Abstract: As time passes, ancient buildings are exposed to different types of erosion. Tous historic mud wall is a part of Tous castle, the oldest building in Mashhad plain, northeast of Iran. This paper deals with evaluation of mineralogical characteristics of the wall construction materials and essential reasons for erosion of this great wall. First of all, the different types of erosion which were observed on the wall were investigated. Then XRD analysis was carried out in order to determine the types of minerals and XRF analysis was done to find out the types and amounts of various elements and oxides in the wall materials. 16 samples were tested in this regard. These specimens were collected from different parts of the wall.

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