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Spinel group
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Spinel group
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The spinels are any of a class of minerals of general formulation AB
2X
4 which crystallise in the cubic (isometric) crystal system, with the X anions (typically chalcogens, like oxygen and sulfur) arranged in a cubic close-packed lattice and the cations A and B occupying some or all of the octahedral and tetrahedral sites in the lattice.[1][2] Although the charges of A and B in the prototypical spinel structure are +2 and +3, respectively (A2+
B3+
2X2−
4), other combinations incorporating divalent, trivalent, or tetravalent cations, including magnesium, zinc, iron, manganese, aluminium, chromium, titanium, and silicon, are also possible. The anion is normally oxygen; when other chalcogenides constitute the anion sublattice the structure is referred to as a thiospinel.

A and B can also be the same metal with different valences, as is the case with magnetite, Fe3O4 (as Fe2+
Fe3+
2O2−
4), which is the most abundant member of the spinel group.[3] It is even possible for them to be alloys, as seen for example in LiNi
0.5Mn
1.5O
4, a material used in some high energy density lithium ion batteries.[4] Spinels are grouped in series by the B cation.

The group is named for spinel (MgAl
2O
4), which was once known as "spinel ruby".[5] (Today the term ruby is used only for corundum.)

Spinel group members

[edit]

Members of the spinel group include:[6]

There are many more compounds with a spinel structure, e.g. the thiospinels and selenospinels, that can be synthesized in the lab or in some cases occur as minerals.

The heterogeneity of spinel group members varies based on composition with ferrous and magnesium based members varying greatly as in solid solution, which requires similarly sized cations. However, ferric and aluminium based spinels are almost entirely homogeneous due to their large size difference.[10]

The spinel structure

[edit]
Crystal structure of spinel

The space group for a spinel group mineral may be Fd3m (the same as for diamond), but in some cases (such as spinel itself, MgAl
2O
4, beyond 452.6 K[11]) it is actually the tetrahedral F43m.[12][13][14] [15]

Normal spinel structures have oxygen ions closely approximating a cubic close-packed latice with eight tetrahedral and four octahedral sites per formula unit (but eight times as many per unit cell). The tetrahedral spaces are smaller than the octahedral spaces. B ions occupy half the octahedral holes, while A ions occupy one-eighth of the tetrahedral holes.[16][17] The mineral spinel MgAl2O4 has a normal spinel structure.

In a normal spinel structure, the ions are in the following positions, where i, j, and k are arbitrary integers and δ, ε, and ζ are small real numbers (note that the unit cell can be chosen differently, giving different coordinates):[18] 

X:
(1/4-δ,   δ,     δ  ) + ((i+j)/2, (j+k)/2, (i+k)/2)
( δ,     1/4-δ,  δ  ) + ((i+j)/2, (j+k)/2, (i+k)/2)
( δ,      δ,   1/4-δ) + ((i+j)/2, (j+k)/2, (i+k)/2)
(1/4-δ, 1/4-δ, 1/4-δ) + ((i+j)/2, (j+k)/2, (i+k)/2)
(3/4+ε, 1/2-ε, 1/2-ε) + ((i+j)/2, (j+k)/2, (i+k)/2)
(1-ε,   1/4+ε, 1/2-ε) + ((i+j)/2, (j+k)/2, (i+k)/2)
(1-ε,   1/2-ε, 1/4+ε) + ((i+j)/2, (j+k)/2, (i+k)/2)
(3/4+ε, 1/4+ε, 1/4+ε) + ((i+j)/2, (j+k)/2, (i+k)/2)
A:
(1/8, 1/8, 1/8) + ((i+j)/2, (j+k)/2, (i+k)/2)
(7/8, 3/8, 3/8) + ((i+j)/2, (j+k)/2, (i+k)/2)
B:
(1/2+ζ,   ζ,     ζ  ) + ((i+j)/2, (j+k)/2, (i+k)/2)
(1/2+ζ, 1/4-ζ, 1/4-ζ) + ((i+j)/2, (j+k)/2, (i+k)/2)
(3/4-ζ, 1/4-ζ,   ζ  ) + ((i+j)/2, (j+k)/2, (i+k)/2)
(3/4-ζ,   ζ,   1/4-ζ) + ((i+j)/2, (j+k)/2, (i+k)/2)

The first four X positions form a tetrahedron around the first A position, and the last four form one around the second A position. When the space group is Fd3m then δ=ε and ζ=0. In this case, a three-fold rotoinversion with axis in the 111 direction is centred on the point (0, 0, 0) (where there is no ion) and can also be centred on the B ion at (1/2, 1/2, 1/2), and in fact every B ion is the centre of a three-fold rotoinversion (point group D3d). Under this space group the two A positions are equivalent. If the space group is F43m then the three-fold rotoinversions become simple three-fold rotations (point group C3v) because the inversion disappears, and the two A positions are no longer equivalent.

