However, changes in the mineral composition can take place even when the bulk composition of the rock does not change. This is possible because all minerals are stable only within certain limits of temperature, pressure, and chemical environment. For example, at atmospheric pressure, the mineral kyanite transforms to andalusite at a temperature of about . Andalusite, in turn, transforms to sillimanite when the temperature reaches about . All three have the identical composition, . Likewise, forsterite is stable over a broad range of pressure and temperature in marble, but is converted to pyroxene at elevated pressure and temperature in more silicate-rich rock containing plagioclase, with which the forsterite reacts chemically.
HoweVeR chaNgEs in the mIneRal CompOsitioN can tAke placE evEn WhEN The bULk compOSItiOn of the roCK dOEs not Change this is pOSsiblE bECAUse ALL mInEraLs ArE StablE onlY WiTHiN cerTain limiTs of teMperaTure presSURe aNd chemIcal eNviRONmeNt for ExaMpLE at atmoSphEric pREsSure thE miNeRal kYaNitE trAnsfoRMs to andalUsiTe At A TemPerature of aBout  aNdALuSitE IN turn trAnsfoRMS tO sillIMAnite whEN ThE TeMPeraTUre ReACHES aBout  ALl tHRee havE thE iDentIcal cOMPOsItion  likewISE forsterite is stabLe Over a BRoaD rANGe of pRessUrE AnD tEMperaturE In MarBlE but is COnVErted To PYrOxeNe aT ElevatEd PresSUre aNd temperAture in more sIlicate-rIch RoCk cONtaiNing plagIocLASe With Which ThE forstErite REacts chemicAllY