Zirak-zigil is the Silvertine mountain, called Celebdil in Sindarin. Underneath it lies the majority of Khazad-dûm, including the path the Fellowship followed to get through Moria. Tolkien translated it as "Silver Spike", with zirak "spike, tine, point" and zigil "silver, the color". Originally, the translations of the two names was reversed. At some point, Tolkien decided that they needed to switch. It may have been when he came up with the name Zigil-nâd for the Silverlode river, which later changed to Kibil-nâla.
Zirak is "spike, tine, point" and would be singular, nominative, composition.
Zigil is "silver, the color" and singular, nominative, indefinite.
Zirak-zigil is thus "Spike of Silver", a construct compound with noun-adjective word order.
The Lord of the Rings, The Fellowship of the Ring, Book II, ch 3
The Treason of Isengard, pg 174-175, 432
Sauron Defeated, pg 45
Parma Eldalamberon XVII: Words, Phrases and Passages, pg 35, 36