Mineralogy of Oxides and Hydroxides

Introduction to Oxide and Hydroxide Mineral Classes

The oxide and hydroxide classes can serve as an instructive connection between primarily metallic opaque and dark-colored native element and sulfide minerals and vitreous, transparent, and light-colored silicate minerals. While studying oxides and hydroxides, we will encounter both opaque dark-colored ore minerals and sparkling transparent gemstones.

There are 395 mineral species in the oxide and hydroxide classes. (Klein & Dutrow, 2007) We will cover the most essential and common membeers. To understand why oxides and hydroxides are studied together as well as their differences, we must consider their chemistry. Take a look at the formulas in the following table. Oxides and hydroxides have identical metallic cations. The only difference is their anions: O^2- for oxides and (OH)^- for hydroxides.

Oxides

Simply put, oxide minerals are compounds of metals with oxygen. To be more exact, oxides are compounds of metal cations bonded with an oxygen anion (O^2-). The general formula is XnOm, where the X position is occupied by Fe, Al, Cr, Ti, Mn, U, or Mg, and n and m are simply integers.

Many oxides are double compounds of the type XO×X₂O₃. Hematite, a simple iron oxide with formula Fe₂O₃, and magnetite, with formula Fe₃O4, are good examples of this. However, as written, the magnetite formula does not reflect a double oxide. The more accurate way to write the magnetite formula is FeO×Fe₂O₃. This formula reflects its double oxide nature. Fe is duplicated here because the iron is different. Iron can occur in two valence states: Fe³⁺ and Fe²⁺. Thus, the more accurate formulas are Fe³⁺O₃ for hematite and Fe²⁺O×Fe³⁺₂O₃. for magnetite.

Mineral Formula and Charge Balance

Studying oxides is an excellent way to learn that a mineral formula should be neutral, which means the charges of anions and cations should be equal. Otherwise, the compound is unstable and incomplete and, therefore, cannot occur in nature. The compound cannot occur in an unbalanced state.

Example 1: As we already know, Fe₂O₃ is a hematite formula. The anion (negatively charged ion) represented by oxygen has a charge of 2^-. The iron cation (positively charged ion) in hematite has a charge of 3⁺. If we write the formula as Fe³⁺O^2-, it is unbalanced. The easiest way to balance the formula is to add integers. We multiply 3⁺ by 2 to get 6⁺. On the other side we have 2^- × 3 = 6^-. In this case, we have a neutral compound Fe³⁺₂O₂^-3.

Example 2: What if two different elements represent a cation? In ilmenite, Fe²⁺Ti⁴⁺O^2-₃, there are two cations: Fe²⁺ and Ti⁴⁺. These make a mutual positive charge equal to 6⁺. Therefore, to compensate, the anion charge of 2^- is multiplied by 3.

Classes and Groups

Oxide minerals are subdivided into classes and groups based on crystal structure and cation site distribution.

The main classes are simple oxides (XO, X₂O, XO₂, and X₂O₃) and complex or multiple oxides (XY₂O₄). (Strunz & Nickel, 2001)

These classes can be further subdivided into six structural groups.This means that the minerals have identical crystal structures or ion arrangements. It does not mean that the minerals crystallize together under the same conditions. This subdivision simply recapitulates the crystal structure of a mineral.

The structural groups are:

• Periclase Group
• Zincite Group
• Hematite Group
• Rutile Group
• Spinel Group
• Goethite Group

Let us describe each oxide class separately, adding a corresponding structural group:

• The XO oxide class is represented by two groups: periclase, MgO, and zincite, ZnO.

• The X₂O oxide class has two mineral examples: cuprite, Cu₂O, and ice, H₂O.

• The XO₂ oxide class includes representatives of the rutile group, which are rutile itself, TiO₂, pyrolusite, MnO₂, cassiterite, SnO₂, and, separately, uraninite, UO₂.

• The X₂O₃ oxide class is represented by three examples of the hematite group: hematite itself, Fe₂O₃, corundum, Al₂O₃, and ilmenite, FeTiO₃.

• The XY₂O₄ oxide class includes several groups: the spinel group represented by spinel itself, MgAl₂O₄, magnetite, Fe²⁺Fe³⁺₂O₄, chromite, FeCr₂O₄, franklinite, (Zn,Fe,Mn)(Fe,Mn)₂O₄, and gahnite, ZnAl₂O₄. Separate groups are chrysoberyl, BeAl₂O₄, and Fe-columbite, (Fe,Mn)Nb₂O₆.

Hydroxides

As the name indicates, hydroxides involve hydrogen or water. Chemically speaking, hydroxide minerals contain an (OH)^- anion in place of all or some O^2- and sometimes water molecules (H₂O). The presence of an (OH)^- anion and water molecules weakens the bonding forces. As a result, the hardness of hydroxides is lowered.

