What is the legally mandated percentage for the obligatory levy on minerals (Ma'adin)?

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The required amount due is one-quarter of one-tenth (rub'u al-'ushr). This ruling is supported by the practice concerning the mines of Bilal ibn al-Harith al-Muzani, from whom only Zakat was taken until the present day, as narrated by Abu Ubayd via Rabia ibn Abi Abd al-Rahman, and corroborated by Ibn Kathir ibn Awf and Al-Darawardi. The rationale is that this levy is a right restricted from the wealthy relatives, thus classifying it as Zakat, similar to the obligation on owned precious metals.