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You will also need to install the Drupal Apache Solr module (https://drupal.org/project/apachesolr).  The Drupal Apache Solr module installation says to copy it's its Solr configuration files to the Solr configuration directory, . Do not do this as you will be overwriting your existing Islandora Solr configuration.  Instead update the existing schema.xml file with the required Drupal fields (it's good practice to backup any files before you edit them if they are not already under a vcs, even better try it on a dev/test install first).

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function bartik_preprocess_islandora_solr(&$variables) {
	$results = &$variables['results'];
		foreach ($results as &$result) {
			if (!empty($result['solr_doc']['bundle_name'])) {
			$path = url('http://aurltothethumbnail'); //could check bundle for type and link to thumbnail for each type
			$options = array('html' => TRUE);
			$options['attributes']['title'] = $result['solr_doc']['label'];
			$image = theme('image', array('path' => $path));
			$key$result['thumbnail'] = l($image, $result['solr_doc']['url']['value'], $options);
		}
	}
}

Multi-site

If you are using a Drupal multi-site setup and you have more than one Drupal site's content in Solr you will probably want to filter on the site field or the hash field to limit results to just the drupal site you are viewing. This can be done in the Islandora Solr Settings -> Query defaults config page or you can setup a custom request handler in the solrconfig.xml.

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