Anisotropy of magnetic susceptibility and magnetic properties of obsidians: volcanic implications

The anisotropy of magnetic susceptibility of uniaxial superparamagnetic particles: Consequences for its interpretation in magnetite and maghemite bearing rocks

“…Theoretical predictions are confirmed by results from obsidians samples, which have a large superparamagnetic and negligible multidomain grains population, and show that AMS axes are consistent with the AIRM axes; hence, maximum anisotropy axes are align to the easy axes. Other similar examples can be found in the literature; Canõń‐Tapia and Castro [] and Canõń‐Tapia and Caŕdenas [], for instance, have reported cases of obsidians where the magnetic mineralogy was identified as a mixture of single‐domain magnetite with a substantial contribution of the superparamagnetic fraction, and none of them shows an inverse AMS pattern.…”

Much More Than a Glass: The Complex Magnetic and Microstructural Properties of Obsidian

“…1 In the literature, most studies about obsidian are dedicated to the application of an analytical technique, both destructive and nondestructive, with the aim of discriminating the main sources and subsources for archeological purposes. Other studies are devoted to the characterization of the obsidian as a material 2−19 dealing with different aspects such as the color or optical properties, 8,9,12,17 the glass structure, 6,10,11 the comparison of its properties with those of impact glasses 13,19 or artificial ones, 2,4 its formation process, 7,16,18 the different coordination of iron ions, 3−5 and the paleointensity studies. 14,15 Previous work has also been devoted to the study of the magnetic properties of obsidian samples of different sources worldwide by magnetometry, 3,14−16,19−26 EPR, 5,8,27,28 and 57 Fe Mossbauer spectroscopy, 3,4,8,[20][21][22]27,29 among which some refer to Sardinian obsidians.…”

“…Some authors underlined the possibility to distinguish magnetite of different sizes from single domain to multidomain. 16,19,21,22,24 However, to date a comprehensive correlation of the magnetic properties with the microstructure and morphology of Monte Arci obsidian is still lacking, despite the evidence for the copresence of a glassy matrix together with iron-containing micro-or nanolites. Indeed, although obsidian is usually defined as a natural glass, it is known from the literature that crystalline inclusions and occlusions, 2−18,20−37 among which, iron-based minerals, are often present, also in the case of the Monte Arci obsidian.…”

“…In the literature, most studies about obsidian are dedicated to the application of an analytical technique, both destructive and nondestructive, with the aim of discriminating the main sources and subsources for archeological purposes. Other studies are devoted to the characterization of the obsidian as a material − dealing with different aspects such as the color or optical properties, ,,, the glass structure, ,, the comparison of its properties with those of impact glasses , or artificial ones, , its formation process, ,, the different coordination of iron ions, − and the paleointensity studies. , Previous work has also been devoted to the study of the magnetic properties of obsidian samples of different sources worldwide by magnetometry, ,− ,− EPR, ,,, and 57 Fe Mössbauer spectroscopy, ,,,− ,, among which some refer to Sardinian obsidians. ,,,− The great majority of the authors evidenced the coexistence of paramagnetism with ferrimagnetism and/or superparamagnetism associated with magnetite nanoparticles. Some authors underlined the possibility to distinguish magnetite of different sizes from single domain to multidomain. ,,,, However, to date a comprehensive correlation of the ...…”

“…Some authors underlined the possibility to distinguish magnetite of different sizes from single domain to multidomain. ,,,, However, to date a comprehensive correlation of the magnetic properties with the microstructure and morphology of Monte Arci obsidian is still lacking, despite the evidence for the copresence of a glassy matrix together with iron-containing micro- or nanolites. Indeed, although obsidian is usually defined as a natural glass, it is known from the literature that crystalline inclusions and occlusions, − ,− among which, iron-based minerals, are often present, also in the case of the Monte Arci obsidian. ,,,,,, Feldspars (KAlSi 3 O 8 – NaAlSi 3 O 8 – CaAl 2 Si 2 O 8 ), ,,,,,,, pyroxenes (XY­(Si,Al) 2 O 6 ), ,,,, biotite (K­(Mg,Fe) 3 AlSi 3 O 10 (OH) 2 ), ,, hornblende ((Ca,Na) 2–3 (Mg,Fe,Al) 5 (Al,Si) 8 O 22 (OH,F) 2 ), ,, silica polymorphs (cristobalite, tridymite, quartz), ,, apatite (Ca 10 (PO 4 ) 6 (OH,F,Cl) 2 ), , zircon (ZrSiO 4 ), , titanomagnetite/magnetite or related spinels (Fe 3 O 4 , Fe 2+ (Fe 3+ ,Ti) 2 O 4 ), ,− ,,…”