Spinel

Spinel is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4 in the cubic crystal system. Its name comes from the Latin word spinella, a diminutive form of spine, in reference to its pointed crystals.

Spinel

General

Category

Oxide minerals

Spinel group

Spinel structural group

Formula MgAl2O4

IMA symbol Spl

Strunz classification 4.BB.05

Crystal system Cubic

Crystal class Hextetrahedral (43m)

H–M symbol: (43m)

Space group F 4 3 m (No. 216)

Unit cell a = 8.0898(9) Å; Z = 8

Identification

Color Various; red, pink, blue, lavender/violet, dark green, brown, black, colourless

Crystal habit Octahedral or flat triangular plates caused by twinning

Twinning common

Cleavage None

Fracture Conchoidal

Mohs scale hardness 7.5–8.0

Luster Vitreous

Streak White

Diaphaneity Transparent to opaque

Specific gravity (Depending on the composition) The rare Zn-rich spinel can be as high as 4.40, otherwise it averages from 3.58 to 3.61.

Optical properties Isotropic

Refractive index 1.719

Pleochroism Absent

Solubility None

Other characteristics Weak to medium magnetic, sometimes fluorescent (red synthetic yes, natural red sometimes)

Properties

Spinel (magnesiospinel) is a frequently occurring mineral from the mineral class of “oxides and hydroxides” with the end-member composition MgAl2O4 and is therefore chemically a magnesium aluminate. Spinel crystallizes in the cubic crystal system. The spinel structure is one of the most important and common structural types, in which 30 other known minerals crystallize. Spinel is therefore also the namesake of the spinel supergroup newly defined by the International Mineralogical Association (IMA).

Spinel predominantly develops octahedral, rarely dodecahedral and cubic crystals and twins, which can grow up to 30 centimeters in size. However, it also occurs in the form of granular to massive mineral aggregates. Due to mixed crystal formation and the frequent admixture of various metal ions such as iron, chromium, zinc, cobalt or manganese, natural spinels come in a wide variety of colors. Since clear and transparent spinels also have a strong, glass-like luster on polished surfaces and are relatively resistant to damage due to their high Mohs hardness of 8, these so-called "noble spinels" are among the most valuable gemstones.

Classification

In the structural classification of the International Mineralogical Association (IMA), spinel is the namesake of the spinel supergroup, where it forms the spinel subgroup within the oxispinels together with chromite, cochromite, coulsonite, cuprospinel, dellagiustaite, deltalumite, franklinite, gahnite, galaxite, guite, hausmannite, hercynite, hetaerolite, jacobsite, maghemite, magnesiochromite, magnesiocoulsonite, magnesioferrite, magnetite, manganochromite, thermaerogenite, titanomaghemite, trevorite, vuorelainenite and zincochromite (as of 2018). Also included in this group are the oxispinels chihmingite and chukochenite, described after 2018, as well as nichromite, whose name has not yet been recognized by the CNMNC of the IMA. 

Already in the 8th edition of the mineral classification according to Strunz, last revised in 1977, the spinel belonged to the mineral class of “oxides and hydroxides” and there to the section “compounds with M 3 O 4 and related compounds”, where it is located together with gahnite, galaxite and hercynite in the group “aluminium spinels ” with the system number IV/B.01a within the “spinel series” (IV/B.01).

In the Lapis classification system by Stefan Weiß, last revised in 2018 and formally based on the 8th edition of Karl Hugo Strunz 's old classification system, the mineral was given the system and mineral number IV/B.01-010. This corresponds to the more precisely defined section "Oxides with a metal: oxygen ratio of 3: 4 (spinel type M 3 O 4 and related compounds)", where spinel, together with gahnite, galaxite, and hercynite, also forms the group of "aluminate spinels" with the system number IV/B.01. 

