Crystal Spinel

Chemical Formula

MgAl₂O₄ 
The Mg may be partially or fully replaced by Fe, Zn, and/or Mn. This leads to a mineral series with the formula (Mg,Fe,Zn,Mn)Al₂O₄. The end-members of the Spinel group are individually categorized:
Spinel (or Magnesium Spinel) - MgAl₂O₄
Gahnite (Zinc Spinel) - ZnAl₂O₄
Hercynite (Iron Spinel) - FeAl₂O₄
Galaxite (Manganese Spinel) - MnAl₂O₄

The above formula for Galaxite is only for pure Galaxite. However, Galaxite will always contain traces of magnesium, iron, and aluminum, so its formula is better recognized as:
(Mn,Fe²⁺,Mg)(Al,Fe³⁺)₂O₄
 

Composition

Series of magnesium, iron, zinc, and/or manganese aluminum oxide.
Spinel is Magnesium aluminum oxide
Gahnite is Zinc aluminum oxide
Hercynite is Iron aluminum oxide
Galaxite is Manganese aluminum oxide, with some magnesium, iron, and aluminum

Variable Formula

(Mg,Mn,Fe²⁺)(Al,Fe³⁺)₂O₄

Color

Red, pink, purple, blue, green, aqua, orange, yellow, brown, gray, and black.

Streak

White

Hardness

7.5 - 8

Crystal System

Isometric

Crystal Forms 
and Aggregates

Spinel occurs in octahedral crystals, often in perfectly shaped octahedrons. Crystals often have parallel layer growths or heavy etchings. Crystals can also be distorted as well as have complex faces. Crystal clusters and twinned octahedrons are very common, especially spinel twinning, which this mineral is famous for. An interesting habit occasionally observed in Spinel is the macle, which is a triangular form of two flattened crystal  twins. Spinel also occurs grainy and as rounded, waterworn pebbles.
 

Transparency

Transparent to opaque

Specific Gravity

3.5 - 4.1

Luster

Vitreous, submetallic, or dull

Cleavage

None, but may exhibit slight octahedral parting

Fracture

Conchoidal

Tenacity

Brittle

Other ID Marks

Red, pink, and violet Spinel often fluoresces red or neon-yellow in shortwave ultraviolet light.

Complex Tests

Insoluble in acids

In Group

Oxides; Multiple Oxides

Striking Features

Crystal habit and high hardness

Environment

In metamorphic marbles and hornfels, as well as in plutonic rocks such as gabbro. Also in alluvial placerdeposits.
 

Rock Type

Igneous, Metamorphic

Popularity (1-4)

1

Prevalence (1-3)

3

Demand (1-3)

1