A thermodynamic state property useful to determine the spontaneity of a reaction within a system (without regard to the surroundings as is the case with entropy changes) and the direction of the reaction. The change in Gibbs energy, ΔG, is equal to ΔH – TΔS, where ΔH is the change in enthalpy (cal/mole), T is the absolute temperature (K), and ΔS is the change in entropy (cal deg^-1 mole^-1). A substance reacts if the change in Gibbs energy is negative going from the initial state to the final state. Gibbs energy is often referred to as Gibbs free energy.
See enthalpy, entropy, state function

See Gibbs energy.

A polymorph of Al(OH)₃ where one third of the octahedral sites are vacant and each OH group is coordinated by two Al cations. Although each layer is approximately closest packed, the stacking of layers is not closest packed, and OH groups in adjacent layers superpose. The interlayer OH to OH distance is relatively small (at ~2.78 Å), indicating a strong hydrogen bond that is enhanced by strong polarization caused by the highly charged Al³⁺ cation.
Cf., bayerite, nordstrandite

A term that is synonymous with the dioctahedral interlayer sheet in chlorite. This sheet is analogous to gibbsite in that gibbsite consists of two (intralayer) planes of closest packed oxygen atoms with two out of three of the octahedral sites between the two planes occupied by trivalent cations, in this case Al. In the gibbsite-like sheet, some of the oxygen atoms are replaced by hydroxyl groups, (OH). This term is only for chlorite. The dioctahedral sheet in a 2:1 layer silicate, such as mica, is very different from gibbsite, whereas the interlayer in a chlorite is quite “gibbsite-like”. For example, in a 2:1 layer octahedral sheet, two thirds of the oxygen anions are apical oxygen atoms whereas only one third are OH groups—very different from a gibbsite-like sheet.

A poorly defined material, possibly muscovite and cordierite.

An obsolete term for muscovite.

A pedofeature forming segregated lumps of material with diverse composition (similar to cutans) as part of a soil groundmass. The non-planar shapes and more distinct outlines differentiate glaebules from cutans. Three common glaebules include mottles, nodules, and concretions.
– nodule : a glaebule that is irregular to nearly spherical, with a massive internal structure.
– concretion : a nodule-shaped glaebule showing an internal structure of concentric layers.
– mottle : poorly differentiated glaebules occurring as diffuse patches in the groundmass.

A chlorite-like mineral containing Na in seven-fold coordination located between the interlayer octahedral sheet and the 2:1 layer. The ideal chemical composition is Na(Mg,Al)₆(Si₃Al)O₁₀(OH,O)₈. Glagolevite was described by Krivovichev et al. (2004) in analogy to chlorite as a tri-trioctahedral chlorite with polytypes IIb-6, IIb-2 and IIb-4. The mineral occurs at the Kovdor Phlogopite quarry, Kovdor massif, Kola peninsula, Russia.

A solid with a degree of order intermediate to the highly ordered arrangement of atoms, molecules, or ions in a “crystalline” solid and the highly disordered arrangement as found in a “gas”. Most glasses are in a metastable state and can be described as a supercooled liquid, which lack a melting point. Rapid cooling from a molten state (e.g., magma, lava) may result in a glass and this commonly depends on the volatile content of the melt. Not all glasses are formed from a melt. Although most opal forms from silica-saturated fluid under near-surface conditions, a rare “non-crystalline opal” (e.g., opal-AN) forms by transport of silica via steam to cold surfaces.

An iron-rich dioctahedral mica that shows K deficiency and limited substitutions of Al in the tetrahedral sites. Glauconite is a series name (Rieder et al., 1998) with a generalized composition of K_0.8R³⁺_1.33R²⁺_0.67(Al_0.65Si_3.87)O₁₀(OH)₂, where ^viR²⁺/(^viR²⁺ + ^viR³⁺) >= 0.15, and ^viAl/(^viAl + ^viFe³⁺) <= 0.5 and shows no compositional overlap with celadonite. Often interstratified with smectite as the mixed-layered mineral glauconite/smectite. When mixed with other minerals or when referring to morphological features, the term “glauconitic” is appropriate. The mode of origin is not a criterion for identification.