An ion with a charge opposite in sign to the charge of the solid interface to which it is attracted.
Cf., co-ion

A chemical bond that involves sharing of one or more electrons, generally an electron pair. Covalent bonds require the appropriate geometric arrangement of coordinating atoms so that orbitals can overlap. Diamond, which involves carbon atoms, exhibits the ideal characteristics of covalent bonds. However, many elemental pairs involve covalent character and other bonding character (i.e., covalent plus ionic character as in the Si-O bond).

a) In soil mechanics or geotechnical engineering, creep refers to the process of slow deformation of saturated material occurring under a constant effective stress, usually after the deformation in response to loading and/or to pore water pressure changes. For clays, if the pore water in the clay is allowed to escape, then the process is drained creep; if the pore water of the clay is confined, then it is undrained creep.

b) In geological sciences, creep is the very slow downslope movement of material.
Cf., secondary compression.

Beta-cristobalite, a high temperature (above 1470^oC, but below liquid at 1727^oC at 1 bar) polymorph of SiO₂, has an ideal basic structure that is polytypic with tridymite. Like tridymite, beta cristobalite has sheets of hexagonal tetrahedral rings with alternate tetrahedra around a ring with apices pointing in opposite directions from adjacent tetrahedra. These sheets have an ABCABC… stacking sequence, which creates an offset such that no channels form as they do in tridymite. Cristobalite is found in volcanic rocks, primarily in a fine groundmass, but also as a lining of cavities and as a devitrification of volcanic glasses.
Cf., opal, tridymite, quartz

In crystal growth and heterogeneous geochemical kinetics theory, the “critical radius” refers to the smallest size that atoms or ions must come together to produce a stable nucleus for crystal growth or to allow bubbles to form.

a) In physics, the critical state (or critical point) is where a substance has two physical states) with the same temperature, pressure and volume. For example, it is where a liquid and gas phase of a substance cannot be differentiated because the volume is the same at a fixed temperature and pressure. The critical state for water occurs at 374^oC at 218 bars, and the liquid state cannot be differentiated from steam.

b) In soil science, the critical state is where the shear stress remains at a constant value (i.e., steady) while the shear strain continues to increase, when a fully saturated clayey soil or sand body is subject to a relatively large shear strain. If the shearing process is drained, the total volume also remains constant.
Syn., constant volume state

Cronstedtite is the Fe-rich serpentine of composition (R²⁺_3-xFe³⁺_x)(Si_2-xFe³⁺_x) O₅(OH)₄. Divalent cations (R²⁺) may include Fe, Mg, Mn, and possibly Ca. It is assumed that the value of x in the formula is equal (and near 0.5 to 1.0) for both the tetrahedral and the octahedral sites. Cronstedtite has a large number of polytypes, with the 1T (space group P31m) most common. The lateral dimensions of the iron-rich tetrahedral sheet are large, but the lateral dimensions of the octahedral sheet are large also, and thus tetrahedral rotation ranges only up to ~8 o . Because most (possibly all) of the iron is ferric, the tetrahedral-octahedral misfit is limited. Fe³⁺ and Si segregate to different T sites in the 2H₂ polytype, but not in the 1T, 3T, 2H₁ , and 6R₂ polytypes. Amesite-2H₂ also shows tetrahedral ordering, but between Al and Si. Cronstedtite forms in low-temperature hydrothermal veins with other iron rich minerals, such as siderite, and in low-temperature metamorphosed iron formations. It is also known to occur in carbonaceous chondrites. The Mn analogue of cronstedtite is guidottiite.
Cf., amesite, greenalite, guidottiite

The original polarizing prisms of petrographic microscopes were the early 19^th century design of William Nicol, and each was referred to as a Nicol prism (Bloss, 1961). When both Nicol prisms were introduced into the optic path in mutually perpendicular orientations, the examination was said to be using “crossed Nicols”. The phrase “crossed nicols” is still used for this geometry of polarized light, regardless of whether the polarizing optics use Nicol prisms or another polarization device.
See petrographic microscope, crossed polarizers

“Polars” or “polarizers” are devices that permit passage of light with vibrations in only one direction. In an optical “polarizing microscope”, two polarizers (the lower device is often referred to as the “polarizer” and the upper device is the “analyzer”) are oriented such that light vibrations are at 90 degrees from each other (and the two polarizers are said to be “crossed”). If nothing is present in the optical path between the two devices to change the light vibrations, all light is prevented from passing and the image is black. Material placed in the optical path changes the behavior of the light rays, with the behavior dependent on the crystallography of the material (e.g., crystal structure, crystallinity), the chemical composition, the thickness of the material, and refractive indices. If the analyzer is removed from the optical path, then the devices are not considered “crossed”.
Syn., crossed nicols, crossed polars

An obsolete varietal term for zinnwaldite, ferroan trilithionite, and ferroan polylithionite