Every ion is on at least three mirror planes and at least one three-fold rotation axis. The structure has tetrahedral symmetry around each A ion, and the A ions are arranged just like the carbon atoms in diamond. There are another eight tetrahedral sites per unit cell that are empty, each one surrounded by a tetrahedron of B as well as a tetrahedron of X ions.

Inverse spinel structures have a different cation distribution in that all of the A cations and half of the B cations occupy octahedral sites, while the other half of the B cations occupy tetrahedral sites. An example of an inverse spinel is Fe3O4, if the Fe2+ (A2+) ions are d6 high-spin and the Fe3+ (B3+) ions are d5 high-spin.

In addition, intermediate cases exist where the cation distribution can be described as (A1−xBx)[Ax⁄2B1−x⁄2]2O4, where parentheses () and brackets [] are used to denote tetrahedral and octahedral sites, respectively. The so-called inversion degree, x, adopts values between 0 (normal) and 1 (inverse), and is equal to 2⁄3 for a completely random cation distribution.

The cation distribution in spinel structures are related to the crystal field stabilization energies (CFSE) of the constituent transition metals. Some ions may have a distinct preference for the octahedral site depending on the d-electron count. If the A2+ ions have a strong preference for the octahedral site, they will displace half of the B3+ ions from the octahedral sites to tetrahedral sites. Similarly, if the B3+ ions have a low or zero octahedral site stabilization energy (OSSE), then they will occupy tetrahedral sites, leaving octahedral sites for the A2+ ions.

Burdett and co-workers proposed an alternative treatment of the problem of spinel inversion, using the relative sizes of the s and p atomic orbitals of the two types of atom to determine their site preferences.[19] This is because the dominant stabilizing interaction in the solids is not the crystal field stabilization energy generated by the interaction of the ligands with the d electrons, but the σ-type interactions between the metal cations and the oxide anions. This rationale can explain anomalies in the spinel structures that crystal-field theory cannot, such as the marked preference of Al3+ cations for octahedral sites or of Zn2+ for tetrahedral sites, which crystal field theory would predict neither has a site preference. Only in cases where this size-based approach indicates no preference for one structure over another do crystal field effects make any difference; in effect they are just a small perturbation that can sometimes affect the relative preferences, but which often do not.

Common uses in industry and technology

[edit]

Spinels commonly form in high temperature processes. Either native oxide scales of metals,[20] or intentional deposition of spinel coatings[21] can be used to protect base metals from oxidation or corrosion. The presence of spinels may hereby serve as thin (few micrometer thick) functional layers, that prevent the diffusion of oxygen (or other atmospheric) ions or specific metal ions such as chromium, which otherwise exhibits a fast diffusion process at high temperatures.

Further reading

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Spinel group

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The Spinel group is a class of minerals characterized by the general formula AB₂X₄, where A represents a divalent cation (such as Mg²⁺, Fe²⁺, or Zn²⁺), B a trivalent cation (such as Al³⁺, Fe³⁺, or Cr³⁺), and X oxygen (O²⁻). These minerals crystallize in the cubic system with Fd³m, forming a distinctive structure based on a face-centered cubic array of anions with cations in tetrahedral and octahedral coordination sites. The group encompasses over 20 recognized members, divided into three primary series: the series (Al-dominant, e.g., MgAl₂O₄), the series (Cr-dominant, e.g., FeCr₂O₄), and the series (Fe³⁺-dominant, e.g., Fe₃O₄). In the spinel structure, normal spinels feature the divalent A cation exclusively in tetrahedral sites and the trivalent B cations in octahedral sites, while inverse spinels have half the B cations in tetrahedral sites and the A cation plus the remaining B cations in octahedral sites; many members exhibit intermediate or mixed distributions. Physical properties vary but generally include high hardness (5.5–8 on the ), specific gravity of 3.5–5.5, vitreous to metallic luster, and no cleavage, with colors ranging from and green in gem-quality spinel to black in opaque ores like . Crystals often form octahedrons or exhibit twinning known as the spinel . Spinel group minerals are widespread as accessory phases in igneous rocks (particularly mafic and ultramafic types like and ), metamorphic rocks (such as marbles, schists, and eclogites), and detrital sediments, where they form during high-temperature processes or . Economically, they are significant: serves as the primary ore for used in and alloys, is a major , and (ZnFe₂O₄) contributes to and production. Additionally, the mineral is prized as a durable , with colors influenced by trace elements like (for reds) or iron (for blues and blacks), sourced from deposits in , , and .

