An element or chemical compound that is normally crystalline and that has been formed as a result of geological processes (quot Nickel, 1995). A mineral forms by a naturally occurring process (e.g., “geological” process); phases formed by the interaction of individuals (even if outdoors under conditions not fully controlled by the individual) are not minerals. Nickel (1995) discussed exceptions to the requirements, such as the equivalence of extraterrestrial and “geological” processes, metamict (non-crystalline) minerals, mercury (a liquid mineral), and others (quot Guggenheim et al., 2006). Some naturally occurring processes, but without a geological component, such as compounds that form biologically (e.g., oxalate crystals in certain plants, marine animal shells), are not minerals. Synthesized materials are not minerals, but may be referred to as “synthetic minerals” (e.g., “synthetic diamond”, “synthetic halite”) because the use of “synthetic” negates the naturally occurring/geological aspects specifically. Likewise, “biomineral” is acceptable for similar reasons as synthetic mineral.
See crystalline,
Cf., biologically controlled mineralization, biologically induced mineralization, biomineralization

A natural solid with insufficient long-range atomic ordering to be classified as a mineral. For example, limonite (FeO . OH . nH₂O) is often considered an amorphous “mineral” or mineraloid.

A poorly defined material, possibly interstratified biotite and vermiculite.

A modulated 2:1 layer silicate with a continuous octahedral sheet and a tetrahedral sheet that forms linked hexagonal 6-fold tetrahedral rings along strips along the [010] direction (Guggenheim and Eggleton, 1986). Some of the tetrahedra are partially inverted to form a chain along the [010], and this chain links adjacent 2:1 layers. There are two varieties of minnesotaite that are based on strip widths and chemical composition: a P cell is Mg-rich and is formed where 10 tetrahedra span 9 octahedra along the [010] whereas a C cell, which is Fe-rich, forms with 9 tetrahedra spanning 8 octahedra. The ideal chemical composition for the P cell is (Fe,Mg)₃₀Si₄₀O₉₆(OH)₂₈ and (Fe,Mg)₂₇Si₂₆O₈₆(OH)₂₆ for the C cell. Early workers incorrectly considered minnesotaite as the Fe analogue of talc. Minnesotaite occurs in low grade metamorphic silicate iron formations.

Or symmetry plane, used to describe a repetition of features whereby identical points occur an equal but opposite distance along any line perpendicular from this imaginary plane. Consequently, an object is “bilateral” in that it shows a matching of features (also referred to as “reflection”) but a change in “handedness”, e.g., most people, standing with arms by their sides and feet side-by-side, have a mirror (bilateral or reflection) relationship between the left side and the right side.
Cf., symmetry, center of symmetry, rotation symmetry

An obsolete term.
See Southern bentonite.

A Fe-,Mg-rich chlorite, heat treated in air for one-hour at 550^oC to produce a chlorite-like structure (Guggenheim and Zhan, 1999) with a strong d(001) peak (14 Å) and weak or absent higher order 00l peaks. The chlorite-to-modified chlorite reaction allows the identification of mixtures of 7 Å phases (e.g., kaolin minerals) and Fe-,Mg-rich chlorite after heating samples of clay mixtures that may contain chlorite with moderate to high amounts of Fe by revealing the possible presence of 7Å peaks in an oriented clay mineral aggregate.

A polymorph of quartz, has lower symmetry than quartz (I2/a) and a triclinic superstructure commonly occurs. The structure was described by Miehe and Graetsch (1992) as comprised of sections of right- and left-handed quartz alternating at the unit cell level to form a framework of corner sharing tetrahedra. The framework has 4-, 6- and 8-fold rings, and there is no open tunnel as found in alpha quartz. The nanoscale alternation follows the Brazil twin law, but because it is periodic at the unit cell level, moganite represents a (metastable) mineral phase. Moganite occurs as intergrowths with (alpha) quartz in chert, quartzine, flint, and chalcedony, thus indicating that these latter varieties are not minerals, but rock names. Any H₂O present in moganite is not structurally required.

Mohr’s circle is a two-dimensional graphical representation of the three-dimensional state of stress of a soil element subject to load in the normal stress – shear stress coordinate system. Components (i.e., orientations of the normal and shear stresses) of applied stress may be graphically obtained across various inclined planes at a given point in a stressed body.
Syn., Mohr circle of stress, stress circle

Mohs’ scale of hardness is composed of a series of common minerals with increasing hardness: (1) talc, (2) gypsum, (3) calcite, (4) fluorite, (5) apatite, (6) orthoclase, (7) quartz, (8) topaz, (9) corundum, and (10) diamond. Hardness is determined by a scratch test, as minerals of greater or equal hardness can scratch those of equal or lesser hardness.
Cf., hardness, Vickers hardness