14.5.2: Spinels and other Oxides with Mixed Coordination

Inverse Spinels
spinel MgAl₂O₄
magnetite Fe₃O₄

Normal Spinels
chromite FeCr₂O₄
franklinite ZnFe₂O₄

Other Oxides
perovskite CaTiO₃
chrysoberyl BeAl₂O₄
uraninite UO₂
thorianite ThO₂
cuprite Cu₂O

In minerals of the spinel group, both tetrahedral and octahedral sites are occupied by metal ions. In what we call normal spinels, each metal species is found in either tetrahedral or octahedral coordination but not both. We can write the formula of normal spinels as XY₂O₄, with X representing the tetrahedral cation and Y the octahedral cation. In inverse spinels, one metal species occupies both coordinations. A general formula is Y[XY]O₄, with the brackets identifying the two different metal ions in octahedral sites.

Mineralogists have identified other oxides that do not fit into the tetrahedral, octahedral, or spinel groups. In uraninite and thorianite, for example, metal atoms occupy cubic sites. In perovskite, Ti and Ca are in 6-fold and 12-fold coordinations, respectively. In cuprite, Cu is in 2-fold coordination; and chrysoberyl is isostructural with olivine.

For more general information about oxides, see Chapter 9: Ore Deposi

Spinel MgAl₂O₄

Origin of Name
From the Latin word spina, meaning “thorn,” a reference to the sharp crystals.

Figure 14.337: Red spinel octahedra on calcite in a marble; crystals are up to 1 cm across

Hand Specimen Identification
The term spinel is used in a generic sense to describe any of the many minerals with spinel structure. The term also refers to a specific mineral with composition MgAl₂O_4. This mineral is recognized by its octahedral crystals, hardness (H = 8), and vitreous luster. Spinel comes in many different colors, but the red color seen in Figure 14.337 is most common and most diagnostic.

Physical Properties

hardness	8
specific gravity	3.5 to 4.0
cleavage/fracture	none/conchoidal
luster/transparency	vitreous/transparent to translucent
color	variable, red, lavender, blue, green, brown, white, or black
streak	white

Properties in Thin Section
Nearly pure MgAl₂O₄ spinel is colorless in thin section but is pleochroic green or blue-green if Fe substitutes for Mg. Octahedral shape and high index of refraction aid identification. Isotropic, n = 1.74.

Crystallography
Spinel is cubic, a = 8.09, Z = 8; space group \(F\dfrac{4}{d}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{d}\overline{3}\dfrac{2}{m}\).

Habit
Spinel typically forms octahedral crystals; twinning and modifying faces are common. Massive forms and irregular grains are also known.

Structure and Composition
Spinel minerals have relatively simple cubic structures. MgO₆ octahedra and AlO₄ tetrahedra share edges and are closest packed. Fe, Zn, and Mn may substitute for Mg. Fe and Cr may substitute for Al.

Occurrence and Associations
Spinel is a high-temperature mineral found in metamorphosed carbonates or schists, as an accessory in mafic igneous rocks, and in placers. Associated minerals include calcite, dolomite, garnet, and Ca-Mg silicates in marbles; garnet, corundum, sillimanite, andalusite and cordierite in highly aluminous rocks; diopside, olivine, chondrodite, in mafic rocks; and other dense minerals in placers.

Varieties
Pleonaste is the name given to intermediate Fe-Mg spinels. Ruby spinel is the name of gemmy-red spinel; various other gem names are used to a lesser extent.

Related Minerals
Spinel is isostructural with other members of the spinel group and with bornhardite, Co₃Se₄; linnaeite, Co₂S4; polydymite, Ni₃S₄; indite, FeIn₂S₄; and greigite, Fe₃S₄. Spinel forms solid solutions with other members of the spinel group, including hercynite, FeAl₂O₄; gahnite, ZnAl₂O₄; galaxite, MnAl₂O₄; zincochromite, ZnCr₂O₄; and magnesiochromite, MgCr₂O₄.