The most common hydroxides are brucite, Mg(OH)₂, manganite, MnO(OH), gibbsite, γ-Al(OH)₃, böhmite, Al(OH)₃, and diaspore, α-Al(OH)₃.

The alteration and weathering of oxides commonly produces hydroxides. Most hydroxides are exogenous and exist only in low-temperature environments. Their occurrence forms also reflect their secondary alteration origins. They commonly occur in aggregate forms, such as botryoidal or stalactitic masses, with concentric or radial fibrous internal structures unable to develop well-formed crystals.

Three hydroxides — gibbsite, böhmite, and diaspore — usually occur together and create bauxite rock, the principal raw material for aluminum production. Bauxite also has minor admixtures of other minerals like kaolinite, quartz, hematite, and goethite. Bauxite is a product of chemical weathering.

​​Geology of Oxides and Hydroxides

The origins of oxides and hydroxides can vary. They can be magmatic, hydrothermal, or metamorphic.

In general, oxides are formed in more high-temperature endogenic environments and in association with igneous and metamorphic rocks. Some resistant oxides like spinel, corundum, rutile, and ilmenite can be transported in the form of detrital grains and create places of secondary accumulation. Thus, gemstones like rubies and sapphires — varieties of corundum — can be mined from alluvial placer deposits where the softer host rock was destroyed by water while the gem crystals remained almost intact.

Hydroxides are formed due to weathering (oxidation and hydration) of minerals of endogenetic origin, especially oxides. They exist in low-temperature environments and are considered secondary minerals. This means they are not formed directly from the melt. Instead, they result from the alteration of other minerals. Hydroxides are of exogenous origin and are pervasive in supergene oxidation zones, soils, and laterites. (Delvigne, 1998)

Oxides and Hydroxides as Ore Minerals

Several oxide minerals have economic significance and serve as sources of essential metals. Oxides have a vast range of applications. For example, as a source of iron, hematite is critical for steel production; aluminum is essential for construction and building materials; titanium and iron oxides are primary minerals for pigments and colorants; titanium dioxide is also a common catalyst widely used in various industrial processes, including plastic and chemical production. The electronics field utilizes many oxide minerals, like tin oxide coatings on solar cells.

Here is a list of critical elements and their primary oxide and hydroxide ore minerals:

• Iron (Fe) — hematite Fe₂O₃, magnetite Fe₃O₄
• Chromium (Cr) — chromite FeCr₂O₄
• Manganese (Mn) — pyrolusite MnO₂
• Zinc (Zn) — zincite ZnO
• Tin (Sn) — cassiterite SnO₂
• Titanium (Ti) — ilmenite FeTiO₃
• Aluminum (Al) — bauxite, a mixture of gibbsite γ-Al(OH)₃, böhmite Al(OH)₃, and diaspore α-Al(OH)₃

Physical Properties of Oxides and Hydroxides

Most oxides, especially ore minerals, are often dark-colored and opaque with metallic luster. They can range from medium hardness (5-6), such as magnetite and hematite, to high hardness (8-9), such as spinel and corundum. They also have mid-range density values. They can occur both in well-crystallized and aggregate forms.

Hydroxides are softer than oxides because of (OH)^- anions and molecular water. As minerals formed due to weathering, hydroxides mainly occur in as aggregates with radiating structures and porous or powdery masses.

Diagnostic Characteristics of Oxides and Hydroxides

Below, you'll find diagnostic characteristics for identifying oxide and hydroxide minerals quickly. Included here are the most ubiquitous and economically significant oxides and hydroxides, such as hematite, magnetite, goethite, chromite, ilmenite, rutile, cassiterite, uraninite, corundum, and spinel. We emphasize how to differentiate them from minerals with very similar appearances. The best diagnostic characteristics are highlighted in bold.

Hematite

Formula: Fe₂O₃ or Fe³⁺₂O₃

Hematite occurs in a range of forms: complex rhombohedral, pseudocubic, prismatic, thin tabular, to micaceous or platy, in rosettes or radiating fibrous aggregates, reniform, botryoidal or stalactitic masses, columnar, earthy, granular, oolitic, and concretionary. Consequently, hematite can be mistaken for almost any other black-colored metallic mineral. However, a distinguishing feature of hematite is its cherry-red or reddish brown streak color.

Magnetite

Formula: FeO×Fe₂O₃, or Fe₃O₄, or Fe²⁺Fe³⁺₂O₄

Ideally, magnetites occur as black, opaque octahedrons with metallic luster. However, in reality, magnetites can form massive aggregates. In these cases, strong magnetism helps identify magnetite. It reacts very strongly with a magnet and sometimes can attract metal flakes.

