Комиссия IMA по новым минералам и названиям минералов: нормы и рекомендации по номенклатуре минералов, 1998 Аморфные веществаAmorphous substances
Amorphous substances are non-crystalline, and therefore do not meet the normal requirements for mineral species. The term “crystalline”, as generally used in mineralogy, means atomic order on a scale that can produce a regular array of diffraction spots when the substance is traversed by a wave of suitable wavelength (X-ray, electrons, neutrons, etc.). However, some geologically derived substances such as gels, glasses and bitumens are non-crystalline. Such substances can be divided into two categories: amorphous, those substances that have never been crystalline and do not diffract, and metamict, those that were crystalline at one time, but whose crystallinity has been destroyed by ionizing radiation. Some mineralogists are reluctant to accept amorphous substances as mineral species because of the difficulty of determining whether the substance is a true chemical compound or a mixture, and the impossibility of characterizing it completely; the term “mineraloid” is sometimes applied to such substances. However, in the past some amorphous substances (e.g., georgeite, calcio-uranoite) have been accepted as mineral species by the CNMMN.
With modern techniques, it is possible to study amorphous phases more effectively than was possible in the past. Spectroscopic methods associated with a complete chemical analysis can in many cases identify an amorphous phase unequivocally. In fact, appropriate spectroscopic techniques (e.g., IR, NMR, Raman, EXAFS, Mössbauer) can reveal the three-dimensional short-range structural environment (chemical bonds) of each atom in the structure. Of course, without the possibility of obtaining a complete crystal-structure analysis, which can give the coordinates and the nature of the atoms, the need for a complete chemical analysis is more stringent with amorphous material than with a crystalline phase.
The basis for accepting a naturally occurring amorphous phase as a mineral species could be a series of complete quantitative chemical analyses that are sufficient to reveal the homogeneous chemical composition of a substantial number of grains in the specimen, and physicochemical data (normally spectroscopic) that prove the uniqueness of the phase.
Metamict substances, if formed by geological processes, are accepted as mineral species if it can be established with reasonable certainty that the original substance (before metamictization) was a crystalline mineral of the same bulk composition. Evidence for this includes the restoration of crystallinity by appropriate heat-treatment and the compatibility of the diffraction pattern of the heat-treated product with the external morphology (if any) of the original crystal, e.g., fergusonite-(Y).
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