Preview index configuration, to be put under App_Config\Sitecore\ContentSearch folder along with other index definitions

Sitecore.ContentSearch.Solr.Index.Preview.config

<?xml version="1.0" encoding="utf-8"?>
<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/" xmlns:role="http://www.sitecore.net/xmlconfig/role/" xmlns:search="http://www.sitecore.net/xmlconfig/search/">
  <sitecore role:require="Standalone or ContentDelivery or ContentManagement" search:require="solr">
    <contentSearch>
      <configuration type="Sitecore.ContentSearch.ContentSearchConfiguration, Sitecore.ContentSearch">
        <indexes hint="list:AddIndex">
          <index id="sitecore_preview_index" type="Sitecore.ContentSearch.SolrProvider.SolrSearchIndex, Sitecore.ContentSearch.SolrProvider">
            <param desc="name">$(id)</param>
            <param desc="core">DigitalEcosystem_preview_index</param>
            <param desc="propertyStore" ref="contentSearch/indexConfigurations/databasePropertyStore" param1="$(id)" />
            <configuration ref="contentSearch/indexConfigurations/defaultSolrIndexConfiguration" />
            <strategies hint="list:AddStrategy">
              <strategy ref="contentSearch/indexConfigurations/indexUpdateStrategies/manual" 
                        role:require="(ContentManagement and !Indexing) or (ContentDelivery and !Indexing)" />
              <strategy ref="contentSearch/indexConfigurations/indexUpdateStrategies/onPublishEndAsyncSingleInstance" 
                        role:require="Standalone or (ContentManagement and Indexing) or (ContentDelivery and Indexing)" />
            </strategies>
            <locations hint="list:AddCrawler">
              <crawler type="Sitecore.ContentSearch.SitecoreItemCrawler, Sitecore.ContentSearch">
                <Database>preview</Database>
                <Root>/sitecore</Root>
              </crawler>
            </locations>
            <enableItemLanguageFallback>false</enableItemLanguageFallback>
            <enableFieldLanguageFallback>false</enableFieldLanguageFallback>
          </index>
        </indexes>
      </configuration>
    </contentSearch>
  </sitecore>
</configuration>