Tusionite is a rare colorless to transparent to translucent yellow brown trigonal borate mineral with chemical formula: MnSn(BO₃)₂. The mineral is composed of 18.86% manganese, 40.76% tin, 7.42% boron, and 32.96% oxygen. It is a late stage hydrothermal mineral and occurs rarely in granite pegmatites in miarolitic cavities.

Tusionite
Tusionite, a rare gem found in Tajikistan
General
Category	Borate minerals
Formula (repeating unit)	MnSn(BO3)2
IMA symbol	Tsn[1]
Strunz classification	6.AA.15
Crystal system	Trigonal
Crystal class	Rhombohedral (3) H-M symbol: (3)
Space group	R3 (no. 148)
Identification
Formula mass	291.26 gm
Color	Colorless, light yellow to yellow brown
Crystal habit	Thin platy crystals in rosettes
Cleavage	[001] Perfect
Mohs scale hardness	5–6
Luster	Vitreous
Streak	white
Diaphaneity	Translucent
Density	4.73
Optical properties	Uniaxial (−)
Refractive index	nω = 1.854 nε = 1.752
Birefringence	δ = 0.102
Pleochroism	Orange yellow
References	[2][3][4][5]

Tusionite was named for the location where the mineral was first discovered and described in 1983 in the Tusion River Valley in the Pamir Mountains of Tajikistan. Tusionite has also been reported from Recice in the Czech Republic and in pegmatites at Thomas Mountain, Riverside County, California.