See hydrotalcite group.

Pyrochroite, Mn(OH)₂, structurally forms sheets of Mn(OH)₆ octahedra and is isostructural with brucite. The structure is hexagonal closest packed. Pyrochroite occurs in low-temperature hydrothermal environments.
Cf., brucite

A property of crystals where an electric dipole moment develops in response to a temperature change. The material cannot exhibit temperature gradients, and the property diminishes over time at temperature. Only polar crystal classes exhibit this property. An analogous magnetic property, “pyromagnetism”, can also exist.

See groutite.

A pyramid with defined dimensions made from certain ceramic materials with different resistances to heat, used as indicators for time-temperature conditions in a kiln. Two or three pyrometric cones are placed in a kiln next to a ceramic material which is to be fired. During heating each cone softens at certain time-temperature conditions leading to a gradual kinking of each cone. Pyrometric cones are often called Seger cones named after the modern inventor of pyrometric cones, Hermann August Seger, a German silicate chemist.

The dioctahedral member of the talc-pyrophyllite group. The ideal composition is Al₂Si₄O₁₀(OH)₂. Pyrophyllite forms as a prominent 1A polytype (where A = anorthic, older literature refers to this polytype as 1Tc) and a less prominent, poorly crystalline 2M polytype. The stacking of 2:1 layers in pyrophyllite (Lee and Guggenheim, 1981) is not constrained by an interlayer cation as in the micas, but is related to Si⁴⁺ to Si⁴⁺ repulsions across the vacant interlayer region. Thus Si tetrahedra between adjacent layers are shifted by ~a/3 so that there are no six-fold or twelve-fold interlayer sites available for interlayer cations, as in mica. Ferripyrophyllite is the ferric iron analogue of pyrophyllite. Pyrophyllite occurs in highly Al-rich metapelites, including metabauxites and metaquartzites, and under hydrothermal conditions.
Cf., talc

An obsolete name for altered material, probably vermiculite.

A modulated 1:1 layer silicate with a continuous, planar octahedral sheet and a general chemical composition of M²⁺₈T₆O₁₅(OH,Cl)₁₀. Pyrosmalite is the M = Mn, Fe series, manganpyrosmalite is M for Mn > Fe, and ferropyrosmalite is M for Fe > Mn. Friedelite is the Mn end member and a disordered (polytypic) equivalent of mcGillite. In addition, mcGillite has several additional polytypes. The pyrosmalite structure has an equal number of tetrahedra coordinating to two adjacent octahedra sheets via tetrahedral apical oxygen atoms (Kato and Takéuchi, 1983). Each tetrahedral sheet is composed of 4-, 6-, and 12-fold tetrahedral rings linked laterally, with half of the tetrahedra in the 4- and 12-fold rings inverted. Schallerite and nelenite are polymorphs and similar to friedelite, but apparently with As₃O₆ molecules within the 12-fold rings. Arsenite analogues of pyrosmalite-type minerals (T = As) occur: manganarsite (analogue manganpyrosmalite), and unnamed arsenite equivalents of schallerite and friedelite. Phase assemblages and occurrences are complex. Pyrosmalite occurs in greenschist facies manganiferous rocks. A near Fe-rich end member was reported from low-grade Fe- and Mn-rich sulfide deposits near Mt. Isa, Queensland, Australia. Friedelite occurs in low-grade metamorphic rocks and is Cl bearing.

The pyroxene group minerals are single-chain silicates with repeat units of two SiO₄ tetrahedra (~ 5.2Å) along the chain direction (c-axis). Chemical formulae, using site nomenclature, are given by: M2M1T₂O₆, where M2 represents medium- to large-size cations, commonly Ca²⁺, Na⁺, Fe²⁺, Mg²⁺, Mn²⁺, and Li⁺; M1 represents small- to medium-size cations like Fe²⁺, Mg²⁺, Al³⁺, Fe³⁺, and Ti⁴⁺; and T represents Si⁴⁺ and Al³⁺ in tetrahedral sites. The minerals develop good {110} cleavage, with cleavage angles near 90 degrees. The pyroxene group is divided further to subgroups according to composition (and symmetry). The common pyroxenes form solid solutions of the Ca-Mg-Fe pyroxenes and are compositionally described (e.g., Morimoto et al., 1988) in the pyroxene quadrilateral with end-members diopside (Di: CaMgSi₂O₆), hedenbergite (Hd: CaFeSi₂O₆), enstatite (En: Mg₂Si₂O₆), and ferrosilite (Fs: Fe₂Si₂O₆). The enstatite-ferrosilite solid solution series forms orthopyroxenes (OPX) with orthorhombic (Pbca) symmetry, whereas, the diopside-hedenbergite solid solution series forms clinopyroxenes (CPX) with monoclinic (C2/c) symmetry. Weathering reactions of pyroxene group minerals often produce clay minerals. Pyroxene end-members are: enstatite Mg₂Si₂O₆ (polymorphs clinoenstatite, orthoenstatite, protoenstatite); ferrosilite Fe₂Si₂O₆; diopside CaMgSi₂O₆; hedenbergite CaFeSi₂O₆; jadeite NaAlSi₂O₆; aegirine NaFeSi₂O₆; spodumene LiAlSi₂O₆; pigeonite (Mg,Fe,Ca)(Mg,Fe)Si₂O₆; augite (Ca,Mg,Fe²⁺,Fe³⁺,Ti,Al)₂(Si,Al)₂O₆; omphacite (Ca,Na)(Mg,Al)Si₂O₆; grossmanite CaTiSiAlO₆. Refer to individual end members for further descriptions.

single-chain silicate minerals with Si-tetrahedral repeats of 3 (e.g., in wollastonite, CaSiO3), 5 (e.g., in rhodonite, MnSiO3), 7 (e.g., in pyroxmangite, FeSiO3), or 9 (e.g., in ferrosilite III). In contrast, the pyroxene tetrahedral repeat is 2. However, both the pyroxenoids and the pyroxenes have octahedrally coordinated cations connecting to the tetrahedral chains in similar ways. Hydrous pyroxenoids exist also, where Na + H substitute for one of the divalent cations, e.g., pectolite, Ca2NaH(SiO3)3. Wollastonite occurs as a contact metamorphic mineral of siliceous dolomites and is used in the manufacture of tile and in glazes. Rhodonite and pyroxmangite occur in manganese deposits and metamorphosed Mn-rich iron formations. Pectolite, commonly associated with zeolites, forms in cavities in basalts as a secondary mineral formed by hydrothermal activity.