Mineral Description
Gibbsite is an aluminum hydroxide mineral of the oxides and hydroxides group, with structural formula [Al(OH)3]. Gibbsite’s structure is made up by the stacking of octahedral sheets of aluminum hydroxide. Each layer consists of octahedrally (six-fold) coordinated Al3+ cations sandwiched between two closed-pack layers of OH–. Because Al is a trivalent cation, each of the hydroxyls is bonded to only two aluminums, and thus, only two-thirds of the available octahedral sites are occupied. This type of octahedral occupancy is called dioctahedral.
For similar structures with divalent cations (e.g., brucite), each hydroxyl is bonded to three cations and all octahedral sites are filled. This is called a trioctahedral mineral. This arrangement results is a neutral sheet with no charge excess or deficit. Therefore, there is no interlayer charge to retain ions between the sheets and to strongly hold the sheets together. The sheets are only held together by weak hydrogen and van der Waals bonds and this results in a very soft and easily cleavable mineral.
In normal gibbsite, the hydroxyl ions are facing each other in successive layers. Layers are slightly offset, to produce monoclinic symetry. Minor substitutions of Fe3+ for Al3+ are common in gibbsite. Crystals of gibbsite are typically-very small (< 2 µm in diameter), tabular and often, foliated, showing pseudohexagonal outline. They may occasionally be granular. Compact lamellar aggregates, or rarely fibrous masses, may occur as whorls or as stalactitic forms. The color may be white, gray, yellow, red, and brown, though most colors other than white or gray are due to traces of iron (hydr)oxides.
Brucite, Mg(OH)2, is a member of the oxides and hydroxides family. It has a monoclinic crystal system with tabular habit. It is not an important mineral in soils, per se, but it presents a model system for a trioctahedral sheet structure that is not attached to a silicate tetrahedral sheet as in the trioctahedral phyllosilicates. Its dioctahedral corollary is gibbsite, Al(OH)3
Mineral Description
Hematite is an iron-oxide mineral of the Oxides and Hydroxides group, with structural formula [alpha-Fe2O3]. The structure is similar to that of corundum, and consists essentially of a dense arrangement of Fe3+ ions in octahedral coordination with oxygen atoms in hexagonal closest-packing. The structure can also be described as the stacking of sheets of octahedrally (six-fold) coordinated Fe3+ ions between two closed-packed layers of oxygens. Since Fe is in a trivalent state (ferric Fe), each of the oxygens is bonded to only two Fe ions, and therefore, only two out of three available oxygen octahedra are occupied. This arrangement makes the structure neutral with no charge excess or deficit. The Fe-O sheets are held together by strong covalent bonds and this results in a very hard and dense structure.
Most hematite is relatively pure, with only very minor inclusions of Fe2+, Ti, Al, and/or Mn. Very limited solid solutions have been reported with magnetite [Fe3O4], ilmenite [FeTiO3], and bixbyite [(Fe,Mn)2O3].
The crystal system of hematite is hexagonal, but crystals appear in a wide variety of forms. Well crystallized forms, also called specularite, tend to develop as flat trigonal crystals. Reniform (kidney ore) or botryoidal forms are common as dehydrated geothite and break down to fibers or splinters. Oolitic and fossiliferous hematite is often found as replacement of carbonate fossils. Crystals may also be massive, or soft and earthy. Hematite is usually opaque, steel-gray, to bright red or brown. The luster is bright metallic to submetallic.
The name “hematite” is from the Greek “haimatites”, meaning blood-like in reference to the bright-red color of the powder hematite. The crystal structure is based on an x-ray diffraction structural refinement of the original structural data of Linus Pauling from 1925.
Kaolinite is a common 1:1 dioctahedral phyllosilicate (clay) mineral found in soils across the world, particularly in highly-weathered environments, as well as scattered monomineralic deposits that are mined for industry. Being a 1:1 mineral, each kaolinite layer has one silica tetrahedral sheet and one alumina octahedral sheet. Individual layers are held together in a crystal by O – H – O bonds between the octahedral sheet of one layer and the tetrahedral sheet of the adjacent layer.
The crystallography of kaolinite played an important role in Linus Pauling’s formulation of the nature of the chemical bond, although for 15 years kaolinite was thought to be monoclinic (crystallographic axes all equal to 90°) instead of triclinic. Crystallographic axes: alpha, 91.8°; beta 104.7°; gamma, 90°).
The crystal structure displayed to the left, including all H atoms, is based on low-temperature (1.5° K) neutron powder diffraction data (Bish, 1993) instead of the more common x-ray diffraction data that has been used since the time of Linus Pauling’s original determination of the crystal structure of kaolinite in 1930.