Overview and Definition

Chemical Composition

The spinel group consists of minerals with the general AB₂O₄, where A represents a divalent cation such as Mg²⁺, Fe²⁺, or Zn²⁺ occupying tetrahedral sites in the normal structure, and B denotes a trivalent cation like Al³⁺, Fe³⁺, or Cr³⁺ in octahedral sites. Some members, such as (Fe³O₄ or Fe²⁺Fe₂³⁺O₄), adopt an inverse spinel configuration where the divalent cation resides in octahedral sites and one trivalent cation occupies tetrahedral sites. Key end-member compositions include (MgAl₂O₄), hercynite (FeAl₂O₄), gahnite (ZnAl₂O₄), (FeCr₂O₄), and (FeFe₂O₄). These end-members define the compositional endpoints of various series within the group, with extensive s forming due to ionic substitutions among similar-sized cations. Notable solid solution series include the spinel- series (MgAl₂O₄–FeFe₂O₄), where Al³⁺ is progressively replaced by Fe³⁺, and the hercynite-gahnite series (FeAl₂O₄–ZnAl₂O₄), involving substitution of Fe²⁺ by Zn²⁺ while maintaining Al³⁺ dominance. The hercynite- series (FeAl₂O₄–MgAl₂O₄) further exemplifies Fe²⁺–Mg²⁺ exchange in the aluminum-dominant subgroup. Compositional variations arise from trace impurities, including elements like Ti, , Cr, Fe, and Co, which substitute into the lattice and influence color and stability. For instance, trace Cr and enhance red hues in spinel, while elevated Fe levels intensify purple tones; these substitutions can also affect phase stability under varying temperature and conditions.

Historical Discovery and Nomenclature

The mineral now known as spinel, with the end-member composition MgAl₂O₄, was first scientifically described in 1546 by the German scholar (Georg Bauer) in his work De natura fossilium, where it was initially confused with due to similarities in color and occurrence in gem deposits. This early misidentification persisted for centuries, as red and pink varieties of spinel were often traded as "Balas rubies" from Asian mines, indistinguishable from true without advanced analysis. In the , distinctions emerged through systematic mineralogical studies; René Just Haüy further differentiated from in his Traité de Minéralogie (1801) based on crystallographic and physical criteria. The formal naming of "" as a distinct occurred in 1783 by French mineralogist Jean-Baptiste Romé de l'Isle, marking its separation from and . The spinel group was established in the as a broader category encompassing structurally related oxides, notably including (Fe₃O₄) and (FeCr₂O₄), formalized by in his A System of Mineralogy (1837), which grouped them by shared cubic crystal symmetry and chemical formulas of the type AB₂O₄. This classification reflected growing recognition of isomorphous series among iron and magnesium-aluminum oxides. Subsequent refinements came through the International Mineralogical Association (IMA), founded in 1958, which has overseen updates to the spinel group's ; for example, galaxite (MnAl₂O₄) was validated as a distinct member in 1964 during IMA proceedings, expanding the group's diversity. The evolution of subgroup terms, such as "oxyspinel" for oxygen-dominant, non-aluminous variants like hausmannite (Mn₃O₄), emerged in the late 20th century to distinguish oxide-based spinels from sulfide analogs, culminating in the IMA's 2019 comprehensive scheme for the supergroup, which recognizes 56 valid members divided into oxyspinel, thiospinel, and selenospinel groups.

