A compositional variation which is the result of an ion or ionic group for another ion or ionic group. Possible types of solid solution include substitutional, interstitial, and omission. Substitutional solid solution is the most common type of ionic substitution. The extent of substitutional solid solution is determined by the ion size, and the temperature. Interstitial solid solution occurs when ions or atoms fill interstitial void which exist between ions or ion groups. Omission solid solution occurs when the presence of vacancies allows a mineral to assume a range of compositions (e.g., pyrrhotite).