Magnetite Fe₃O₄

Origin of Name
Named after Magnesia, near Macedonia in Thessaly, where the Greeks found this mineral.

Figure 14.338: Magnetite is one of the few very magnetic minerals

Figure 14.339: Octahedral magnetite crystals from the Potosi Department, Brazil; the specimen is 6 cm tall

Hand Specimen Identification
Magnetite, a member of the spinel group, is similar to other dense, hard, dark-colored minerals. It is, however, characterized by its strong magnetism, which distinguishes it from ilmenite, chromite, and other similar looking minerals (Figure 14.338). Macroscopic euhedral crystals are uncommon, but form octahedra (Figure 14.339).

Physical Properties

hardness	6
specific gravity	5.20
cleavage/fracture	none/subconchoidal
luster/transparency	metallic/opaque
color	black
streak	black

Crystallography
Magnetite is a cubic mineral, a = 8.397, Z = 8; space group \(F\dfrac{4}{d}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{d}\overline{3}\dfrac{2}{m}\).

Habit
Magnetite crystals are octahedra that sometimes display contact or lamellar twins. Magnetite is also common as massive or granular aggregates or disseminated as fine grains.

Structure and Composition
Magnetite has the spinel structure (see spinel structure, above). Some Ti is usually present in magnetite; at high temperature a complete solid solution to Fe₂TiO₄ (ulvöspinel) is possible. Minor amounts of Mg, Mn, Ni, Al, Cr, and V may substitute for Fe.

Occurrence and Associations
Magnetite is common and widespread. It is found as an accessory in many types of igneous, metamorphic, and sedimentary rocks and in unconsolidated sediments. It may be concentrated to form ore bodies by magmatic, metamorphic, or sedimentary processes.

Related Minerals
Magnetite forms solid solutions with ulvöspinel, Fe₂TiO₄; magnesioferrite, MgFe₂O₄; jacobsite, MnFe₂O₄; and to a lesser extent with maghemite, Fe₂O₃.

Chromite FeCr₂O₄

Origin of Name
The name chromite refers to this mineral’s composition.

Figure 14.340: 7-cm wide specimen of chromite with interstitial plagioclase, from Zimbabwe

Figure 14.341: Typical example of coarse- and fine-grained chromite ore

Hand Specimen Identification
Black color, high density, black/brown streak, metallic luster, and association distinguish chromite. It may be slightly magnetic and is sometimes confused with magnetite or ilmenite. The photos in these two photos show typical examples.

Physical Properties

hardness	5.5
specific gravity	4.5-4.8
cleavage/fracture	none/conchoidal
luster/transparency	metallic/subtranslucent; opaque
color	black, brownish black
streak	brown, dark brown

Crystallography
Chromite is a cubic mineral, a = 8.37, Z = 8; space group \(F\dfrac{4}{d}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{d}\overline{3}\dfrac{2}{m}\).

Habit
Rare euhedral crystals are octahedral; chromite is generally massive or granular.

Structure and Composition
The structure of chromite is the same as that of all the spinel minerals (see spinel structure). Mg, Fe, Al, and Zn are typical impurities.

Occurrence and Associations
Primary chromite is found with olivine, pyroxene, spinel, magnetite, and sulfides in ultramafic rocks. It is also found in placers and black sands. Chromite is commonly a minor accessory mineral but may be concentrated by gravity or magmatic processes.

Related Minerals
Chromite has one polymorph, donathite. Chromite forms solid solutions with magnesiochromite, MgCr₂O₄, and hercynite, FeAl₂O₄, and to lesser extent with other spinel minerals.

Franklinite ZnFe₂O₄

Origin of Name
Named after Franklin, New Jersey, the type locality where this mineral is found.