Crystal Structure and Properties

The Spinel Crystal Structure

The spinel crystal structure belongs to the and is described by the Fd\overline{3}m (No. 227), with a lattice parameter of approximately 8.08 for the ideal end-member MgAl₂O₄. This structure accommodates the general formula AB₂O₄, where A and B represent cations of different valences and ionic radii, influencing site preferences within the framework. The conventional is relatively large, containing 8 formula units (Z = 8). At the core of the structure is a close-packed array of 32 oxygen anions arranged in a slightly distorted face-centered cubic (fcc) sublattice, which defines the overall framework and provides interstitial sites for cation placement. These oxygen anions create two primary types of coordination polyhedra: tetrahedral sites () and octahedral sites (). The oxygen positions are specified by the with a positional parameter u ≈ 0.385, which governs the distortion from ideal close-packing and affects polyhedral geometries. In the normal spinel configuration, the divalent A²⁺ cations (such as Mg²⁺) occupy 1/8 of the available tetrahedral interstices (8 cations total per ), while the trivalent B³⁺ cations (such as Al³⁺) fill 1/2 of the octahedral interstices (16 cations total per ), resulting in 8 A, 16 B, and 32 O atoms per . This arrangement yields average tetrahedral A–O bond lengths of about 1.94 and octahedral B–O bond lengths of about 1.92 , with the oxygen framework exhibiting characteristic O–O distances of approximately 2.96 that reflect the shared edges between adjacent polyhedra.

Physical and Optical Properties

Properties of spinel group minerals vary significantly with composition across the Al-, Cr-, and Fe³⁺-dominant series. The Mohs hardness ranges from 5.5 to 8, with Al-dominant members like spinel reaching 7.5-8 and Fe-dominant magnetite at 5.5-6.5, making them relatively resistant to scratching and suitable for various applications. Their specific gravity varies between 3.5 and 5.5, influenced by compositional differences such as the incorporation of heavier elements like iron or . Optically, these minerals are isotropic due to their cubic , resulting in a single ranging from approximately 1.71 (for MgAl₂O₄ ) to 2.50 (for ), with no observed. Pleochroism is absent, though rare anomalous effects may occur in imperfect due to or impurities. Color variations arise primarily from trace transition metals; for instance, classic red owes its hue to , black to iron, and green gahnite to iron (Fe²⁺) impurities. Spinel group minerals demonstrate high thermal stability, remaining intact up to approximately 1800°C in environments, which stems from their robust framework. Electrically, they range from insulators, as in pure MgAl₂O₄ , to semiconductors, particularly in iron-rich members like , depending on cation ordering and electronic hopping mechanisms.

Members of the Spinel Group

Classification by Cation Arrangement

The spinel group minerals and compounds are classified based on the ordering of cations in their tetrahedral and octahedral coordination sites within the cubic spinel structure. In normal spinels, the divalent cations (A²⁺) occupy all tetrahedral sites, while the trivalent cations (B³⁺) fill the octahedral sites, following the general distribution [A²⁺]ᵀᵉₜ[B³⁺₂]ᴼᶜₜO₄. A representative example is magnesiochromite (MgCr₂O₄), where Mg²⁺ resides in tetrahedral positions and Cr³⁺ in octahedral ones. Inverse spinels feature a reversed cation arrangement, with trivalent cations (B³⁺) occupying the tetrahedral sites and a mixture of divalent (A²⁺) and remaining trivalent (B³⁺) cations in the octahedral sites, described as [B³⁺]ᵀᵉₜ[A²⁺ B³⁺]ᴼᶜₜO₄. Magnetite (Fe₃O₄) exemplifies this type, exhibiting a high degree of inversion approaching 1, where Fe³⁺ fills tetrahedral sites and both Fe²⁺ and Fe³⁺ occupy octahedral sites. Many spinels adopt a mixed or partially inverse configuration, where cations are distributed between sites with an inversion parameter δ (0 < δ < 1), expressed by the formula [A¹⁻δ Bδ]ᵀᵉₜ[Aδ B₂⁻δ]ᴼᶜₜO₄, indicating the fraction of B³⁺ ions in tetrahedral sites. For instance, certain nickel ferrites like NiFe₂O₄ display partial inversion with δ ≈ 0.8, resulting in random partial occupancy across sites. The degree of inversion in spinels is governed by several factors, including the ionic radii and electronegativities of the constituent cations, which determine site preferences and lattice stability. Smaller trivalent cations with higher electronegativities tend to favor octahedral coordination, promoting structures, while larger or more electropositive divalent cations may drive inversion. Additionally, temperature plays a key role; in systems like MgAl₂O₄, increasing temperature from 600°C to 1100°C raises the inversion degree from 0.18 to 0.29, favoring partial inverse ordering, whereas in NiMn₂O₄, higher temperatures above 300°C reduce inversion, shifting toward a more arrangement.

