Classification of Coated Grains

The term “coated grains” was coined by Wolf (1960) as a substitute for Folk’s (1959) “oolites”, “to include other concentrically formed materials such as pisolites” (Wolf 1960, p. 1415). Among the different terms proposed for specific kinds of these grains the term “ooid” has unquestionably the longest tradition in geological literature, going back to the 18th century, although the orthography as used here is of later date, Having been introduced by Kalkowsky (1908). As ooids, “small egg-like grains, resembling the roe of a fish, each of which has usually a small fragment of sand as a nucleus, around which concentric layers of calcareous matter have accumulated” (Lyell 1855, p. 12) have been named. In 1916 Heim introduced a new term: “oncoid” (in original German orthography: Onkoid; “onchos” in Greek means “nodules”), for denoting a type of coated grain found by him in Mesozoic deposits of the eastern Swiss Alps which differed from an ooid in that the coated grain called oncoid possessed (Heim 1916, p. 566): (1) non-concentric overlapping of separate envelopes, (2) not exclusively rounded form, (3) compact structure, with or without nucleus, and (4) sharp or indistinct boundaries. As noted by Heim (1916), transitions between ooids and oncoids exist, and this is expressed in encrustation of ooids by irregular calcareous envelopes.