The spinel mineral series

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. Some minerals in the Spinel group are individually categorized:
Common Spinel or Spinel (Magnesium Spinel) - MgAl₂O₄
Gahnite (Zinc Spinel) - ZnAl₂O₄
Hercynite (Iron Spinel) - FeAl₂O₄
Galaxite (Manganese Spinel) - MnAl₂O₄
The above formula is for Galaxite is only for pure Galaxite. However, Galaxite almost always contains magnesium, iron, and aluminum, so its formula is scientifically recognized as:
(Mn,Fe²⁺,Mg)(Al,Fe³⁺)₂O₄

Composition

Magnesium, iron, zinc, and/or manganese aluminum oxide:
Common Spinel is Magnesium aluminum oxide
Gahnite is Zinc aluminum oxide
Hercynite is Iron aluminum oxide
Galaxite is Manganese aluminum oxide, usually with some magnesium, iron, and aluminum

Color

Red, pink, violet, blue, green, aqua, orange, yellow, brown, and black. Rarely white or colorless.

Streak

White

Hardness

7½ - 8

Crystal Forms
and Aggregates

(Isometric) Octahedral crystals are most frequent; seldom occurs in cubic and dodecahedral crystals. Crystal clusters and twinned octahedrons are very common. Also occurs grainy, as deformed crystals, 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 is often fluorescent red or neon-yellow in shortwave ultraviolet light

Varieties

Ceylonite - Magnesium Spinel containing traces of iron, which gives it a dark green to black color.

Chlorospinel - Grass green Magnesium Spinel; the color caused by traces of iron and copper.

Chrome-spinel - Synonym of Picotite

Chromohercynite - Chromium rich variety of Hercynite. Also a variety of Picotite.

Gahnospinel - Mixture of Magnesium Spinel and Gahnite. Formula = (Mg,Zn)Al₂O₄

Pleonast - Opaque, dark blue or green to black variety of Spinel.

Chrysomelane - synonym of Hercynite

There are varieties of Spinel which were given names based on their color. These names are only used on the gem market:
Ruby Spinel - ruby-red color
Rubicelle - yellow to orange color
Balas Ruby - pink to pale red color
Almandine Spinel - violet color
Blue Spinel - light to dark blue color
Pleonast - black color
Picotite - brown color

In Group

Oxides ; Multiple oxides ; Spinel group

All About

Spinel is the most common and well-known member of the Spinel series. The other members (Gahnite, Hercynite, and Galaxite) are very rare and basically unheard of. The remainder of this section only deals with the mineral Spinel, not the other minerals in the Spinel series.

Spinel occurs in almost every color, and creates gems of all colors. It is not a well known gem, although a good specimen can attain a very high price. Bright red Spinel, known as Ruby Spinel, is the most valuable Spinel. In the past, there was no distinction between Ruby and Ruby Spinel, as they look identical and are found in the same localities. Nowadays, distinctions can be made through hardness and x-ray tests. Many famous old "Rubies" were found to be Spinel. The enormous "Ruby" that forms the centerpiece of the royal crown of Great Britain (the Black Prince's Ruby) was recently discovered to be a Spinel.

Uses

Spinel is a rare mineral, and makes a rare gem. The rarity as well as color make Spinel a gemstone very much in demand.

Also see the gemstone section on Spinel

Striking Features

Crystal habit and high hardness

Complex Tests

Insoluble in acids

Popularity (1-4)

Prevalence (1-3)

Demand (1-3)

Distinguishing
Similar Minerals

Corundum (Ruby) - harder (9)
Magnetite - softer (5½ - 6½), attracted to magnets
Zircon - different crystal system (tetragonal)
Chromite - softer (5½), heavier (4.1 - 5.0)
Garnet - different crystal forms

Commonly
Occurs With

Calcite, Corundum, Hornblende, Olivine, Phologopite, Chondrodite (Marble, Limestone)

Noteworthy
Localities

Most gem grade Spinel comes from Sri Lanka, particularly in Ratnapura, and from Mogok, Myanmar (Burma). Ambatomainty, Madagascar has produced large clusters of black crystals. Spinel has also been found in Canada, in Ross Township, Renfrew Co., Ontario. In the U.S., the finest crystals came from a limestone belt streching from Sussex Co., New Jersey to Orange Co., New York. Those areas include Sparta and Ogdensburg, Sussex Co., New Jersey, together with Amity, Edenville, and Warwick, Orange Co., New York.
Spinel was also found at Bolton, Worcester Co., Massachusetts, and opaque blue crystal clusters occur in Lewis and Clark Co., Montana.

Picture Icon
Links

Picture Links

1. "Ruby Spinel"
2. Blue Spinel octahedrons