Gahnite is the zinc end-member of the spinel group, a Zn-Al oxide producing dark green to bluish-black sharp octahedral crystals. It is a key indicator of zinc-rich metamorphic environments and a Franklin/Sterling Hill suite member.
Properties
- Formula: ZnAl2O4
- Crystal system: Isometric (Cubic)
- Hardness: 7.5 – 8
- Color: Dark green, bluish-green, blue-black
- Streak: Gray
- Luster: Vitreous
- Cleavage: None, parting on {111}
- Density: 4.3 – 4.6 g/cm³
Occurrence
Type locality: Falun (Sweden). Excellent crystals from Charlemont (Massachusetts), Franklin (New Jersey), Broken Hill (Australia) and Bodenmais (Germany). Chinese gahnite is minor and largely metamorphic.
Identification
Sharp dark-green octahedra + very high hardness (~8) + association with zinc-rich gneiss/skarn. Distinguish from common spinel (red/pink/blue, lower density) by chemistry — gahnite has the characteristic dark blue-green color and higher density.
Collector Notes
Charlemont gahnite octahedra in mica schist are the American classic. Closes the Franklin spinel-suite trio with franklinite and ordinary spinel.
Found at these Localities
- Broken Hill (布罗肯希尔铅锌银矿)
- Franklin and Sterling Hill (富兰克林矿(新泽西))
