Fukuchilite
Fukuchilite
General
Category	Sulfide mineral
Formula (repeating unit)	Cu3FeS8
IMA symbol	Fuk[1]
Strunz classification	2.EB.05a
Crystal system	Cubic
Crystal class	Diploidal (m3) H-M symbol: (2/m 3)
Space group	Pa3
Unit cell	a = 5.58 Å: Z = 1
Identification
Color	Dark brownish gray; pinkish brown in polished section
Crystal habit	As intergrowths with pyrite and covellite, grains less than 1 μm
Mohs scale hardness	4 - 6
Luster	Submetallic
Diaphaneity	Opaque
Specific gravity	4.86
Optical properties	Isotropic
References	[2][3][4]

Fukuchilite, Cu
₃FeS
₈, is a copper iron sulfide named after the Japanese mineralogist Nobuyo Fukuchi (1877–1934), that occurs in ore bodies of gypsum-anhydrite at the intersection points of small masses of barite, covellite, gypsum and pyrite, and is mostly found in the Hanawa mine in the Akita prefecture of Honshū, Japan where it was first discovered in 1969. It occurs in masses within the third geologic unit of the Kuroko type deposits within the mine.

As a copper, iron sulfide, it is placed in the same group as bornite and chalcopyrite, and most fukuchilite locations are found in relatively close proximity to these minerals. Fukuchilite was found to have a reflection color very similar to bornite and bright pinkish brown in air, while being a purplish brown in oil. Also, it was found to have a reactivity lower than pyrite, but distinctly higher than bornite.^[5] It has a Mohs hardness of 4–6, a specific gravity of 4.9, and a sub metallic luster, composed of 11.1% iron, 37.9% copper, and 51.00% of sulfur.^[6]

It is in the isotropic cubic crystal system with symmetry: (2/m3), space group P a3. Much relating to the structure of the mineral is still under debate, and some believe that fukuchilite might actually be a form of villamaninite (Cu,Ni,Co,Fe)S
₂,^[7] but fukuchilite currently still holds its mineral status as there is currently not enough evidence to discredit an already accepted and titled mineral.^[6]

References