[root@arch mnt]# gdisk /dev/sdb
GPT fdisk (gdisk) version 1.0.1

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): n
Partition number (1-128, default 1): 
First sector (34-30662622, default = 2048) or {+-}size{KMGTP}: 
Last sector (2048-30662622, default = 30662622) or {+-}size{KMGTP}: +512M
Current type is 'Linux filesystem'
Hex code or GUID (L to show codes, Enter = 8300): ef00
Changed type of partition to 'EFI System'

Command (? for help): n
Partition number (2-128, default 2): 
First sector (34-30662622, default = 1050624) or {+-}size{KMGTP}: 
Last sector (1050624-30662622, default = 30662622) or {+-}size{KMGTP}: +1024M
Current type is 'Linux filesystem'
Hex code or GUID (L to show codes, Enter = 8300): 
Changed type of partition to 'Linux filesystem'

Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): y
OK; writing new GUID partition table (GPT) to /dev/sdb.
The operation has completed successfully.
[root@arch mnt]# cryptsetup luksFormat /dev/sdb2 --cipher serpent-cbc-essiv:sha256 --hash tiger2

WARNING!
========
This will overwrite data on /dev/sdb2 irrevocably.

Are you sure? (Type uppercase yes): YES
Enter passphrase: 
Verify passphrase: 
[root@arch mnt]# ls
boot  efi
[root@arch mnt]# mount /dev/sdb1 efi
mount: unknown filesystem type 'crypto_LUKS'