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.

Wiluite
Wiluite crystal from the Wilui River, Siberia
General
Category	Sorosilicates
Formula (repeating unit)	Ca19(Al,Mg,Fe,Ti)13(B,Al,[ ])5Si18O68(O,OH)10
Strunz classification	9.BG.35
Crystal system	Tetragonal
Crystal class	Ditetragonal dipyramidal (4/mmm) H-M symbol: (4/m 2/m 2/m)
Space group	P4/nnc
Unit cell	a = 15.716, c = 11.704 [Å]; Z = 2
Identification
Color	Dark green
Crystal habit	Typically as euhedral crystals
Cleavage	{100} poor
Fracture	Irregular
Mohs scale hardness	6
Luster	Vitreous
Specific gravity	3.36
Optical properties	Uniaxial (+)
Refractive index	nω = 1.721 nε = 1.725
Birefringence	δ = 0.004
References	[1][2]

The minerals that wiluite and viluite refer to have often been confused, and may refer to grossular,[3][4] or wiluite.[5][6][7] 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.[8]