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Excel XML is known as SpreadsheetML

FAIR WARNING:

• SpreadsheetML file sizes will be about 3-4 times larger than native XLS files. Not really a problem for small files, but with large data sets this could eat up your system resources to generate the file.
• SpreadsheetML will only be readable by Microsoft Office. Other mainstream office suites will open it as an XML document.

The CSV approach fits my needs 9 times out of 10. However if you require a native Excel format, please use a gem and avoid using the SpreadsheetML method.