Variable Formula
Describes the variable formula of a mineral, meaning the possibilities of common elemental replacements of the mineral. Underlined elements in the variable formula denote elements that can be absent and must have a smaller percentage than the element(s) not underlined in the same set of parenthesis. Any element not underlined must be present in an appreciable quantity.

Note that the variable formula field only displays elements that commonly replace part of an element in a given mineral. Other possible element replacements not mentioned may exist but only occur insignificantly or rarely.