14.6: Hydroxide Minerals

VI. Hydroxides
gibbsite Al(OH)₃
brucite Mg(OH)₂
manganite MnO(OH)
goethite FeO(OH)
diaspore AlO(OH)
romanechite BaMn₉O₁₆(OH)₄

The hydroxide minerals all contain OH^– as an essential anion species. Some, such as diaspore also contain O^2- as well. The atomic arrangements are generally simple; brucite and gibbsite have layered structures equivalent to the trioctahedral and dioctahedral layers in micas. Romanechite (also called psilomelane) has a structure related to rutile and spinel. Only limited solid solution occurs between the various hydroxides, but they are often found in intimate mixtures with each other and with oxide minerals.

Bauxite refers to a mixture of Al oxides and hydroxides, limonite to a mixture of Fe oxides and hydroxides, and wad to a mixture of Mn oxides and hydroxides.

For more general information about oxides, see Section 9.2.3 in Chapter 9.

Gibbsite Al(OH)₃

Origin of Name
Named after Colonel G. S. Gibbs (1777–1834), a mineral collector.

Figure 14.350: Typical light-green gibbsite, from the Yunnan Province, China; FOV is 11.1 cm across

Figure 14.351: Turquoise gibbsite from the Yunnan Province, China; the sample is 6.1 cm across

Hand Specimen Identification
Earthy smell and appearance, and sometimes green or blue color help identify gibbsite. When clear, it looks like other soft clear minerals. It may occasionally be confused with kaolinite, talc, or brucite when uncolored. When light green, it looks like smithsonite.

Figures 14.350 and 14.351 show two samples of gibbsite from the same place in China – one green and one turquoise blue.

Physical Properties

hardness	2.5 to 3
specific gravity	2.40
cleavage/fracture	perfect (001)/uneven
luster/transparency	vitreous, pearly, earthy/ transparent to translucent
color	greenish or aquamarine blue; less commonly white or gray
streak	white

Properties in Thin Section
Gibbsite is usually colorless in thin section; maximum interference colors are upper first order. It is difficult to distinguish from clay minerals. Biaxial (+), α = 1.57 , β = 1.57, γ = 1.59, δ = 0.02, 2V = 0° to 40°

Crystallography
Gibbsite is monoclinic, a = 8.641, b = 5.07, c = 9.719, β = 94.57°, Z = 8; space group \(P\dfrac{2_1}{n}\); point group \(\dfrac{2}{m}\).

Habit
Gibbsite may form foliated or tabular crystals, typically very small with a pseudohexagonal shape. Granular aggregates, colloform or radiating masses, and coatings are most common.

Structure and Composition
In the layered gibbsite structure, octahedral Al³⁺ occupies two of the three sites between OH sheets. Small amounts of Fe may replace Al.

Occurrence and Associations
Gibbsite is a secondary mineral associated with aluminum deposits, bauxites, and laterites. Diaspore and böhmite, other aluminum hydroxides, are typically intimate associates.

Related Minerals
Gibbsite is similar in structure to brucite, Mg(OH)₂. It has several polymorphs, including bayerite, doyleite, and nordstrandite. Other related minerals include diaspore and böhmite, both AlO(OH), and bauxite, a mixture of gibbsite, böhmite, and diaspore.

Brucite Mg(OH)₂

Origin of Name
Archibald Bruce (1777–1818), an early American mineralogist, was the inspiration for this mineral’s name.

Figure 14.352: Translucent white brucite from the Wood’s Chrome Mine, Texas; the specimen is 15 cm across

Figure 14.353: Green-blue brucite from the Palobora Mine, South Africa; the specimen is 8.4 cm across

Hand Specimen Identification
Good platy or micaceous cleavage, flexible sheets, clear or light color, and luster help identify brucite. It may be confused with gypsum or gibbsite, other soft light minerals that may be clear. Color is variable, but translucent white, green, or blue crystals are most common. The two photos above show examples of brucite; Figure 8.76 shows another example.

Physical Properties

hardness	2.5
specific gravity	2.4 to 2.5
cleavage/fracture	perfect basal (001)/sectile
luster/transparency	vitreous, pearly/transparent to translucent
color	white, light green or blue, or gray
streak	white

Properties in Thin Section
Brucite is colorless in thin section and displays anomalous second-order red or blue interference colors. Brucite may be mistaken for talc, white mica, or gypsum, but they are all biaxial. Uniaxial (+), ω = 1.57, ε = 1.58, δ = 0.02.

Crystallography
Brucite belongs to the trigonal crystal system, a = 3.147, c = 4.769, Z = 1; space group \(P\overline{3}\dfrac{2}{m}\); point group \(\overline{3}\dfrac{2}{m}\).

Habit
Broad platy, foliated, or tabular crystals are typical for brucite. Massive or fibrous aggregates and foliated masses are common.

