An interesting question. The short and simple answer would appear to be "no" because a mineral is supposed to be a unique designation. However, minerals are made up of many atoms and even single element minerals like gold,iron, or silver, may contain inclusions that with very sophisticated tools might show up in chemical analysis. So,in fact, minerals do vary in chemical composition. This fact is often used to determine the genesis and subsequent metamorphism of minerals. The effect of different compositions is within very narrow ranges and usually in very small amounts but make for some fascinating studies.