gems, jade, amphibole jade, actinolite, tremolite, amphibole group, mineralogy, nomenclature, crystal chemistry, classification, New Zealand, pounamu, Maori

New Zealand “jade” — the pounamu of the Māori — refers to two varieties of green “jade” rock which, in a strict mineralogical definition, are: (1) rocks composed essentially of actinolite [Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂; in weight-percent oxides: 59.17% SiO₂; 34.82% (MgO + xFeO); 13.81% CaO; 2.22% H₂O], and (2) serpentine [(Mg,Fe)₃Si₂O₅(OH)₄, antigorite variety; in weight-percent oxides: 43.37% SiO₂; 43.63% (MgO + xFeO); 13.00% H₂O]. The actinolite variety of pounamu is commonly referred to as nephrite, following Alexis Damour’s original 1846 classification of jade into nephrite and jadeite types, but among the Māori it is known by several names based principally on colour — kahurangi, kawakawa, inanga, and others (see Beck 2010). It has been recommended that the name nephrite be replaced by actinolite (Leake 1978), which, in the International Mineralogical Association nomenclature of amphiboles (Leake et al. 1997), forms part of the tremolite–actinolite–ferro-actinolite calcium-amphibole solid-solution series: Ca₂Mg₅Si₈O₂₂(OH)₂ — Ca₂Fe₅Si₈O₂₂(OH)₂. Within this series, the tremolite end-member occupies only a narrow compositional range and is defined by a Mg/(Mg+Fe²⁺) ratio ≥ 0.9; actinolite has a ratio between 0.9 and 0.5; and ferro-actinolite has a ratio below 0.5 (Fig. 1). These limits were designed to preserve the distinction between green actinolite and colourless tremolite (Leake et al. 1997), the intensity of the green coloration increasing with increasing FeO content. Pounamu actinolite possesses a high XMg value of 0.87–0.90 and therefore has a composition close to, or directly upon, the tremolite–actinolite boundary (Berwerth 1880a; Grapes and Yun 2010) (inset in Fig. 1). The serpentine variety of pounamu is known as tangiwai, or takiwai in the southern Māori dialect. It has variously been designated noble serpentine² (Hector and Skey 1866), marmolite³ (Hutton and Ulrich 1875), and bowenite⁴ (Berwerth 1880b; Marshall 1904; Finlayson 1909, and in most subsequent references to this pounamu variety) (see also Adkin 1956; Reed 1957, 1962). Grapes and Nolden (2021) have recommended, on the basis of origin, geological association, and cultural considerations, that the term “bowenite” be replaced by the earlier proposed term (1869) tangiwaite. Although nephrite was regarded as a mineral in early mineralogical classifications, it is in fact a rock, typically composed predominantly of actinolite or tremolite and usually containing small quantities of other minerals as impurities. In actinolite pounamu, these accessory phases include chlorite, serpentine, talc, diopside, chromium-bearing calcium garnet, chromium muscovite, titanite, magnesite, chromite, pyrite, chalcopyrite, and, more rarely, green-brown hornblende, margarite, and quartz (e.g., Turner 1935; Grapes, unpublished data). Depending upon their composition and abundance, such accessory minerals “dilute” the ideal actinolite composition given above, producing the relatively wide spread of bulk-rock pounamu compositions expressed in mol.% CaO–MgO–FeO and extending above and below the tremolite–ferro-actinolite tie-line (Fig. 2).