An obsolete term for nontronite.

A poorly defined material, possibly a pseudomorph mostly of mica after cordierite, nepheline, or scapolite.

An obsolete term for saponite.

Plagioclase feldspar minerals occur in the solid solution series between albite [Ab: Na(Si₃Al)O₈] and anorthite [An: Ca(Si₂Al₂)O₈] end-members. Ordering between Al and Si is very sluggish in the plagioclase feldspars and requires a coupled substitution between (Si + Na) and (Al + Ca). The plagioclase feldspar minerals are the most abundant minerals in the earth’s crust. Plagioclase mineral names are divided into albite (An_0-10), oligoclase (An_10-30), andesine (An_30-50), labradorite (An_50-70), bytownite (An_70-90), and anorthite (An_90-100), but it is increasingly common to designate molecular percentages (i.e., Or₁₀Ab₆₀An₃₀, where Or = orthoclase and Ab and An are defined above). Labradorite feldspar may show iridescent colors at compositions of An₅₀–₆₀ and with Or₂–₅ where semi-periodic exsolution lamellae occur with periodicity of ~100 nm to ~300 nm. Labrodorite commonly occurs in gabbroic rocks or anorthosites (plagioclase-dominated rocks) that experienced extremely slow cooling history. Plagioclase feldspars are used in glass making and ceramics, paints, rubber, and plastics, although the alkali feldspars are more commonly used in industry.
Cf., alkali feldspar

In phyllosilicate mineralogy, a set of one or more types of atoms (e.g., a plane of Si and Al atoms, a plane of basal oxygen atoms) that form a two-dimensional flat or nearly flat surface.
See Guggenheim et al. (2006) and references therein.
Cf., sheet, layer

Polarized light, or plane light, is light that vibrates within a single plane (“plane of vibration”) which is defined as the plane parallel to the ray’s path and its vibration direction. The polarized light results from the introduction of a single polarizing device into the optic path of a petrographic (polarized light) microscope, with the plane of vibration obtained from the privileged direction of the polarizer.
See petrographic microscope. Abbr. PL, or PPL (plane polarized light).

A mixture of lime, sand, cement, and water often used to protect ceilings or walls. The mixture hardens upon drying to form a hard surface in preparation for final finishing.

One of the Atterberg Limit tests. The water content of an homogenous, fine-grained soil/water mixture where the mixture begins to exhibit plastic behavior upon deformation, as defined by the test method described in ASTM Standard D4318 – 05.
Syn. “lower plastic limit”.
See Mitchell (1993).
See also activity, Atterberg Limits, consistency number, liquid limit, plasticity index, shrinkage limit.

When referring to clay, this is a property where moistened material, when deformed under the application of pressure, will retain the induced deformed shape when the applied pressure is removed.

The numerical difference between the liquid limit and the plastic limit; i.e., the difference between the water content of a fine-grained soil/water mixture at the boundary between its liquid and plastic states and the boundary between its plastic and brittle states, based on tests outlined by Atterberg and standardized by ASTM Standard D4318 – 05.
Syn. Plasticity number.
See Mitchell (1993).
See also activity, Atterberg limits, consistency number, liquid limit, plastic limit, shrinkage limit.