Zincite

Zincite is the mineral form of zinc oxide (ZnO). Its crystal form is rare in nature; a notable exception to this is at the Franklin and Sterling Hill Mines in New Jersey, an area also famed for its many fluorescent minerals. It has a hexagonal crystal structure and a color that depends on the presence of impurities. The zincite found at the Franklin Furnace is red-colored, mostly due to iron and manganese dopants, and associated with willemite and franklinite.

Microscopic image of Zincite and Franklinite under normal light

Zincite
Crystal blades of zincite
General
Category	Oxide mineral
Formula (repeating unit)	(Zn,Mn)O
IMA symbol	Znc[1]
Strunz classification	4.AB.20
Dana classification	04.02.02.01
Crystal system	Hexagonal
Crystal class	Dihexagonal pyramidal (6mm) H-M symbol: (6mm)
Space group	P63mc
Identification
Color	Orange, yellow-orange to deep red, red, rarely yellow, rarely green and colorless to white
Crystal habit	Disseminated – occurs in small, distinct particles dispersed in matrix.
Twinning	On {0001}
Cleavage	On {1010}, perfect; parting on {0001}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	4
Luster	Subadamantine to resinous
Streak	Yellowish orange
Diaphaneity	Translucent, transparent in thin fragments
Specific gravity	5.64–5.68
Optical properties	Uniaxial (+)
Refractive index	nω = 2.013, nε = 2.029
Birefringence	δ = 0.016
References	[2][3]

Zincite crystals can be grown artificially, and synthetic zincite crystals are available as a by-product of zinc smelting. Synthetic crystals can be colorless or can range in color from dark red, orange, or yellow to light green.

Synthetic zincite crystals

Both natural and synthetic zincite crystals are significant for their early use as semiconductor crystal detectors in the early development of crystal radios before the advent of vacuum tubes. As an early radio detector it was used in conjunction with another mineral, galena, and this device was known as the cat's-whisker detector.