Feldspathoids and their relationships to zeolites

摘要

The feldspathoids are defined as anhydrous framework aluminosilicates that contain alkali and alkali-earth elements, but contain no volatile anions and are similar in composition to the feldspars (i.e., they contain the same or similar atoms), but contain less silica than the alkali feldspar. Thus the feldspathoids have an Si/(Si + Al) ratio less than 0.75. Nepheline and leucite fit this definition. Sodalites, cancrinites, and scapolites are commonly considered as feldspathoids. However, these are the only framework aluminosilicates that contain large volatile anions such as Cl~-, SO_4~(2-), CO_3~(2-) OH~-, and H_2O. These minerals have chemistries and structures that are quite different from that of the feldspars or the feldspathoids, proper. However, some of their structures are closely related to the zeolites, but they are not zeolites, proper. The structures of the cancrinite-group minerals are characterized by stacking of six-membered rings in an AB ... sequence. This stacking gives rise to large continuous channels that are formed by twelve-membered rings of alternating AlO_4 and SiO_4 tetrahedra. These are among the largest channels known, even for zeolites. The cancrinite structure also consists of small cages, known as ε-cages in zeolite chemistry. The sodalite-group minerals have strong structural similarities to the cancrinite-group minerals, but the former have an ABC ... stacking sequence that leads to cubic instead of hexagonal symmetry. This sequence leads to an offset of the C-type layer and gives rise to a network of large β-cages in the sodalite-group minerals. Some cancrinite-type minerals have structures that are based on more complicated stacking of six-membered rings. Examples of these minerals and their stacking sequences are liottite (ABABAC ...), and afghanite (ABABACAC ...). Some zeolites also have structures that are based on similar stacking of six-membered rings, e.g., offretite (AAB ...), erionite (AABAAC ...), levynite (AABCCABBC...), and chabazite (AABBCC ...). The ε- and β-cages are also used as building blocks in many zeolite structures such as zeolite A, offretite, and faujasite. As a family, sodalite, cancrinite, and scapolite, have a unique chemistry in that they are the only aluminosilicate minerals that contain large volatile anions and they have a close structural relationship to the zeolites, but not to the feldspars or the feldspathoids, proper. Therefore, a new group name, e.g., zeoloids, may be appropriate for these minerals because they are not feldspathoids.