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An analysis of the magnetic behavior of olivine and garnet substitutional solid solutions

Authors
  • Geiger, Charles A.
  • Grodzicki, Michael
  • Dachs, Edgar
Type
Published Article
Journal
American Mineralogist
Publisher
Mineralogical Society of America
Publication Date
Aug 28, 2019
Volume
104
Issue
9
Pages
1246–1255
Identifiers
DOI: 10.2138/am-2019-6839CCBYNCND
Source
De Gruyter
Keywords
License
Yellow

Abstract

The low-temperature magnetic properties and Néel temperature, TN, behavior of four silicate substitutional solid solutions containing paramagnetic ions are analyzed. The four systems are: fayalite-forsterite olivine [Fe22+SiO4-Mg2SiO4], and the garnet series, grossular-andradite [Ca3(Alx,Fe31+ –x)2Si3O12], grossular-spessartine [(Cax,Mn21+ –x)3Al2Si3O12], and almandine-spessartine [(Fex2+,Mn21+ –x)3Al2Si3O12]. Local magnetic behavior of the transition-metal-bearing end-members is taken from published neutron diffraction results and computational studies. TN values are from calorimetric heat capacity, C P, and magnetic susceptibility measurements. These end-members, along with more transition-metal-rich solid solutions, show a paramagnetic to antiferromagnetic phase transition. It is marked by a C P l-anomaly that decreases in temperature and magnitude with increasing substitution of the diamagnetic component. For olivines, TN varies between 65 and 18 K and TN for the various garnets is less than 12 K. Local magnetic behavior can involve one or more superexchange interactions mediated through oxygen atoms. TN behavior shows a quasi-plateau-like effect for the systems fayalite-forsterite, grossular-andradite, and grossular-spessartine. More transition-metal-rich crystals show a stronger TN dependence compared to transition-metal-poor ones. The latter may possibly show superparamagnetic behavior. (Fex2+,Mn21+ –x)3Al2Si3O12 garnets show fundamentally different magnetic behavior. End-member almandine and spessartine have different and complex interacting local superexchange mechanisms and intermediate compositions show a double-exchange magnetic mechanism. For the latter, TN values show negative deviations from linear interpolated TN values between the end-members. Double exchange seldom occurs in oxides, and this may be the first documentation of this magnetic mechanism in a silicate. TN behavior may possibly be used to better understand the nature of macroscopic thermodynamic functions, C P and S °, of both end-member and substitutional solid-solution phases.

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