Figure 14.342: Octahedral franklinite crystals from Franklin, New Jersey; the specimen is 10 cm across

Physical Properties

hardness	6
specific gravity	5.32
cleavage/fracture	none/conchoidal
luster/transparency	metallic/opaque
color	black or iron-black
streak	black, reddish brown to dark brown

Crystallography
Franklinite is cubic, a = 8.43, Z = 8; space group \(F\dfrac{4}{d}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{m}\overline{3}\dfrac{2}{m}\).

Habit
Octahedral franklinite crystals, often with modifying faces, are common in Franklin, New Jersey, the only place where this mineral is found in large quantities. Franklinite also occurs as discrete rounded grains, as granular masses, or in massive lenses.

Structure and Composition
Franklinite‘s structure is the same as that of other spinel minerals (see spinel structure). Franklinite normally contains substantial Mn substituting for Zn. Mn may also substitute for Fe. Minor Mg, Cr, and V also may be present.

Occurrence and Associations
Franklinite is associated with zincite and willemite, two other zinc minerals, in zinc ore deposits at Franklin, New Jersey. The host rock is a coarse-grained limestone.

Related Minerals
Franklinite is similar in many ways to other dark-colored spinel minerals. It forms minor solid solutions with most of them.

Chrysoberyl BeAl₂O₄

Origin of Name
From the Greek words meaning “golden beryl.”

Figure 14.343: Light-green chrysoberyl crystal on quartz; minor black tourmaline is present

Figure 14.344: Chrysoberyl displaying cyclic twinning, from Minas Gerais, Brazil; 7.3 mm across,

Hand Specimen Identification
Translucent to transparent character, light color, vitreous luster, extreme hardness (H = 8.5), and common twinning characterize chrysoberyl. Chrysoberyl looks superficially like any of a number of other light-colored translucent/transparent minerals, but is significantly harder than most. Greenish to yellow colors are typical, like the specimens in these two figures, but other hues are known.

When visible, chrysoberyl‘s orthorhombic symmetry helps identify chrysoberyl. Cyclic twinning, such as seen in Figure 14.344, is classic for this mineral.

Physical Properties

hardness	8.5
specific gravity	3.7 to 3.8
cleavage/fracture	good but indistinct prismatic {011}, poor (010)/ subconchoidal
luster/transparency	vitreous/transparent to translucent
color	yellow, green, or brown
streak	white

Properties in Thin Section
Chrysoberyl is biaxial (+), α = 1.747 , β = 1.748, γ = 1.757, δ = 0.010, 2V = 45°.

Crystallography
Chrysoberyl is orthorhombic, a = 4.24, b = 9.39, c = 5.47, Z = 4; space group \(P\dfrac{2_1}{b}\dfrac{2_1}{n}\dfrac{2_1}{m}\); point group \(\dfrac{2}{m}\dfrac{2}{}\dfrac{2}{m}\).

Habit
Chrysoberyl is generally tabular and sometimes heart shaped or pseudohexagonal due to cyclic twinning. Faces are often striated.

Structure and Composition
Chrysoberyl‘s structure, similar to that of olivine, contains hexagonal closest packed oxygens with Be in tetrahedral sites and Al in octahedral sites.

Occurrence and Associations
Chrysoberyl is a rare mineral occurring in granites, pegmatites, mica schists, and some placers.

Varieties
Cat’s eye (cymophane) is a green chatoyant gem variety of chrysoberyl. Alexandrite is an emerald-green gem variety that appears red under artificial light. Both are very valuable.

Related Minerals
Chrysoberyl is isostructural with olivine minerals. It is chemically similar to members of the spinel group but has a different structure.

Uraninite UO₂

Origin of Name
The name uraninite refers to this mineral’s composition.

Figure 14.345: Typical anhedral sample of uraninite

Figure 14.346: Uraninite crystals from Topsham, Maine; 2-3 cm in long dimension

Figure 14.347: Botryoidal uraninite from the Czech Republic, 6 cm tall

Hand Specimen Identification
Uraninite is characterized by its radioactivity, association with other radioactive minerals, high specific gravity, brown to black streak, black color, and luster. It can be confused with other dense, dark-colored minerals.

