Rheniite

Rheniite is a very rare rhenium sulfide mineral (ReS₂). It forms metallic, silver grey platey crystals in the triclinic - pinacoidal class.[1] It has a specific gravity of 7.5.

Rheniite
Rheniite, hardened lava on the backside, from the Kudriavy Volcano, Kurile Islands
General
Category	Sulfide mineral
Formula (repeating unit)	ReS2
Strunz classification	2.EB.35
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 6.47 Å, b = 6.368 Å c = 6.401 Å; α = 105° β = 91.59°, γ = 118.9°; Z = 4
Identification
Color	Black, red translucent
Crystal habit	Platey
Luster	Metallic
Streak	Black
Diaphaneity	Opaque, translucent in thin fragments
References	[1][2]

It was discovered at the Kudriavy Volcano, Iturup Island in the Kurile Islands, Russia[3] and approved in 2004. It is found in active hot fumaroles on the volcano.

Rheniite is the first mineral of the element rhenium to be found. The second known approved rhenium mineral is tarkianite, being also a sulfide. Almost all commercially mined rhenium is retrieved as a by-product of molybdenum mining as rhenium occurs in amounts up to 0.2% in the mineral molybdenite. A discredited rhenium sulfide known as zappinite does not appear to be valid.

Rheniite has also been reported in the Pagoni Rachi Mo–Cu–Te–Ag–Au deposit in northeastern Greece where it occurs with molybdenite in quartz veins associated with an epithermal system in a dacite porphyry.[4]