Montanari and Koeberl (2000) define distal ejecta as material deposited at a distance of over 5 crater radii, with proximal ejecta being deposited at distances less than 5 crater radii. Stöffler and Grieve (2007) offer a classification of ‘impactites’, which is a collective term for all rocks affected by one or more hypervelocity impact(s) resulting from collision(s) of planetary bodies. Impactites are sub-divided into shocked rocks, impact breccias and impact melt rocks. We are interested in the ‘impact melt rocks’, which are further subdivided by Stöffler and Grieve (2007) into glassy, hypocrystalline (part glass, part crystalline) and holocrystalline (wholly crystalline).
Tektite-like objects fall into the holohyaline (glassy) and hypohyaline / hypocrystalline (part glass, part crystalline) impact melt rocks traditionally described as distal ejecta. The author prefers to replace the phrase 'distal ejecta' with 'ballistic ejecta'. Bodies should be classified on physical and chemical properties and not on distance from a (possibly undiscovered / undefined) crater. Of these tektite-like bodies a sub-division can quickly be made separating out the macroscopically heterogeneous Muong Nong-type layered impact glasses which formed 'bombs' from the macroscopically homogenous droplets principally controlled by cohesive forces (surface tension) See Table 2.1). These droplets can be collectively termed impact spherules.
Impact spherules fall into two textural categories: Holohyaline (wholly glass) and hypohyaline / hypocrystalline (part glass, part crystalline). Glass and Burns (1987, 1988) formerly differentiated the holohyaline microtektites from the crystallite / microlite-bearing (hypohyaline to hypocrystalline) microkrystites. The principal control on whether an impact spherule contains cystallites, and therefore whether it is categorised as a tektite or krystite, is chemistry. Krystites are typified by a more basic composition, which may be derived from the source rock or by combination of meteoritic component. This more basic melt is inconsistent with optimal glass formation, hence the presence of crystallites / microlites.
A krystite can be defined in the same was as a tektite: Krystites are naturally occurring hypohyaline to hypocrystalline (crystallites within a glass groundmass, appearing macroscopically homogeneous) droplets formed by the vapourisation and condensation (and melting and ballistic ejection) of silica-rich rock by cosmic impacts.
Krystites and tektites can then each be further subdivided based on whether they are derived from a melt or whether they are vapour condensates (Glass and Simonson, 2013). The melt vs. vapour condensate derivation can be determined by microscopic analayses of physical characteristics and confirmed by geochemical attributes as outlined below (with reference to Glass, 2000 and Glass and Simonson, 2013):
a) Lechatelierite particles (SiO2 ribbons formed from detrital grains) would be expected in melts but not in condensates. Many individual microtektites do not, however, contain lechatelierite (Glass and Simonson, 2013).
b) Melts may theoretically contain other detrital minerals, particularly in closer proximity to the source crater. Detrital grains are more characteristic of lower temperature Muong Nong-type impact glass melts.
c) Bubbles / vesicles would be expected in melts but should be absent from condensates (some crystalline phases may be removed from microkrystites by dissolution, thus creating voids).
d) Melts should contain rotational forms (spheres, dumbbells and teardrops) whereas condensates should be only spheres.
e) Condensates should have lost a greater volatile content than melts.
f) A detectable, but not obvious meteoritic component should be present in melts. In contrast, a more obvious meteoritic component is typically present in vapour condensates.
At this point one has divided tektite-like objects into five principal categories: Impact 'bombs' (Muong Nong-type layered impact glass); melt-drop tektites; condensate tektites; melt-drop krystites and condensate krystites (see Table 2.1, principally based on Glass and Simonson, 2013).
The final classification method employed by Glass and Simonson (2013) is a size division. All bodies are divided into either micro- (under 1 mm diameter), mini- (1 to 10 mm diameter) or macro- (over 10 mm diameter). If no suffix is applied one can assume the body is macroscopic (see Table 2.1). It is notable that the largest spherical melt-form tektite is 111 mm diameter and that elongate forms (dumbbells, teardrops) are typically around 160 mm in maximum length. Melt-form macrokrystites are not known, but might theoretically exist.
Over geological time the tektites or krystites may interact with the host environment, resulting in alteration products. The tektite may still be recognizable as a pseudomorph, whereby the shape is maintained but the composition/chemistry is partially or fully altered. If it is still possible to determine whether the impact spherules are tektites or krystites then they could be called 'altered tektites' or 'altered krystites'. If the sub-division cannot be deciphered the the general term impact spherule is suitable for use in this case. If the origin of the spherule is uncertain (e.g. it might be volcanic of impact derived) then the more general term 'spherule' should be applied.