Figure 14.345 shows a typical nondescript sample of uraninite. In contrast, Figure 14.346 show a spectacular euhedral crystal cluster. Figure 14.347 is a photo of botryoidal uraninite. Each of the spheres grew outward from seeds at their centers and subsequently grew together to produce the cluster seen.

Physical Properties

hardness	5.5
specific gravity	7 to 9.5
cleavage/fracture	none/conchoidal
luster/transparency	pitchy dull to submetallic/opaque
color	black
streak	brown to black

Crystallography
Uraninite is cubic, a = 5.4682, Z = 4; space group \(F\dfrac{4}{m}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{m}\overline{3}\dfrac{2}{m}\).

Habit
Individual crystals of uraninite are rare but may form cubes, octahedra, or combinations. Massive, colloform, or botryoidal forms are more typical.

Structure and Composition
Uraninite is isostructural with fluorite (see fluorite structure, above). Uraninite is isostructural with thorianite, ThO₂; cerianite, (Ce,Th)O₂; and fluorite (CaF₂). It is closely related to sellaite, MgF₂, and frankdicksonite, BaF₂.U valence and U:O ratios are somewhat variable; uraninite is really a mixture of UO₂ and U₃O₈. Th may substitute for U; N, Ar, Fe, Ca, Zr, and rare earths are also commonly present. Pb and Ra are always present as radioactive decay products.

Occurrence and Associations
Uraninite occurs in granitic pegmatites, in veins, and in sandstones. Associated minerals include quartz, K-feldspar, zircon, tourmaline, and monazite in pegmatites; cassiterite, galena, sulfides, and arsenides in veins; and quartz and various other secondary minerals in sandstones. Uraninite also concentrates in coal and other organic debris in sediments and related sedimentary rocks.

Varieties
Massive forms of uraninite are called pitchblende.

Related Minerals
Uraninite is isostructural with fluorite, CaF₂, and cerianite, (Ce,Th)O₂. It forms complete solid solution with thorianite, ThO₂. Other related minerals include baddeleyite, ZrO₂.

Cuprite Cu₂O

Origin of Name
From the Latin word cuprum, meaning “copper.”

Figure 14.348: Cuprite octahedra (red) with malachite (green) from the Democratic Republic of the Congo

Figure 14.349: Cuprite crystals from Kazakhstan; the specimen is 2 cm wide

Hand Specimen Identification
Red- or black-colored crystals, sometime showing internal reflection, octahedral crystal shape, adamantine luster, brownish red streak, and association all help identify cuprite. It may be confused with cassiterite, hematite, and cinnabar, all red minerals. It can sometimes also be confused with rutile and other red-black oxides but is denser than most of them. Figures 14.348 and 14.349 show two examples of cuprite crystals.

Physical Properties

hardness	3.5 to 4
specific gravity	6.14
cleavage/fracture	imperfect {111}/conchoidal
luster/transparency	adamantine, submetallic or earthy/often translucent to transparent
color	dark red, sometimes almost black
streak	brownish red, metallic

Crystallography
Cuprite is cubic, a = 4.27, Z = 2; space group \(F\dfrac{4}{m}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{m}\overline{3}\dfrac{2}{m}\).

Habit
Octahedral, cubic, and dodecahedral forms, often in combination, are common. Cuprite may occur as elongated capillary crystals called chalcotrichite.

Structure and Composition
Oxygen, in tetrahedral groups, is arranged in a body centered cubic array. Each Cu is bonded to two O. Cuprite is generally close to end-member composition; Fe is a common minor impurity.

Occurrence and Associations
Cuprite is a secondary mineral found in the oxidized zones of copper deposits. Native copper, limonite, and secondary copper minerals such as malachite, azurite, and chrysocolla are typically associated minerals.

Related Minerals
Tenorite, CuO, is similar in composition and occurrence.