The key thing here is to know the actual partition that is encrypted.
Often in Linux Mint's installer that ends up being partition 5 or /dev/sda5
sudo cryptsetup luksOpen /dev/sda5 anynamehere
You will then be prompted for your irrecoverable passphrase:
Enter passphrase for /dev/sda5:
If all goes well it won't say anything further. If it says ""No key available with this passphr........