A tale of two garnets: The role of solid solution in the development toward a modern mineralogy

Abstract

This article reviews the development of mineralogy as a science by focusing largely on the common silicate garnets of general formula {X 3 }[Y 2 ](Si 3)O 12. It tells of important discoveries, analyses, and proposals by various scientists relating to crystallography, crystal structures, isomorphism, and solid solution starting in Europe in the late 1700s. The critical recognition of the importance of ionic size of atoms in determining crystal-chemical properties and solid-solution behavior is emphasized. The two garnet species "pyralspite" and "(u)grandite," which were considered to represent two independent solid-solution series, were introduced by N.H. Winchell and A.N. Winchell (1927) in their well-known book Elements of Optical Mineralogy. Critical comments on the assumptions behind the classification scheme have been pointed out for at least 50 yr, but it remains in use. There is more, though, behind this garnet classification scheme than just simple terminology. There are a long series of scientific discoveries and advances that are largely forgotten by the broader mineralogical community. They begin, here, with the work of the "father of crystallography," René-Just Haüy, concerning the microscopic nature of crystals around 1780 and include later discoveries and proposals by Mitscherlich, Beudant, Wollaston, and Kopp relating to isomorphism and solid-solution behavior all before 1850. A second key era started with the discovery of X-ray diffraction in 1912 that allowed the atomic structures of crystals and, furthermore, atomic and ion radii to be determined. In terms of isomorphism and solid solution, the proposals and studies of Vegard, Zambonini, Wherry, A.N. Winchell, and the "father of crystal chemistry" Goldschmidt are briefly discussed. The recognition of the sizes of atoms and ions, along with an understanding of chemical bonding behavior in crystals, was critical in the establishment of what can be termed "modern mineralogy," a quantitative science as it is largely understood today that emerged by the mid-1930s. The silicate garnet system pyrope-almandine-spessartine-grossularandradite-uvarovite shows extensive homovalent substitutional solid solution over two structural sites and complete compositional variation between "pyralspite species" and "ugrandite species" has been documented. Thus, the prerequisites behind the terms "pyralspite" and "(u)grandite," as originally formulated and often accepted even today, are incorrect and use of this classification is not recommended. Diffraction determinations of the volumes of garnet end-members and volumes of mixing of garnet solid solutions give physical insight into solid-solution behavior. Today, investigations of local structural and crystal-chemical properties, together with determinations of lattice strain and thermodynamic mixing properties, of silicate solid solutions are leading to an ever more quantitative understanding of mineral behavior from the microscopic to macroscopic level.

Provenance of Neogene sandstones in western Taiwan traced with garnet geochemistry and zircon geochronology

“…Common silicate garnets have a general formula {X 3 }[Y 2 ](Z 3 )O 12 , where {X}, [Y], and (Z) represent the dodecahedral, octahedral, and tetrahedral crystallographic cation sites and their polyhedral coordination in space group Ia3d (Geiger, 2008, 2016). The main end members are almandine (Fe 3 Al 2 Si 3 O 12 ), pyrope (Mg 3 Al 2 Si 3 O 12 ), spessartine (Mn 3 Al 2 Si 3 O 12 ), grossular (Ca 3 Al 2 Si 3 O 12 ), andradite Ca 3 Fe 2 Si 3 O 12 ), and rarer uvarovite (Ca 3 Cr 2 Si 3 O 12 ), traditionally distinguished in the two series pyralspites and (u)grandites (Winchell & Winchell, 1927).…”

“…The results of Raman analysis are summarised in Table 1 and provided in full in Table A4. (Geiger, 2008(Geiger, , 2016 (Winchell & Winchell, 1927).…”

Calibration of the garnet–biotite–Al2SiO5–quartz geobarometer for metapelites

“…It is well known that the reciprocal solid solution of garnet can be described as , in which A, M and T refer to the hexahedral, octahedral and tetrahedral crystallographic sites respectively. According to the mixing sites, the garnet solid solution was classified into two series: the pyralspite series, ferrous cations mixing on the hexahedral site; and the ugrandite series, ferric cations mixing on the octahedral site, although such a classification scheme is not tentatively recommended today (Geiger, ). For garnet formed in metapelites, mixing of cations occurs mainly on the hexahedral site and mixing on the octahedral and tetrahedral sites is essentially negligible.…”

New Insights into Coloration Mechanism in Violet-Red Pyrope-Almandine

“…The shared corners of both [YO 6 ] octahedrons and isolated silicon-oxygen tetrahedrons make it workable to form a dodecahedron vacuum between them. This vacuum is centered on X 2+ , shaped as a distorted cubic [3]. Most of the ions can be substituted by similar particles, which is due to the fact that garnet is a complete isomorphous series [4].…”