Formula: Al₂(Si₂O₅)(OH)₄

Species:

Colour: White to tan, sometimes greenish or bluish, also chocolate brown to reddish.

Lustre: Waxy, Earthy

Hardness: 1 – 2

Specific Gravity: 2 – 2.65

Crystal System: Monoclinic

Member of: Kaolinite Subgroup > Kaolinite-Serpentine Group

Name: Originally named by French geologist and mining engineer Pierre Berthier in 1826 after the Belgian geologist Jean-Baptiste Julien d’Omalius d’Halloy (1783-1875), who discovered the mineral. He was a nobleman, a statesman, and pioneer of modern geology in Belgium. After the discovery of more hydrated species, the name metahalloysite was sometimes used for this species  ). The name was changed by the IMA to halloysite-7Å in 1980   where the suffix refers to the 7-Ångström layer thickness that is identified in XRD patterns. The species name was reverted back to halloysite, without the suffix, in 2022.

Polymorph of: Dickite, Kaolinite, Nacrite

Typically found as earthy to somewhat waxy looking extremely fine-grained clayey masses, white to tan and also occasionally bluish or greenish.