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Silica polymorphs occur under various pressures and temperature conditions, and their characteristics can be used to better understand the complex metamorphic history of planetary materials. Here, we conducted isothermal heating experiments of silica polymorphs in basaltic eucrites to assess their formation and stability. We revealed that each silica polymorph exhibits different metamorphic responses: (1) Quartz recrystallizes into cristobalite when heated at ≥ 1040 °C. (2) Monoclinic (MC) tridymite recrystallizes into no other polymorphs when heated at ≤ 1070 °C. (3) Silica glass recrystallizes into quartz when heated at 900-1010 °C, and recrystallize into cristobalite when heated at ≥ 1040 °C. These results suggest that MC tridymite in eucrites does not recrystallize into other polymorphs during the reheating events, nor does it recrystallize from other silica phases below the solidus temperature of eucrite (~ 1060 °C). Additionally, we found that pseudo-orthorhombic (PO) tridymite crystallizes from quenched melts in the samples heated at ≥ 1070 °C. Previously, cristobalite has been considered as the initial silica phase, which crystallizes from eucritic magma. Our findings suggest that the first crystallizing silica minerals may not always be cristobalite. These require a reconsideration of the formation process of silica minerals in eucrites., (© 2024. The Author(s).)

Chibaite, a silica-framework structure with cage-like voids occupied by gaseous molecules, was found in marine sediments. Its formation age could be evaluated using electron spin resonance (ESR) if the radicals formed by natural radiation can be assumed to accumulate over time. To investigate whether hydrogen transfer reactions, where organic radicals withdraw hydrogen atoms from other molecules in adjacent cages, occur in chibaite and affect ESR dating, gamma-irradiated chibaite was measured by ESR. Methyl, ethyl, n-propyl, isopropyl, tert-butyl radicals and hydrogen atoms were created by gamma irradiation at 77 K. The amount of tert-butyl radicals increased around 240 K and the similar amount of the other organic radicals decreased simultaneously, implying that hydrogen transfer reactions occur between isobutane and the organic radicals in chibaite around 240 K and therefore would have no influence on ESR dating because the reactions are completed at the environmental temperature., (© The Author(s) 2023. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)