Key Mineral Species

The spinel group encompasses 26 IMA-recognized mineral (as per the 2019 nomenclature) characterized by the general formula AB₂O₄, where A is a divalent cation and B is trivalent, exhibiting normal or inverse spinel structures such as in . These minerals typically exhibit cubic symmetry, though some display lower symmetry (e.g., tetragonal in hausmannite due to ), and are distinguished by their dominant cations, which impart unique chemical and physical properties. The primary are well-known end-members, while rarer ones highlight compositional diversity within igneous, metamorphic, and mantle-derived environments. Key representatives are often classified into three series based on the dominant trivalent cation: the series (Al-dominant), series (Cr-dominant), and series (Fe³⁺-dominant). In the spinel series, (MgAl₂O₄) is the magnesium-aluminum end-member that defines the group and occurs as colorless to red octahedra, often valued as a due to its (Mohs 7.5–8). Hercynite (FeAl₂O₄) features iron substituting for magnesium, resulting in black crystals with a metallic luster, commonly found in metamorphosed iron-rich sediments. Gahnite (ZnAl₂O₄) incorporates , yielding dark green to black grains that are zinc indicators in granitic pegmatites and metamorphosed zinc deposits. Among rarer species in this series, galaxite (MnAl₂O₄) is a manganese-aluminum variant forming pinkish to reddish crystals in metamorphosed manganese deposits, distinguished by its relatively low density (3.62 g/cm³). The series includes (FeCr₂O₄), notable for its high content (up to 62 wt% Cr₂O₃), forming black, metallic grains essential for production and serving as the primary . (MgCr₂O₄) is a magnesium- variant intermediate between and , often co-occurring in ultramafic rocks. In the series, (Fe³O₄), an inverse with ferric iron dominance, exhibits strong due to its ordered cation distribution, appearing as black, magnetic octahedra in diverse igneous and sedimentary rocks. Other significant members include (ZnFe₂O₄), a zinc-iron with brownish-black color from metamorphosed zinc ; and (NiFe₂O₄), a nickel-iron reaffirmed as a valid IMA in the nomenclature update, appearing as greenish-black masses in nickel-bearing laterites. (MnFe₂O₄) is another Fe-dominant member. Rarer species outside the main series include coulsonite (FeV₂O₄), with replacing aluminum or , occurring as tiny black grains in vanadium-rich iron formations, notable for its rarity and vanadium content exceeding 40 wt% V₂O₃. Hausmannite (Mn³Mn²O₄) is a end-member with tetragonal distortion in some samples but cubic in ideal form, known for its black, submetallic luster. These species, along with less common ones like hetaerolite (ZnMn₂O₄), illustrate the group's extensive solid-solution series driven by cation substitutions.

Occurrence and Synthesis

Natural Geological Occurrence

Spinel group minerals primarily form in igneous, metamorphic, and derived sedimentary environments, often as accessory phases in magnesium- and aluminum-rich rocks. In igneous settings, they occur in and ultramafic rocks through processes such as fractional of mantle-derived magmas. For instance, , a key member of the group, is abundant in peridotites within complexes, where it crystallizes early during the differentiation of ultramafic melts in . Similarly, picotite—a chromium-bearing variety of —appears as an accessory mineral in basalts, associated with phenocrysts in low-silica, alkali-rich magmas. In metamorphic settings, spinel group minerals develop under high-temperature and high-pressure conditions, often via metasomatic reactions involving fluid infiltration. They are common in granulites, where high-pressure metamorphism of pelitic or mafic protoliths produces spinel in association with garnet and orthopyroxene. Skarn deposits, formed at contacts between intrusive rocks and carbonate sequences, host spinel through calc-silicate metasomatism, as seen in associations with wollastonite and diopside. Spinel also occurs in marble xenoliths, where it forms during contact metamorphism of impure limestones, often alongside phlogopite and calcite. Sedimentary derivatives of spinel group minerals are found in placer deposits, resulting from the and concentration of primary sources in alluvial gravels. Gem-quality spinels, particularly from metamorphic origins, accumulate in such settings in ( Valley), (), and . These minerals commonly exhibit parageneses with and in ultramafic assemblages or with in aluminous metamorphic rocks, reflecting formation through or magmatic differentiation. Major localities include the Bushveld Complex in , a rich in layers from fractional crystallization, and the Badakhshan region in , known for noble (gem) spinels in marble-hosted deposits.

