The in-house developed software package XRDUA was used for the processing of all XRPD data. XRDUA provides the necessary tools for extracting crystalline-specific distributions from the large number of 2D diffraction patterns obtained during XRPD imaging experiments (34). The procedure to obtain compound-specific images from raw diffraction data is visualized in fig. S4. The acquired diffraction patterns were converted into 1D diffractograms by means of azimuthal integration after which the crystalline phases present in the diffractogram were identified. After integration, whole-pattern fitting was performed on the 1D diffractograms using a Rietveld model containing all the identified crystalline phases. The structural information was obtained from the American Mineralogist Crystal Structure Database (35). The relative positions and intensities of the diffraction peaks for every compound were kept fixed during the fitting, while the angle-independent parameter W of the Caglioti width function was used to model the peak width. Refined values for the scaling, displacement, and width parameters were obtained for each compound. The intensity scaling parameters obtained in each point of the image were used to visualize the spatial distribution of the different compounds (fig. S4). A more detailed explanation of the modeling procedure and the functionalities of XRDUA is offered elsewhere (34, 36). Weight fractions were calculated using the aforementioned scaling parameters (fig. S4). The 2D distributions shown throughout this work are either based on the global scaling factor or the weight fraction data obtained from this fitting procedure. For the average μ-XRPD weight ratios, an estimated error of ±10% was used on the basis of the analysis of model samples with a known composition and the SD calculated for different microscopic areas within the same cross-sectional layer. These model samples were created by mixing known amounts of hydrocerussite and cerussite into a homogeneous powder, after which the aforementioned fitting procedure was applied to calculate the weight ratios. For the average weight ratios obtained with MA-XRPD, an error of ±2σ, derived from the Gaussian profiles used in the histograms, was used.

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