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geothermometer An indicator of the temperature, or range of temperatures, at which a geologic event (e.g. the crystallization of a magma or the metamorphism of pre-existing rocks) occurred. Apart from the presence or absence of minerals or mineral assemblages known to be stable within certain temperature ranges, among the most widely used indicators are: (a) stable-isotope distribution, e.g. the ratios of 18O to 16O between different mineral pairs varies according to temperature (see also OXYGEN-ISOTOPE ANALYSIS); (b) mineral transformations or inversions known to be temperature dependent, e.g. the transition of α quartz to β quartz at 573°C; (c) liquid-vapour homogenization points in fluid inclusions (subject to certain assumptions, the temperature of formation of a crystal is indicated by the temperature at which the vapour bubble co-existing in the inclusion disappears upon heating); (d) unmixing or exsolution lamellae of mineral pairs below a particular temperature, e.g. chalcopyrite—bornite at 500°C; (e) temperature-dependent element distribution between co-existing minerals, e.g. iron—titanium oxide distribution between the co-existing mineral pairs magnetite—ulvöspinel and ilmenite—hematite (in this instance subject also to oxygen fugacity).