Schon, Journey's longtime lead guitarist, announced Tickner's passing Thursday with a social media post saying, "Journey Junkies, I have some very sad news. George Tickner, Journey’s original rhythm guitarist and songwriting contributor on their first three albums, has passed away."

He added, "Our condolences to his family and friends, and to all past and present band members. So heartbreaking. I think we need to do a group hug."

Tickner was the original rhythm guitarist, when he formed Journey in 1973 ... along with Schon and bassist Ross Valory.

Interestingly, he bowed out of the band in 1975 -- before they became hitmakers, selling more than 100 million records -- because he wanted to pursue a career in medicine, and he was serious about it. George earned a scholarship to attend Stanford Medical School.

He never fully gave up on music, though ... he was reportedly writing and doing local performances as a hobby.

George rejoined his Journey bandmates -- and subsequent members like Steve Perry -- back in 2005 when the group got its star on the Hollywood Walk of Fame. That's him rocking that sweet handlebar mustache.

Prior to Journey, George was a member of Frumious Bandersnatch, a psychedelic rock band formed in Berkeley in the late '60s.

Neal's post didn't mention a cause of death. George was 76.