Structure and Composition
Brucite‘s structure is similar to that of gibbsite: sheets of OH sandwich Mg in octahedral coordination. Fe, Mn, and Zn may substitute in minor amounts for Mg.

Occurrence and Associations
Brucite occurs in veins of mafic rocks, serpentinite, or talc–chlorite schists, and in metamorphosed carbonates or marls. It also occurs as a secondary mineral that form from periclase. Associated minerals include chlorite and other secondary magnesium minerals in mafic rocks, and calcite, dolomite, talc, magnesite, and periclase in carbonates.

Related Minerals
Brucite is isostructural with gibbsite, Al(OH)₃. It is isotypical with pyrochroite, Mn(OH)₂; amakinite, (Fe,Mg)(OH)₂; portlandite, Ca(OH)₂; and theophrastite, Ni(OH)₂.

Manganite MnO(OH)

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

Figure 14.354: Manganite from the Wessels Mine, South Africa; the specimen is 4.3 cm tall

Hand Specimen Identification
High density, prismatic crystals that may be striated, and black color characterize manganite. It may be confused with pyrolusite, with which it is frequently found, but is significantly harder and has a brown streak.

hardness	4
specific gravity	4.2 to 4.4
cleavage/fracture	perfect basal (010), good {110}/sectile
luster/transparency	submetallic/opaque
color	steel gray to black
streak	dark gray, black, sometimes red-brown

Crystallography
Manganite is monoclinic, a = 8.84 , β = 5.23, c = 5.74, β = 90.0°, Z = 8; space group \(B\dfrac{2_1}{d}\); point group \(\dfrac{2}{m}\).

Habit
Manganite crystals are prismatic, sometimes twinned, and may show striations and complicated terminations. Bundled, stalactitic, columnar, bladed, or fibrous aggregates are common.

Structure and Composition
In manganite, O and OH are nearly hexagonal closest packed. Mn(O,OH)₆ octahedra share corners to make a three-dimensional framework. Fe and Mg may replace Mn in small amounts.

Occurrence and Associations
Manganite is an uncommon secondary mineral found in veins with other Mn oxides or hydroxides, carbonate minerals, limonite, and barite.

Related Minerals
A number of manganese oxides and hydroxides are closely related, including pyrolusite, MnO₂; partridgite and bixbyite, both Mn₂O₃; hausmannite, Mn₃O₄; hollandite, Ba₂Mn₈O₁₆; romanechite, BaMn₉O₁₆(OH)₄; pyrochroite, Mn(OH)₂; vernadite, Mn(OH)₄; and takanelite, (Mn,Ca)Mn₄O₉ · H₂O. Wad is a mixture of these manganese minerals.

Goethite FeO(OH)

Origin of Name
Named after J. W. Goethe (1749–1832), a German poet and scientist.

Figure 14.355: Typical fine-grained goethite

Figure 14.356: Black goethite crystals from the Mont-Roc Mine, France

Figure 14.357: Spray of goethite crystals from Teller County, Colorado; the specimen is 5.9 cm wide

Hand Specimen Identification
Brown color, sometimes earthy appearance, habit, and streak identify goethite. It is sometimes confused with hematite but has a brownish yellow streak, in contrast with hematite’s red streak. Most samples of goethite are fine-grained, rusty looking, and unexciting.

Figure 14.355 shows typical fine-grained earthy reddish goethite. In contrast, Figures 14.356 and 14.357 are photos of black euhedral goethite crystals. Goethite crystals such as these may be hard to distinguish from other black oxides and hydroxides unless orthorhombic symmetry is apparent. See also the photo of black goethite with crocoite in Figure 14.426.

Physical Properties

hardness	5 to 5.5
specific gravity	4.3
cleavage/fracture	perfect but rarely seen (010)/uneven
luster/transparency	subadamantine to earthy/subtranslucent to opaque
color	yellow-brown to dark brown or black
streak	brown-yellow

Properties in Thin Section
Goethite is pleochroic yellow, orange-red, or various shades of brown in thin section. It has high positive relief and extremely high birefringence that may be masked by its color. Goethite is difficult to distinguish from other Fe oxides. Biaxial (-), α = 2.26 to 2.26 , β = 2.39 to 2.41, = 2.40 to 2.52, δ = 0.15, 2V = 0° to 27°.

Crystallography
Goethite is orthorhombic, a = 4.65, b = 10.02, c = 3.04, Z = 4; space group \(P\dfrac{2_1}{b}\dfrac{2_1}{n}\dfrac{2_1}{m}\); point group \(\dfrac{2}{m}\dfrac{2}{m}\dfrac{2}{m}\).

Habit
Euhedral goethite crystals are rare; it is usually platy, prismatic, fibrous, botryoidal, or mammillary. Concentric growth bands are common.

Structure and Composition
Goethite is isostructural with diaspore. Edge-sharing Fe(O,OH)₆ octahedra form bands running parallel to the c-axis. Perpendicular to c, the bands form a checkerboard pattern, leaving empty channels between them. Up to several weight percent of Mn and absorbed water are often present.