Synthetic Production Methods

Synthetic spinel group materials are produced through various and industrial techniques that enable control over composition, purity, and microstructure, often yielding properties superior to natural counterparts for specific applications. These methods typically involve high-temperature reactions or solution-based processes to form the characteristic AB₂O₄ spinel structure, where A and B are divalent and trivalent cations, respectively. One of the earliest and most established industrial methods for producing gem-quality synthetic spinel is the flame fusion process, also known as the , developed in the early and applied to spinel production in the 1920s. In this technique, finely powdered oxides of aluminum and magnesium (or other cations) are fed through an flame at approximately 2000°C, where they melt and crystallize onto a to form a boule-shaped . This process allows for the incorporation of dopants, such as for blue hues, and has been widely used since its adaptation for spinel to produce colorless or colored varieties mimicking natural gems. Solid-state sintering represents a key method for synthesizing high-density, transparent ceramics like MgAl₂O₄ spinel, particularly for optical and refractory uses. It involves mixing stoichiometric oxide precursors, such as MgO and Al₂O₃, compacting them into a green body, and heating at 1400–1600°C in a controlled atmosphere to promote diffusion and densification while minimizing porosity. This approach achieves near-theoretical transparency when combined with additives like LiF or hot isostatic pressing, as demonstrated in studies on microstructure development. Hydrothermal synthesis is employed for producing spinel nanocrystals with uniform size and morphology, suitable for advanced . This method utilizes sealed autoclaves to react metal salts or hydroxides in aqueous solutions under autogenous pressure at 200–400°C for several hours, facilitating and growth in a supersaturated environment. For instance, ferrite (CoFe₂O₄) nanocrystals in the 3–30 nm range have been synthesized by varying temperature and reaction time, yielding phase-pure s with tailored magnetic properties. Sol-gel and co-precipitation techniques are versatile for synthesizing doped powders, enabling precise control over cation distribution and particle size at lower temperatures than methods. In sol-gel processing, metal alkoxides or nitrates are hydrolyzed to form a sol, which gels and is calcined at 600–1000°C to yield nanoscale particles; co-precipitation involves adding precipitants to aqueous salt solutions to form hydroxides that are subsequently annealed. These methods are particularly effective for incorporating dopants like into Al₂O₃-based spinels (e.g., CoAl₂O₄), producing intense due to tetrahedral Co²⁺ coordination in the lattice. Recent advances in (CVD) have enabled the fabrication of high-purity thin films for electronic and magnetic devices, with significant progress since 2010. In CVD, volatile metalorganic precursors are decomposed on a heated substrate (typically 400–800°C) under reduced , depositing conformal films of spinels like CoFe₂O₄ with thicknesses of 100–500 nm and growth rates up to 200 nm/h. Techniques such as direct liquid injection CVD have produced epitaxial films on substrates like MgAl₂O₄, enhancing properties like for spintronic applications.

