For iron sulfide, two elements (sulfur and iron) are heated together so that they form the compound iron sulfide. Although iron sulfide contains both sulfur and iron, it is not a mixture: it is a pure substance. Both the sulfur and the iron have 'given up' their individual properties to become a compound.
Gneiss. Gneiss is a hard rock that has a mineral composition similar to granite as it contains the feldspar, mica, and quartz. This is a rock that is formed from pre-existing igneous rocks such as granite that have been subject to conditions of high pressure and temperature.ranite is a hard igneous rock that is mostly made up of feldspar and quartz. Granite has a crystalline structure, and it can have a wide range of colors from grey to pink. The color of a granite rock is dependent upon the composition of minerals.
Gneiss is a high grade metamorphic rock, meaning that it has been subjected to higher temperatures and pressures than schist. It is formed by the metamorphosis of granite, or sedimentary rock. Gneiss displays distinct foliation, representing alternating layers composed of different minerals.neiss is typically associated with major mountain building episodes. During these episodes, sedimentary or felsic igneous rocks are subjected to great pressures and temperatures generated by great depth of burial, proximity to igneous intrusions and the tectonic forces generated during such episodes.