillings are retained here, pending newer data.
- Elements
- Al, B, Ca, H, Mg, O, Si
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