The 9th edition of Strunz's mineral classification, last updated in 2009 by the International Mineralogical Association (IMA), also classifies spinel in the division "Metal: Oxygen = 3: 4 and comparable". This division is further subdivided according to the relative size of the cations involved, so that the mineral can be found in the subdivision "With exclusively medium-sized cations" according to its composition, where it forms the "spinel group" with the system number 4.BB.05 together with chromite, cochromite, coulsonite, cuprospinel, filipstadite, franklinite, gahnite, galaxite, hercynite, jacobsite, magnesiochromite, magnesiocoulsonite, magnesioferrite, magnetite, manganochromite, qandilite, trevorite, ulvöspinel, vuorelainenite and zincochromite.

In the Dana classification of minerals, which is primarily used in English-speaking countries, spinel has the classification and mineral number 07.02.01.01. This also corresponds to the class of "Oxides and Hydroxides" and the division "Multiple Oxides." Here, it is found within the subdivision "Multiple Oxides (A + B 2+) 2 X 4, Spinel Group" in the "Aluminum Subgroup," which also includes galaxite, hercynite, and gahnite.

Color

The colors and shades in which spinel is found have given rise to different varieties:

Gahnoespinel - Variety of an attractive blue color due to the presence of iron, zinc and, less frequently, cobalt.

Pleonast - The iron content of this variety gives it a black to dark green color. It is also known as Ceylonite, named after the island of Ceylon (present-day Sri Lanka), where significant deposits exist.

Red spinel - It is the variety of intense red color, related to the presence of chromium, which can be exchanged with aluminum.

Crystal structure

Spinel crystallizes isotypically with magnetite in the cubic crystal system in the space group Fd 3 m (space group no. 227) with the lattice parameter a = 8.09 Å and eight formula units per unit cell. 

Cut spinel

Spinel crystallizes in the isometric system; common crystal forms are octahedra, usually twinned. It has no true cleavage, but shows an octahedral parting and a conchoidal fracture. Its hardness is 8, its specific gravity is 3.5–4.1, and it is transparent to opaque with a vitreous to dull luster. It may be colorless, but is usually various shades of red, lavender, blue, green, brown, black, or yellow. Chromium(III) causes the red color in spinel from Burma. Some spinels are among the most famous gemstones; among them are the Black Prince's Ruby and the "Timur ruby" in the British Crown Jewels, and the "Côte de Bretagne", formerly from the French Crown jewels. The Samarian Spinel is the largest known spinel in the world, weighing 500 carats (100 g).

The transparent red spinels were called spinel-rubies or balas rubies. In the past, before the arrival of modern science, spinels and rubies were equally known as rubies. After the 18th century, the word ruby was only used for the red gem variety of the mineral corundum, and the word spinel came to be used. "Balas" is derived from Balascia, the ancient name for Badakhshan, a region in central Asia situated in the upper valley of the Panj River, one of the principal tributaries of the Oxus River. However, "Balascia" itself may be derived from Sanskrit bālasūryaka, which translates as "crimson-coloured morning sun". Mines in the Gorno Badakhshan region of Tajikistan constituted for centuries the main source for red and pink spinels.

Modifications and varieties

Pure spinel is colorless. However, the stoichiometry of the chemical formula varies widely, meaning that magnesium or aluminum can be present in varying proportions or even replaced by cations that are not part of the formula. Particularly large excesses of aluminum cations can be dissolved in it. A magnesium excess is only possible at extremely high temperatures (above approximately 1500 °C).

This results in a wide range of possible colors, ranging from violet to red to pink, yellow, green to blue, brown to black. Some color varieties have their own names and are also common in the gemstone trade.

“Noble spinel” or “ruby spinel” gets its strong light to dark red color through diadochic replacement of Al 2 O 3 by traces of Cr 2 O 3. 

"Almandine spinel" is a 19th-century term for dark red spinels with a blue or violet tinge. 

"Balas ruby" or "Balas ruby" is an outdated term for a pink to pale red gemstone that became widespread around 1200 AD and served to distinguish a "carbuncle" or ruby of lesser beauty. Because this term is misleading, it has since been rejected by the CIBJO and, in contrast to the alternative term "Balas spinel", is rarely used in the gemstone trade.

