Hello Vik,
Pistacite is not recommended to be used as a name but rather as a way of expressing the Fe3+ content in the epidote. Your sample looks like a dead on for epidote (pistachio green), the pistacite content would need to be determined with microprobe or something comparable. Hello Vik,
Pistacite is not recomended to be used as a name but rather as a may of expressing the Fe3+ content in the epidote. Your smple looks like a dead on for epidote, the pistacite content would need to be determined with microbrobe or something comprable.
Epidote group minerals are great at incorporating impurities as substitutions for Fe3+ and Ca2+ and can be fascinating petrologically because of this.
Cheers.
Ian Merkel
There are two lattice sites where Fe can go, but only one of them makes epidote out of clinozoisite. If there is less than 5% Fe by wt then there is not enough Fe to make epidote and so it must be Clinozoisite. I don't know if the Fe preferentially occupies one or the other of its possible lattice postions, but there will be a wt % of Fe that allows for filling the irrelvant postion completely and just more than half the species determining site. Any iron content above that % will definately be epidote. Anything in between must be determined by single xl x-ray work to find the species. However this range can be narrowed somewhat as Cinozoisite is optically positive and Epidote is optically negative, but that may not be the case close to the midpoint between the species and there single xl work really is necesary. I think this is a silly use of the "dominant lattice site" rule and there should only be one species and two colour varieties.
Rob Woodside
Yes, clinozoisite is a member of the epidote group - essentially epidote with very little Fe. A lot of mineralogists would consider that epidote should be called ferroan clinozoisite but its been around a long time. So the difference, whether by Fe content or optics or colour is quite arbitrary really, and the names are almost interchangable to many mineralogists and collectors. I don't think that pistacite (i.e. an epidote with Fe>Al) has yet been found in nature, even synthetically (but someone may correct me?) it only exists as a hypothetical endmember. I dont think there is any significant difference in Fe/Al partitioning between the sites else someone would have tried to define an ordered epidote as a new mineral.
"optically positive/negative" refers to optical effects under polarised light microscopy, not very useful info unless you have a petrological microscope, and not used so much these days with microprobes etc so accessible.
Ralph Bottrill
Rock Currier
Crystals not pistols.
