Refinement of the crystal structure of christofschäferite-(Ce) and the modular aspect of the chevkinite polysomatic series with the general formula of {A&lt;sub&gt;4&lt;/sub&gt;B(T&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;}{C&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;8&lt;/sub&gt;}&lt;sub&gt;m&lt;/sub&gt; (m = 1, 2)
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