"Rubicell" or "Rubacell" as a diminutive from the French rubis or rubace (ruby) has been in use since the 17th century as a name for yellow-orange to yellow-red, hyacinth-like varieties. 

Blue spinel, also known as sapphire spinel, gets its blue color by replacing MgO with FeO up to 3.5%. 

In green spinel or “chlorospinel” (from ancient Greek χλωρός chlōrós “light green, fresh”, after Gustav Rose 1840), parts of MgO and Al 2 O 3 are replaced by traces of CuO and Fe 2 O 3 of up to 15 %.

Pleonast (after René-Just Haüy), also ceylanite (after Jean-Claude Delamétherie 1793), gets its brown to black color from larger admixtures of FeO and Fe 2 O 3. It is also usually opaque.

Occurrence

Geologic occurrence

Spinel is found as a metamorphic mineral in metamorphosed limestones and silica-poor mudstones. It also occurs as a primary mineral in rare mafic igneous rocks; in these igneous rocks, the magmas are relatively deficient in alkalis relative to aluminium, and aluminium oxide may form as the mineral corundum or may combine with magnesia to form spinel. This is why spinel and ruby are often found together. The spinel petrogenesis in mafic magmatic rocks is strongly debated, but certainly results from mafic magma interaction with more evolved magma or rock (e.g. gabbro, troctolite).

Spinel, (Mg,Fe)(Al,Cr)2O4, is common in peridotite in the uppermost Earth's mantle, between approximately 20 km to approximately 120 km, possibly to lower depths depending on the chromium content. At significantly shallower depths, above the Moho, calcic plagioclase is the more stable aluminous mineral in peridotite while garnet is the stable phase deeper in the mantle below the spinel stability region.

Spinel, (Mg,Fe)Al2O4, is a common mineral in the Ca-Al-rich inclusions (CAIs) in some chondritic meteorites.

Geographical occurrence

Spinel has long been found in the gemstone-bearing gravel of Sri Lanka and in limestones of the Badakshan Province in modern-day Afghanistan and Tajikistan; and of Mogok in Myanmar. Over the last decades gem quality spinels are found in the marbles of Lục Yên District (Vietnam), Mahenge and Matombo (Tanzania), Tsavo (Kenya) and in the gravels of Tunduru (Tanzania) and Ilakaka (Madagascar).

Since 2000, in several locations around the world, spinels have been discovered with unusual vivid pink or blue colors. Such "glowing" spinels are known from Mogok (Myanmar), Mahenge plateau (Tanzania), Lục Yên District (Vietnam) and some more localities. In 2018 bright blue spinels have been reported also in the southern part of Baffin Island (Canada). The pure blue coloration of spinel is caused by small additions of cobalt.

Use

Spinel is a precious stone mined primarily for jewelry, where it is used as a cutting stone mounted on rings, bracelets, earrings, and necklaces. Specimens that cannot be used as gemstones are used, thanks to the stone's hardness, to manufacture watch movements, abrasive powders, and superelastic ceramics (capable of stretching to twice their original size without changing their physical properties) through the process of plastic deformation.

Flawless spinels are sought-after but rare gemstones. Red spinels bear a physical resemblance to rubies. For example, the "Black Prince's Ruby" in the Imperial State Crown, long thought to be a ruby, and the "Timur Ruby" in a necklace from the British Crown Jewels, as well as some pear-shaped gemstones in the Wittelsbach Crown, have been identified as spinels. 

There are spinels that occupy a place of honor in history. The most famous is a deep red stone that adorns the crown of Catherine II of Russia. Next in line are two magnificent spinels that were once set as rubies in the Imperial State Crown: the 170- carat Black Prince Ruby and the 352- carat Timur Ruby, engraved with the names of several Mongol emperors.

