Both Uses of

phenomenon
in
The Brothers Karamazov

And then they do say that it all comes from terror at the menacing phenomena of nature, and that none of it's real.†
Chpt 2 *

phenomena = things that exists or happened -- often of special interest

He was interested in it as a social phenomenon, in its classification and its character as a product of our social conditions, as typical of the national character, and so on, and so on.†
Chpt 12

phenomenon = something that exists or happened -- often of special interest

Definitions:

(1) (phenomenon)
something that exists or happened -- especially something of special interest -- sometimes someone or something that is extraordinary

"Phenomenons" and "phenomena" are both appropriate plural forms of this noun. "Phenomena" is generally used in scientific or philosophical contexts.
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

In philosophy, a phenomenon is something as known through the senses. It is contrasted with a noumenon.