Hafnium (Hf) – element-mineral links of Hf

Although it may not appear as so, hafnium (Hf) is an important element in chemostratigraphy. So, which are the element-mineral links of Hf?

Hafnium (symbol Hf; atomic number 72; relative atomic mass 178.49) [Link to webelements.com]

Hf has one oxidation state. +4.

Hafnium is present in five naturally occurring isotopes (¹⁷⁶Hf, ¹⁷⁷Hf, ¹⁷⁸Hf, ¹⁷⁹Hf, and ¹⁸⁰Hf), of which the most abundant ones are ¹⁸⁰Hf > ¹⁷⁸Hf > ¹⁷⁷Hf.

Hf commonly substitutes for zirconium (Zr), due to their very similar ionic radii (Hf⁴⁺ = 71 pm, Zr⁴⁺ = 72 pm) and chemical behavior.

In geochemical analysis, hafnium is commonly reported in its element form in parts per million [ppm]. However, some analyses report it in its oxide form as HfO₂.

Common element-mineral links of Hf

Most common element-mineral link of Hf: zircon (ZrSiO₄) {or (Zr,Hf)SiO₄}

Hf in siliciclastic rocks

Hafnium is closely associated with Zr minerals and no pure Hf minerals are known.

Zircon (ZrSiO4) {or (Zr,Hf)SiO4} is the most common Zr mineral. Baddeleyite (ZrO₂) {or (Zr,Hf)O₂} is much rarer.

Siliciclastic sedimentary rocks typically show similar Hf concentrations (commonly 2.5 to 6.5 ppm). Thus, ratios of Zr/Hf or Hf/Zr are commonly relatively constant through siliciclastic successions. Small variations may occur due to grain-size variations, as zircons are often zoned, and these zones may show varying Hf concentrations. Strong systematic variations between siliciclastic successions, however, may indicate different provenance. For instance, such as Nepheline-Syenite, have (very) high Hf concentrations, which thus are expressed in the Zr/Hf or Hf/Zr ratios.

Hf in carbonate and evaporite rocks

In carbonates, Hf concentrations are commonly very low and hosted in the non-carbonate fraction. It possibly can get enriched in stylolites. It may also be present as part of volcanic ashes. Hence, Hf may have a high potential for chemostratigraphic correlation (marker horizon).

Hf in igneous rocks

Felsic igneous rocks (especially intrusive ones) contain generally more Hf (and Zr) than mafic and ultramafic rocks. Kimberlite, carbonatite, and alkali-rich lavas can be enriched in Hf.