Mindat is a very useful and important tool for mineral enthusiasts. But it has a bad habit of going down from time to time.
So what can you do when this happens?
Well, you can wait, and sooner or later, it will return.
Otherwise, do what I do. Go to Mindat on archive.org. It won’t be totally up to date, but most of the time, you don’t need “perfect”.
Generally what I do is a Google search for say, a specific locality, then copy the url that comes up in the search results, and put that into the Internet Wayback Machine (archive.org). Not ideal, but when needs must…
Example: Search for (say) mindat cornwall. You will get a result screen like this:
Copy the link for the locality you want. In this case, the first link is https://www.mindat.org/loc-840.html
Then paste it into the Wayback Machine like this:
You might have to go back to an early date as Mindat no longer allows indexing from archive.org. But there is a good chance you will find the information you are after.
This one is from 2016:
And when you scroll down, you can click on a link such as Penberthycroftite, and you will get this:
https://www.mineralienatlas.de/lexikon/index.php/Chapter/Mineralogy
Mineralienatlas is a very useful site when Mindat is down. Up to date species information and worldwide locality information, and lots of good mineral photos. Mostly in German language, but there is a translate button (click on the British flag), and the photos speak for themselves.