1) What does the commar represent in the formula for Olivine (Mg,Fe)2 Si04?
2) As quartz is not completely pure why are’nt impurities such as Ti or Al included in its chemical formula?
3) What is meant by the term mineral end members?
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1) The ratio of magnesium and iron varies between the two endmembers of the solid solution series: forsterite (Mg-endmember) and fayalite (Fe-endmember). Compositions of olivine are commonly expressed as molar percentages of forsterite (Fo) and fayalite (Fa) (e.g., Fo70Fa30). meaning that between the two element their ratios very but that their is a molar ratio of two in the empirical formula.
3)An endmember in mineralogy is a mineral that is at the extreme end of a mineral series in terms of purity. Minerals often can be described as solid solutions with varying compositions of some chemical elements, rather than as substances with an exact chemical formula. There may be two or more endmembers in a group or series of minerals.
For example, the endmembers of the pyralspite garnets are almandine [Fe3Al2(SiO4)3], pyrope [Mg3Al2(SiO4)3] and spessartine [Mn3Al2(SiO4)3], A specific pyralspite garnet can have varying quantities of iron (Fe), magnesium (Mg) and manganese (Mn).
As another example, the plagioclase series of tectosilicate minerals within the feldspar group ranges from albite to anorthite (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral’s crystal lattice structure. Thus, both albite and anorthite are endmembers of the plagioclase series. Albite is also the sodium endmember of the alkali feldspars whose series ranges from pure NaAlSi3O8 to pure KAlSi3O8.
The comma is there to show you that there can be a varying amount of either Mg or Fe in the mineral.
Quartz can be complete. It is rare in nature but where it has impurities in the PPM range, they are not listed.
Mineral end members. Sorry . Don’t know this.