Presentation Information
[R1-P-15]Four new allanite-group minerals from the Mogurazawa mine, Gunma, Japan
*Mariko NAGASHIMA1, Daisuke Hamane2, Masayuki Ohnishi, Hiroshi Miyajima, Akira Harada (1. Yamaguchi Univ. Sci., 2. Univ. Tokyo, ISSP)
Keywords:
Akasakaite,New mineral,Mogurazawa,Vanadium,Rare earth element
The new allanite-group minerals, akasakaite-(Ce) (IMA No. 2025-001), akasakaite-(La) (2025-002), vanadoakasakaite-(Ce) (2024-044), and vanadoakasakaite-(La) (2025-003), associated with small quartz lenses in rhodonite-enriched rocks of the stratiform manganese deposits at the Mogurazawa mine, Gunma, Japan, were investigated. Akasakaite is defined by the ideal formula A1CaA2REE3+M1Me3+M2AlM3Mn2+(SiO4)(Si2O7)O(OH) (Z = 2). In our studied new minerals, the dominant cation at A2 is Ce or La, and Me3+ at M1 is Al or V3+.
In the epidote supergroup, the variations in the a-, b-, and c-dimensions commonly reflect the cation distribution in the M1 and M3 sites[1], whereas the β-angles are known to mainly reflect the cation distribution in the A1 and A2 sites[2,3]. Specifically, it has been suggested that an increase in REE content at the A2 site[2] and an increase in Mn2+ content at the A1 site[3,4,5] both reduce the β angle. In fact, the β angle of our specimens (113.6-113.8º), where Ce and La occupy more than 70% at the A2 site and Mn2+ is the second dominant cation at the A1 site (0.40-0.46 apfu), is similar to that of androsite (113.1-113.9º[5,6]). The unit-cell volume of M1V3+-dominant species is larger than the others. It is mainly explained by the elongation of <M1-O> due to V3+ substitution for Al at M1.
The positive correlation between δ[(A1-O6)-(A1-O5)] (Å) and Mn2+ at A1 (apfu) has been noted in some previous studies[4]. The observed distances in this study support this trend, but they are longer than expected. This is attributed to the distribution of cations at the M1 site and can be interpreted as the elongation of M1-O5, which promotes the shortening of A1-O5, leading to an increase in the δ[(A1-O6)-(A1-O5)] value.
[1] e.g., Franz & Liebscher, RMG56, 1-82 (2004), [2] e.g., Gieré & Sorensen, RMG56, 431-493 (2004), [3] Nagashima et al. Miner. Mag. 79, 735-753 (2015), [4] e.g., Bonazzi et al. Amer. Miner. 81, 735-742 (1996), [5] Biagioni et al. Minerals, 9, 353 (2019), [6] Cenki-Tok et al. Eur. J. Miner., 18, 569-582 (2006).
In the epidote supergroup, the variations in the a-, b-, and c-dimensions commonly reflect the cation distribution in the M1 and M3 sites[1], whereas the β-angles are known to mainly reflect the cation distribution in the A1 and A2 sites[2,3]. Specifically, it has been suggested that an increase in REE content at the A2 site[2] and an increase in Mn2+ content at the A1 site[3,4,5] both reduce the β angle. In fact, the β angle of our specimens (113.6-113.8º), where Ce and La occupy more than 70% at the A2 site and Mn2+ is the second dominant cation at the A1 site (0.40-0.46 apfu), is similar to that of androsite (113.1-113.9º[5,6]). The unit-cell volume of M1V3+-dominant species is larger than the others. It is mainly explained by the elongation of <M1-O> due to V3+ substitution for Al at M1.
The positive correlation between δ[(A1-O6)-(A1-O5)] (Å) and Mn2+ at A1 (apfu) has been noted in some previous studies[4]. The observed distances in this study support this trend, but they are longer than expected. This is attributed to the distribution of cations at the M1 site and can be interpreted as the elongation of M1-O5, which promotes the shortening of A1-O5, leading to an increase in the δ[(A1-O6)-(A1-O5)] value.
[1] e.g., Franz & Liebscher, RMG56, 1-82 (2004), [2] e.g., Gieré & Sorensen, RMG56, 431-493 (2004), [3] Nagashima et al. Miner. Mag. 79, 735-753 (2015), [4] e.g., Bonazzi et al. Amer. Miner. 81, 735-742 (1996), [5] Biagioni et al. Minerals, 9, 353 (2019), [6] Cenki-Tok et al. Eur. J. Miner., 18, 569-582 (2006).
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