Google uses an XML schema to define the elements and attributes that can
appear in your Sitemap file. You can download this schema from the links below:
For Sitemaps:
http://www.google.com/schemas/sitemap/0.84/sitemap.xsd
For Sitemap index files:
http://www.google.com/schemas/sitemap/0.84/siteindex.xsd
For more information on validating a Sitemap against a schema, see:
http://www.google.com/webmasters/sitemaps/docs/en/protocol.html#sitemapValidation

