Widgiemoolthalite is a rare hydrated nickel(II) carbonate mineral with the chemical formula (Ni,Mg)5(CO3)4(OH)2·5H2O.

Present on gaspéite surfaces, widgiemoolthalite has a Mohs scale hardness of 3.5 and an unknown though likely disordered crystal structure.

It was named the following year by the three researchers who first reported its existence, Ernest H. Nickel, Bruce W. Robinson, and William G. Mumme.

The mineral was discovered in 1992 and was first reported in American Mineralogist in 1993 by Ernest H. Nickel, Bruce W. Robinson, and William G. Mumme, when it received its name for its type locality.

It is found overlaying nickel sulfide that has undergone weathering, often in hollow spaces on gaspéite surfaces, and often exhibiting fibrous and rarely massive crystal habits.

[2] Other minerals associated with widgiemoolthalite include annabergite, carrboydite, dolomite, glaukosphaerite, hydrohonessite, kambaldaite, magnesite, nepouite, nullaginite, olivenite, otwayite, paratacamite, pecoraite, reevesite, retgersite, and takovite.

[9] In support of the designation of an Anthropocene epoch, the existence and provenance of widgiemoolthalite, along with 207 other mineral species, have been cited as evidence of uniquely human action upon global stratigraphy.

[14][15] Under an optical microscope, Nickel, Robinson, and Mumme reported difficulty discerning individual crystals as their lateral dimensions were too small.