Applications and Uses

Gemological and Jewelry Uses

Spinel is prized in for its vivid colors and durability, with a Mohs hardness of 8 that renders it suitable for everyday jewelry wear. Transparent varieties, particularly those exhibiting intense reds, pinks, and blues, have been cut into faceted gems since ancient times. Noble spinel denotes high-quality red and pink specimens, often sourced from Myanmar's region, where content imparts their characteristic hues. Historically, red spinels were misidentified as rubies and termed balas rubies, a nomenclature originating from deposits in what is now and . The gem's historical role in jewelry underscores its prestige among royalty, frequently serving as substitutes for rarer until advancements in clarified its identity. Iconic examples include the , a 361-carat polished red engraved with 14th- to 19th-century Persian inscriptions, now set in a necklace within the British Crown Jewels. Similarly, the —a 170-carat octagonal red —adorns England's , acquired in the and long believed to be . These pieces highlight 's use in crowns and regalia across empires from the Timurids to the Mughals, passing as war spoils until French mineralogist Jean-Baptiste Louis Romé de l'Isle distinguished it from in 1783 via . Treatments play a key role in enhancing spinel's appeal for jewelry, with low-temperature (around 950–1150°C) commonly used to remove brownish or orangey overtones, yielding purer reds and pinks without altering transparency. Such enhancements are detectable through spectroscopic , where chromium-bearing spinels display diagnostic absorption bands at approximately 410 nm, 560 nm, and a doublet near 690 nm, absent or altered in heavily treated or synthetic imitations. Imitation detection further relies on these Cr³⁺ features, which differentiate genuine noble from simulants. Valuation of spinel gems hinges on color intensity and hue—vivid reds and pinks from command premiums, alongside blues and attractive to lilac tones—coupled with clarity and carat weight. Eye-clean stones over 5 carats are rare and highly sought, as larger sizes amplify brilliance in cuts like ovals and cushions. Fine-quality untreated pieces can reach up to $5,000 per carat, though prices vary by origin and demand, with blues occasionally exceeding this for exceptional vividness. In the contemporary market, spinel enjoys renewed popularity due to its affordability relative to and ethical sourcing from alluvial deposits in , , and , where small-scale supports traceability initiatives like verification. Synthetics, produced via flame fusion since the mid-20th century but proliferating in the , now flood the market, often mimicking natural colors and necessitating gemological certification for authentic jewelry pieces. This surge underscores spinel's versatility in modern designs, from engagement rings to statement necklaces.

Industrial and Technological Applications

Synthetic magnesium aluminate (MgAl₂O₄) is widely used in bricks for furnaces due to its high thermal stability and resistance to , withstanding temperatures up to 1700°C. These bricks form protective layers during operation, minimizing penetration and erosion from molten in ladles and converters. Spinel group minerals serve as catalysts in petrochemical processes, with (FeCr₂O₄)-based materials facilitating reactions like oxidative dehydrogenation for production. (Fe₃O₄), another spinel ferrite, is employed in for , generating hydroxyl radicals to degrade organic pollutants efficiently. In , synthetic Ni-Zn ferrites, which adopt the structure, are essential for high-frequency applications such as devices and transformers, offering low losses and high permeability above 1 MHz. These materials enable compact designs in inductors and absorbers for and systems. Transparent polycrystalline aluminum oxynitride (AlON) spinel, developed in the 1990s, provides durable windows for military (IR) sensors, transmitting from visible to mid-IR wavelengths (0.4–5 μm) while resisting ballistic impacts. Its optical clarity and hardness make it suitable for canopies and domes. Recent advancements include spinel-structured LiMn₂O₄ cathodes in lithium-ion batteries, offering a theoretical capacity of approximately 100 mAh/g and commercial adoption since the for high-power applications like electric vehicles and power tools. These cathodes provide cost-effective alternatives to cobalt-based materials, with improved cycle life through doping strategies.

References

  1. https://www.sciencedirect.com/topics/physics-and-astronomy/spinel
  2. https://www.mindat.org/min-29156.html
  3. https://geo.libretexts.org/Bookshelves/Geology/Mineralogy_(Perkins_et_al.)/14:_Mineral_Descriptions/14.05:_Oxide_Minerals/14.5.02:_Spinels_and_other_Oxides_with_Mixed_Coordination
  4. https://www.gia.edu/spinel-description
  5. https://www.mdpi.com/2075-163X/12/8/928
  6. https://mgc-labs.ru/en/encyclopedia/
  7. https://www.gia.edu/spinel-history-lore
  8. http://www.minsocam.org/msa/collectors_corner/arc/hauyv.htm
  9. https://www.efstathiakosmima.gr/en/POLUTIMOI-LITHOI/Spinellios/
  10. https://mineralogy-ima.org/docs/IMA_Meetings/Dehli_1964.pdf
  11. https://rruff.info/uploads/EJM31_183.pdf
  12. https://materialsproject.org/materials/mp-3536
  13. https://www.geo.arizona.edu/xtal/group/pdf/AM92_1031.pdf
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