Metamict phase

The relative quantity of the metamict phase in the bulk quartz is insignificant therefore, a component corresponding to this denser modification could not been singled out in the Si2p spectra of bulk quartz. 

Irradiation by fast neutrons causes a densification of vitreous silica that reaches a maximum value of 2.26 g/cm3, ie, an increase of approximately 3%, after a dose of 1 x 1020 neutrons per square centimeter. Doses of up to 2 x 1020 n/cm2 do not further affect this density value (190). Quartz, tridymite, and cristobalite attain the same density after heavy neutron irradiation, which means a density decrease of 14.7% for quartz and 0.26% for cristobalite (191). The resulting glass-like material is the same in each case, and shows no x-ray diffraction pattern but has identical density, thermal expansion (192), and elastic properties (193). Other properties are also affected, ie, the heat capacity is lower than that of vitreous silica (194), the thermal conductivity increases by a factor of two (195), and the refractive index, increases to 1.4690 (196). The new phase is called amorphous silica M, after metamict, a word used to designate mineral disordered by radiation in the geological past (197). 

In this section, we summarize radiation damage data for minerals however, we focus only on those minerals for which substantial amounts of data are available or which are of considerable geologic interest. We do not include a discussion of phases that contain actinides but are not metamict (i.e. AO2 A = Zr baddeleyite Th thorianite U uraninite). 

Robinson MT (1994) Basic physics of radiation damage production. J Nucl Mater 216 1-28 Salje EKH, Chrosch J, Ewing RC (1999) Is metamictization of zircon a phase transition Am Mineral 84 1107-1116... 

The early work on phosphate glasses led to the idea that crystalline phosphates might make extremely durable waste forms, particularly for actinides. The earliest suggestion was for the use of monazite (Boatner 1978, Boatner et al. 1980, McCarthy et al. 1978, 1980). The attractive qualities of monazite as a nuclear waste form are (1) a high solubility for actinides and rare earths (10 to 20 wt %) (2) evidence from natural occurrences of good chemical durability (3) an apparent resistance to radiation damage, as natural monazites are seldom found in the metamict state, despite very high alpha-decay event doses (Boatner and Sales 1988). There have been extensive studies of monazite and apatite as potential waste form phases, and a considerable amount of work on a number of synthetic phosphate phases has been completed. 

Crystallographic studies are impeded by the fact that some rare earth minerals always occur in the metamict state, which is an amorphous state mainly caused by radiation damage to the crystal structure caused by radioactive decay of elements such as uranium and thorium. Crystallographic data and crystal structures of metamict minerals were therefore determined using samples recrystallized by annealing. In such cases, we must note that there is some doubt about the identity of the crystal structure of the recrystallized phase and the original structure of pre-metamict minerals. 

Fitzpatrick and Pabst (1986) suggested that yttrialite-(Y) may be identical with metamict thalenite-(Y) in their description of thalenite-(Y) from Arizona. YttriaUte-(Y) is known only as a metamict mineral. The crystal structure of thalenite- Y) reported by Kornev et al. (1972) does not resemble the structure of the high-temperature y-phase of yttrialite-(Y). 

The similarity between the X-ray diffraction patterns of rowlandite-(Y) and yttrial-ite-(Y) heated under identical conditions suggests only that the phase assemblages that result from recrystallization are similar, not that the rowlandite-(Y) and yttrialite-(Y) structures are similar. The crystal structure of rowlandite-(Y) has never been analyzed because it occurs in the metamict state. 

Metamiction (or metamictization) is a mineralogical term that describes the gradual and ultimately complete destruction of a crystal structure to yield an amorphous phase (i.e.. 

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