Question about mineral compositions?

Question

If we take a hand sample from one area and another hand sample from a completely different area of say Biotite or Olivine for example will their chemical compositions be exactly the same? That is will each hand sample contain the same % of each of the Biotite or Olivine end members or would the % compositions of end members vary from hand sample to hand sample?

Answer 1

They could be, and you could buy a lottery ticket and win $390,000,000.00. Hey! It could happen! Olivine and biotite are mineral families. Olivine is a magnesium, iron, calcium nesosilicate (Fe,Mg,Ca)SiO4 and biotite is a hydrous to fluorous, potassium, magnesium, iron, aluminum phyllosilicate K(Mg,Fe)3(Al,Fe)Si3O10(OH,F)2. The parentheses with elements separated by commas indicate a solid solution, where anywhere from 0% to 100% of one element can be present and from 100% to 0% of the other element is then present. As you can see, with such wide variability it is unlikely to find identical minerals from different locations.

Answer 2

I know olivine can be different because magnesuim and iron can trade places in the molecular structure of olivine. So your olivine mineral can fall anywhere along the fayalite/fosterrite line in reference to being more iron or magnesium rich. When you say hand sample do you mean a mineral handsample or rock one? If it is a rock the percent of a certain mineral will definitly change depending on condition it was formed under ex. magma mixing or fractional crystalization of magmas before they erupt or cool. Minerals can also alter to other minerals. Biotite alters to cholorite I think though I could be wronge. Olivine can alter to iddingsite... though im not sure if your teacher is counting alteration products as a different mineral or not.