Occurrence and Associations
Goethite is a widely distributed secondary mineral formed by the weathering of Fe-rich compounds. It concentrates in sediments, gossans, and laterites. Common associated minerals include siderite, pyrite, magnetite, and many residual weathering products.

Varieties
Bog ore is a porous, poorly consolidated form of goethite. Limonite refers to a mixture of hydrous iron oxides of variable chemistry and crystallinity.

Related Minerals
Lepidocrocite, akaganeite, and feroxyhyte are all rare polymorphs of goethite. Diaspore has the same structure except that Al replaces two-thirds of the Fe. Other related minerals are manganite, MnO(OH); heterogenite, CoO(OH); and montroseite, (V, Fe)O(OH).

Diaspore AlO(OH)

Origin of Name
From the Greek word diaspora, meaning “to scatter,” referring to its decrepitation when heated.

Figure 14.358: Translucent cleavage fragments of diaspore

Figure 14.359: White translucent diaspore on top of a chlorite-magnetite mix, from Chester, Massachusetts, FOV is 10.8 cm across

Hand Specimen Identification
Bladed habit, hardness, good cleavage, and luster identify diaspore. It is occasionally confused with brucite, but brucite is much softer.

Diaspore has one excellent cleavage, and Figure 14.358 shows typical cleavage fragments. The specimen in Figure 14.359 contains translucent gray diaspore with green chlorite and specks of magnetite from a classic collecting spot near Chester, Massachusetts.

Physical Properties

hardness	6.5 to 7
specific gravity	3.2 to 3.5
cleavage/fracture	one perfect (010), poor (210)/conchoidal
luster/transparency	pearly, vitreous/translucent
color	colorless, yellow, gray, white or green
streak	white-yellow

Properties in Thin Section
Diaspore has high relief, is colorless or pale, displays parallel extinction, and shows up to third-order interference colors. It may be confused with gibbsite or sillimanite, but gibbsite has lower relief and inclined extinction, and sillimanite has lower relief and birefringence. Biaxial (+), α = 1.68 to 1.71 , β = 1.71 to 1.72, γ = 1.73 to 1.75, δ = 0.04, 2V = 85°.

Crystallography
Diaspore is orthorhombic, a = 4.42, b = 9.40, c = 2.84, Z = 4; space group \(P\dfrac{2_1}{b}\dfrac{2_1}{n}\dfrac{2_1}{m}\); point group \(\dfrac{2}{m}\dfrac{2}{m}\dfrac{2}{m}\).

Habit
Platy, tabular, or acicular crystals of diaspore are common. Massive forms or foliated aggregates are also common.

Structure and Composition
Diaspore structure is the same as that of goethite, except that Al replaces two-thirds of the Fe (see goethite structure, above). Diaspore is generally close to end-member composition, but minor Fe or Mn may replace Al.

Occurrence and Associations
Diaspore is found in emery deposits with corundum, magnetite, spinel, and chlorite; in bauxites with other aluminum oxides and hydroxides; and as a rare mineral in some pegmatites.

Related Minerals
Böhmite is a polymorph of diaspore. Diaspore is isostructural with goethite and close in composition to gibbsite, Al(OH)₃.

Romanechite (Psilomelane) BaMn₉O₁₆(OH)₄

Origin of Name
From the original locality in Romaneche, France.

Figure 15.360: Fine-grained and botryoidal romanechite

Hand Specimen Identification
Black color, submetallic to dull luster, and habit identify romanechite. Figure 14.360 shows the two most common habits for this mineral.

Romanechite is sometimes confused with pyrolusite, but pyrolusite is harder. A brown-black streak distinguishes romanechite from limonite and other hydrous iron oxides.

Physical Properties

hardness	5 to 6
specific gravity	3.5 to 4.7
luster/transparency	submetallic, dull/opaque
color	black
streak	brown-black

Crystallography
Romanechite is monoclinic, a = 9.56, b = 2.88, c = 13.85, β = 90.5°, Z = 2; space group \(A\dfrac{2}{m}\); point group \(\dfrac{2}{m}\).

Habit
Romanechite is typically in reniform, botryoidal, or dendritic masses. Individual euhedral crystals are not known.

Structure and Composition
Romanechite‘s structure is related to the structures of rutile and spinel. Mn(O,OH)₆ octahedra form distorted chains; Ba and H₂O occupy holes between the chains. Many other elements, including As, V, W, Co, Cu, Ni, Mg, Ca, and alkalis may be present in small or trace amounts.

Occurrence and Associations
Romanechite is a rare secondary mineral associated with pyrolusite, manganite, calcite, and hematite.

Related Minerals
Related manganese minerals include pyrolusite, MnO₂; manganite, MnO(OH); cryptomelane, KMn₈O₁₆; hollandite BaMn₈O₁₆; and wad, a mixture of manganese oxides and hydroxides.