The group name for a set of anhydrous framework silicate minerals. The most common feldspar minerals belong to the ternary system of end members of albite [Ab: Na(Si₃Al)O₈], anorthite [An: Ca(Si₂Al₂)O₈], and orthoclase [Or: K(Si₃Al)O₈]. Si and Al atoms are in tetrahedral sites of the aluminosilicate framework. Large monovalent or divalent cations (Na, K, Ca, Ba) occupy interstices of the framework to maintain charge balance. In general, cleavage along the {001} is perfect and good on the {010} planes. The feldspar group minerals are separated into two series, alkali (K- to Na-rich) feldspars and plagioclase (Na- to Ca-rich) feldspars. At high temperature, solid solutions are common in both alkali feldspar (between K and Na) series and plagioclase feldspar (between Na and Ca, and Si and Al) series, but not between K- and Ca-rich feldspars. During cooling, homogeneous compositions may exsolve to form perthite (alkali feldspar with plagioclase exsolution) or antiperthite (albite with “orthoclase” exsolution). Feldspar minerals readily weather near the surface of the earth to produce clay minerals. In addition, ground feldspar and quartz may be mixed with clays and/or clay minerals (e.g., kaolin) and fused to create porcelain and glazes, with the feldspar acting as a binder material.
Cf., alkali feldspar

A geologic term describing magmas, igneous rocks and silicate minerals that have a relatively high concentration of lighter elements such as oxygen, sodium, aluminum, silicon and potassium. Felsic magmas typically are highly viscous when compared to mafic magmas. Felsic rocks are typically light colored when compared to mafic rocks.
Cf., mafic

A poorly defined, fine grained, poorly crystalline, and non-magnetic form of gamma- FeO(OH). Feroxyhite transforms in air to goethite, but it has been found in ocean-floor iron-manganese concretions and in some soils with high iron hydroxide content. Drits et al. (1993) examined several structural models and suggested from X-ray data that iron atoms occupy octahedral sites with distributions of face-sharing octahedral pairs alternating with vacant octahedral pairs along the c axis.
Syn., feroxyhyte

An obsolete varietal term for ferrian muscovite.

An obsolete term for tetra-ferri-annite.

An obsolete varietal term for biotite.

See hollandite.

Ferrihydrite is metastable, fine grained, and poorly crystalline and occurs as brown gel-like precipitates and as friable crusts in soil matrix in podzols or lithosols, and in environments associated with acid mine drainage. The chemical composition of ferrihydrite is very approximately Fe₂O₃.2FeO(OH).2.6H₂O, and it is believed to be a precursor of hematite and possibly goethite (by dissolution). Surface adsorption of organic molecules and (heavy and other foreign) metals may occur and affect ferrihydrite stability. The gel-like forms, when studied by powder X-ray diffraction, produces patterns with two broad diffraction peaks (“two line ferrihydrate”) and the crust produces five or more lines (“five line ferrihydrate” and sometimes as “six line ferrihydrate”). Ageing promotes formation of the “five line ferrihydrite” from the two line form. Michel et al. (2007) suggested a single-phase model for ferrihydrite that is based on a cluster of 13 Fe and 40 oxygen atoms: central FeO₄ tetrahedra surrounded by 12 FeO₆ octahedra, with an ideal chemical composition of Fe₁₀O₁₄(OH)₂. The model is affected by surface bound H₂O, surface effects, strain and defects, particle size and shape, etc.

A property of material where there is an antiparallel alignment of magnetic moments but these alignments are unbalanced either because one direction has stronger moments or a larger number of moments. Inverse spinel structures (“ferrites”) are often ferrimagnetic, where magnetic moments of ferric iron cations in the tetrahedral sites and one-half the cations in the octahedral sites (also ferric iron) are opposed and cancel, whereas the remaining occupied octahedral sites can have either balanced (e.g., Mg, Cd, Zn, zero unpaired electrons) or unbalanced (e.g., Mn, Fe²⁺, Co, Ni, Cu) magnetic moments, approximately proportional to the number of unpaired electrons.

An invalid name for a hypothetical end member.