CHEMICAL VARIATION IN VESUVIANITE

Microprobe analyses of 44 vesuvianite specimens of different color, morphology, and occurrence indicate the chemical variation of the species. Vesuvianites can be divided into four types, based on chemistry and color; most vesuvianites can be assigned to one of these types with confidence. Type 1 vesuvianites contain approximately 2 atoms of Mg, <0.25 atoms of Ti, and variable Fe, apparently trivalent, in a formula based on 50 non-H cations. They are dark to pale green, white, or pink. Type 2 vesuvianites contain >2.5 atoms of Mg, variable (divalent?) Fe, and <0.5 atoms of Ti. In color they are yellow, yellow-brown, or yellow-green. Type 3 vesuvianites contain divalent Fe, 0.5 to 1.5 atoms of Ti and close to 18 atoms of Si (full occupancy of tetrahedral sites). They are yellow, brown, red-brown, or black. Type 4 vesuvianites are manganoan or cuprian; the studied samples are from Pajsberg, Sweden, Franklin, New Jersey, and Telemark, Norway. In color they are red-brown, purple, blue ("cyprine"), or green. Iron and much of the Mn is apparently trivalent. Unlike garnets, which they resemble structurally, vesuvianites contain almost exclusively Ca in the eight-fold sites in the structure. Silicon occupies 95% or more of the tetrahedral sites. Aluminum fills the smaller octahedral site, A. Chemical variation occurs predominantly in the more open, octahedral, general or G site and in the five-coordinated B site. Simple substitutions in G include Fe3+ or Mn3+ for Al3+, and Fe2+, Mn2+, or Zn2+ for Mg2+. Coupled substitutions include TiO = AlOH, MgTi = AlAl and AlAl = MgSi. The B site may contain CU2+, Fe2+, Fe3+, or Al3+. Changes in the amounts of O and OH in two different positions give a range of anion charge from 146(O67OH12) to about 148 (O69OH10).