Goethite

Formula: α-Fe³⁺O(OH)

Generally, goethite forms dense, porous, or powdery masses or pseudomorphs after various Fe-minerals. Sometimes, the term limonite is used for iron hydroxide because limonite is a collective term for amorphous to cryptocrystalline mixtures of goethite and lepidocrocite (polymorphic modification of goethite with formula γ-FeOOH). Because differentiating goethite and lepidocrocite with the naked eye is hard, their mixture is called limonite, which commonly contains some relicts of hematite and variable amounts of water.

When goethite crystals are black, the streak color will help differentiate goethite from hematite.

Chromite

Formula: Fe²⁺Cr₂O₄

Chromite rarely occurs in a well-shaped octahedral form. When dealing with chromite's massive and compact form, you can differentiate it from magnetite due to the absence of magnetic properties. Chromite has a dark brown streak, which distinguishes it from hematite. You may also note that chromite's metallic luster is less evident than that of other oxides and sulfides. It is sometimes described as "sub-metallic."

Ilmenite

Formula: Fe²⁺TiO₃

Ilmenite is another black, opaque metallic mineral. In massive form, it can be mistaken for hematite, magnetite, and chromite. Ilmenite tends to have a black to reddish brown streak color, which helps differentiate it from hematite and chromite. Occasionally, weakly magnetic ilmenite forms. This property is noticeably weaker than that of magnetite.

Rutile

Formula: TiO₂

Rutiles are not typically found in massive forms. They commonly have visible crystal faces with obvious striation along the direction of crystal elongation. However, accessory rutile crystals are tiny, and a magnifying glass will be handy. Reddish hues are commonly found. Another typical feature of rutile is that it often occurs in the form of contact twins when several crystals form elbow-type or cyclic twins. Rutile can be mistaken for cassiterite. However, cyclic twins are atypical to cassiterite; the color is more brownish and orangey than reddish, and striation is perpendicular to elongation. Also, cassiterite feels heavier than the same-size rutile sample because of its higher density.

Cassiterite

Formula: SnO₂

Cassiterite looks quite similar to rutile. However, you can differentiate it from rutile due to its higher density (it feels heavier than it looks) and typical color. Cassiterites are usually more brownish, while rutiles are typically more reddish.

Uraninite

Formula: UO₂

Uraninite occurs mainly in massive form. It differs from other oxide minerals due to its low, dull luster. Because of its radioactivity, uraninite can be easily identified with a Geiger counter. If you don't have a Geiger counter, look for mineral associations. You will likely find some secondary uranium minerals with vivid yellow colors.

Corundum

Formula: Al₂O₃

The best distinguishing feature of corundum is its hardness of 9 on the Mohs scale. Barrel shapes and prominent horizontal striation are also very typical for corundum. Minerals with a hardness greater than 6-7 commonly have a white streak. This holds true for corundum. Corundum's streak will remain white regardless of body color.

Spinel

Formula: MgAl₂O₄

Spinel can be perfectly transparent to nearly opaque and occurs in a variety of colors. However, one of spinel's best diagnostic features is its hardness of 8 on the Mohs scale. This means spinel is harder than minerals from many other groups and harder than any other oxide except corundum. Spinel crystallizes in the cubic crystal system and, therefore, lacks any pleochroism. Thus, you can also distinguish spinel from corundum by using a dichroscope. (Corundum will show pleochroic colors; spinel will not). Regardless of color, spinel will always have a white streak. Spinel crystals commonly occur in octahedral form.

References for Oxides and Hydroxides

• Anthony, J. W., Bideaux,R. A., Bladh, K. W., & Nichols, M C. (2001). Handbook of Mineralogy (Vol.1), Mineralogical Society of America, Chantilly, VA 20151-1110, USA. http://www.handbookofmineralogy.org/.

• Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals. Mineralogical Society of Great Britain and Ireland, 498 p. https://doi.org/10.1180/DHZ

• Delvigne, J. E. (1998). Atlas of micromorphology of mineral alteration and weathering. Mineralogical Association of Canada, Ottawa. 509 p.

• Klein, C., & Dutrow, B. (2007). Manual of mineral science. John Wiley & Sons, 704 p.

• Nadoll, P. (2017). Oxide Minerals. Encyclopedia of Engineering Geology, 1-4. doi:10.1007/978-3-319-39193-9_345-1.

• Okrusch, M., & Frimmel, H. E. (2020). Mineralogy: An introduction to minerals, rocks, and mineral deposits. Springer Nature, 719 p. https://doi.org/10.1007/978-3-662-57316-7

• Strunz, H., & Nickel, E. H .(2001). Strunz mineralogical tables. Schweizerbart, Stuttgart. 869 p.

Olena Rybnikova, PhD

Olena Rybnikova is a gemologist and mineralogist. She has a PhD in mineralogy and petrology specializing in beryllium minerals and is a certified Applied Jewelry Professional accredited by the Gemological Institute of America. Her passion is actively promoting knowledge and appreciation of nature, geology, and gemstones.

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