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Crystal chemistry of spinels from xenoliths of the Alban Hills volcanic region

Sergio Lucchesi, Monica Amoriello and Antonio Della Giusta
Crystal chemistry of spinels from xenoliths of the Alban Hills volcanic region
European Journal of Mineralogy (June 1998) 10 (3): 473-482


EPMA and XRD results are given for 21 spinel crystals from salic to ultramafic xenoliths in pyroclastic flows (lahars) around craters in the Alban Hills, central Italy. These crystals along belong to the Mg-rich side of the MgAl (sub 2) O (sub 4) -FeAl (sub 2) O (sub 4) binary with Fe (super 2+) < or = 0.15 pfu; Fe (super 3+) is always present, ranging from 0.05 to 0.13 pfu. Unit cell parameters range from 8.098 to 8.115 Aa and the oxygen coordinate from 0.2624 to 0.2616, the latter value being particularly low for natural spinels of this paragenesis. Despite the small variations in bulk chemistry, the distribution of a and the oxygen coordinate values allows two series to be distinguished. The cation distribution (obtained by a minimization procedure taking into account chemical and structural data) shows a high degree of disorder, with (super [4]) Al < or = 0.22 pfu, (super [4]) Fe (super 3+) < or = 0.06 pfu and Fe (super 2+) almost exclusively ordered in the tetrahedral site. The variations in tetrahedral bond distances in both series are attributed to two contrasting effects: substitution of (super [4]) Fe (super 2+) for (super [4]) Mg, which causes T-O lengthening, and substitution of Al for (super [4]) Mg (inversion), which results in T-O shortening. The octahedral bond distances are linked to the cooperative effects of inversion and substitution of (super [6]) Fe (super 3+) for (super [6]) Al.

ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 10
Serial Issue: 3
Title: Crystal chemistry of spinels from xenoliths of the Alban Hills volcanic region
Affiliation: Universita di Roma "La Sapienza", Dipartimento di Scienze della Terra, Rome, Italy
Pages: 473-482
Published: 199806
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 23
Accession Number: 1998-046905
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables
N41°40'00" - N41°49'60", E12°34'60" - E12°49'60"
Secondary Affiliation: Universita di Padova, ITA, Italy
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United KingdomTwickenhamUKUnited Kingdom
Update Code: 199818
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