Mineral Information and Data:

ZnAl2O4 [CNMNC approved formula]

Named for the Swedish chemist and mineralogist, Johan Gottlieb Gahn (1745–1818), who discovered the mineral.

IMA status : Valid species (Pre-IMA 1807) Grandfathered
(8th edition) :  
  (07) Multiple Oxides
  (07.02) (A+ B++)2 X4 Spinel group
  (07.02.01) (Aluminum subgroup)
(10th edition) :
  (04) Oxides and Hydroxides
  (04.B) Metal:Oxygen = 3:4 and similar
  (04.BB) With only medium-sized cations
Crystal system: Cubic System
Point group (H-M): m3m (or 4/m 3 2/m) — hexoctahedral
Unit cell: a = 8.062Å
Crystal Habit: Crystals octahedral, rarely cubic or dodecahedral; massive, coarse granular to compact; rounded grains
Twinning: On {111} as both twin and composition plane, common
Color: Greenish black, bluish black, dark bluish green, brown, yellowish
Diaphaneity: Translucent to nearly opaque
Luster: Vitreous to greasy
Hardness (Mohs): 7.5 to 8
Measured Density: 4.61 g/cm3
Cleavage: 1; {111} parting indistinct
Tenacity: Brittle
Fracture: Conchoidal
Streak: Gray
Geologic Setting: An accessory mineral in granites and granite pegmatites; in medium- to high-grade metamorphic rocks and metamorphosed base-metal sulfide deposits; in diasporites, formed by the low-grade metamorphism of bauxites; a detrital mineral in placers.
Mineral Association: Rhodonite, franklinite, calcite, andradite, willemite (Franklin, New Jersey, USA); corundum, pyrrhotite, h¨ogbomite, nigerite, phlogopite, staurolite, cordierite, pyrite, chalcopyrite (Geco mine, Canada)
Synonyms/varieties: Automalite, Zinc Spinel, Dysluite
Other Properties: Gemstone Extremely Rare.
Comments: Forms two series, with spinel, and with hercynite


Origin: Franklin, Sussex Co., New Jersey, U.S.A.
Sample size: 2.3 x 2.2 x 1.7 cm
Photo: Rob Lavinsky


Origin: Alto do Giz pegmatite, near Parelhas, Rio Grande do Norte, Brazil
Sample size:3.8 x 2.7 x 2.1 cm
Photo: Dan Weinrich


Gahnite on a quartz rich matrix
Origin: Davis Pyrite Mine, Rowe, Franklin Co., Massachusetts, U.S.A.
Sample size: 7.1 x 8.4 x 4.7 cm
Photo: Dan Weinrich


(zoom-in of above)