Sinoite

Sinoite
Crystal structure of sinoite. Atoms: red – O, blue – N, gray – Si.[1]
General
Category	Silicate mineral
Formula (repeating unit)	Si2N2O
IMA symbol	Sno[2]
Strunz classification	1.DB.10
Dana classification	1.3.9.1
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) H-M symbol: (mm2)
Space group	Cmc21
Unit cell	a = 8.84 Å, b = 5.47 Å c = 4.83 Å, Z = 4
Identification
Color	From colorless to light gray
Cleavage	None
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.83
Optical properties	Biaxial (-)
Refractive index	nα = 1.740 nβ = 1.855 nγ = 1.855
Birefringence	δ = 0.115
References	[3][4][5]

Sinoite is a rare mineral with the chemical formula Si₂N₂O. It was first found in 1905 in chondrite meteorites and identified as a distinct mineral in 1965. Sinoite crystallizes upon meteorite impact as grains smaller than 0.2 mm surrounded by Fe-Ni alloys and the mineral enstatite. It is named after its SiNO composition and can be prepared in the laboratory as a silicon oxynitride ceramic.^[6]

The crystalline structure of silicon oxynitride is built by SiN₃O tetrahedra connected through oxygen atoms along the c axis and through nitrogen atoms perpendicular to it. The strong covalent bonding of this structure results in high flexural strength and resistance to heating and oxidation up to temperatures of about 1600 °C.^[7]