Although octahedral spinel crystals were once set in jewelry uncut or simply polished, today we find this gem cut in very different ways. It is generally cut using mixed cuts, table cuts, or oval brilliant cuts. Mixed cuts can be octagonal, cushion, or step cuts. When stones exhibiting asterism are cut en cabochon, they display a four-pointed star.

Due to its high hardness and chemical resistance as well as its high melting point of 2135 °C, synthetic spinels are used for refractory and gas-tight technical ceramics. 

Plastic deformation of spinel

The fundamental element of the spinel forging process is obtaining a supersaturated solution of extremely fine grains of the ceramic in question, so that these grains cannot form microcracks at the grain boundaries when stress is applied to the ceramic element. Furthermore, larger grains can only relieve stress by cracking at the contact surface. The planes of very fine grains tend to slide under small, continuous stresses, making the process more economically viable; high, continuous stresses could break the forged dies.

In tests, spinel deforms under high stresses with strain rates of 10-5 to 10-3 Sg-l and temperatures between 1450 °C and 1612 °C. Spinel becomes ductile at low strain rates (low rates of stress application) and high temperatures, especially at temperatures close to the melting point.

Although elongations several times greater than the original length of the test piece are obtained, the shape and size of the crystals remain equiaxed and equal.

Superplastic forging of spinel has already been tested to improve its reliability due to failures caused by structural defects.

Historical uses

A magical stone that wards off melancholy, the red spinel has long been considered a carbuncle, a name given to precious stones that, like the ruby or garnet, were supposed to glow in the dark like a burning coal.

In ancient times, spinel was frequently used in magical practices as a substitute for ruby, as both stones were considered to share the same positive energetic vibrations. Spinel was also often used in noble and ecclesiastical jewelry instead of ruby (in many cases in good faith, as no distinction was made between the two stones).

Similarities and differences with rubies

It is not uncommon for a quality red spinel to surpass rubies in beauty in daylight, which appear more violet. Under artificial light, however, rubies regain their splendor, while spinels appear duller and more similar to garnets.

Manipulations and imitations

In contrast to corundums (sapphire and the red variety ruby), spinels are only rarely heated, as their colors change either insignificantly or not in the desired direction. Due to the fact that typical gemstone qualities of spinel have comparatively fewer clouding inclusions, another effect of heating, namely the improvement in transparency through the melting of the included foreign crystals, has only been documented in a few cases. In the specialist trade, natural spinels are generally still considered untreated. Exceptions do exist, however, in the premium sector for certain color variants from certain localities. High-priced spinels are usually certified by a gemmological laboratory, where both heated and synthetic spinels can be reliably distinguished from untreated spinels using Raman spectroscopy due to the associated structural change, which begins as early as 750 °C. 

Synthetic spinels for the jewelry industry have been produced since the 1920s using the Verneuil process, in which aluminum oxide (corundum, Al 2 O 3) and magnesium oxide (periclase, MgO) react to form spinel (MgAl 2 O 4). With the help of further additives, the desired color varieties can be created, such as Co 2 O 3 for blue and Ni 2 O 3 for light green spinels, to imitate, for example, aquamarine, garnet, ruby, sapphire, emerald, various tourmalines, and even moonstone. Since the 1950s, lapis lazuli has also been imitated using synthetic sintered spinel. 

Synthetic spinel can be produced by similar means to synthetic corundum, including the Verneuil method and the flux method pioneered by Edmond Frémy. It is widely used as an inexpensive cut gem in birthstone jewelry for the month of August. Light blue synthetic spinel is a good imitation of aquamarine beryl, and green synthetic spinel is used as an emerald or tourmaline simulant. By 2015, transparent spinel was being made in sheets and other shapes through sintering. Synthetic spinel, which looks like glass but has notably higher strength against pressure, can also have applications in military and commercial use.

However, under the polarization microscope, spinels synthesized using the Verneuil method always show a typical anomalous stress birefringence and can thus be distinguished from natural spinels. 

Sourced from Wikipedia