"Rock of the Month # 270, posted for December 2023" ---

Tranquillityite, a "well-travelled" mineral, if rather rare on Earth

In one short paragraph: Tranquillityite is an iron- titanium- zirconium- yttrium silicate. It is dark brownish-red to nearly opaque, and occurs as thin laths. Found in lunar basalts brought back by the Apollo 11 and Apollo 12 missions (the landing sites on Mare Tranquillitatis and Mare Procellarum: Lovering et al., 1971; see also Fleischer, 1973; Bernard and Hyrsl, 2015, p.691). Lunar occurrences from Apollo 12 are described by, e.g., Baldridge et al. (1979) and Beaty et al. (1979) and from Apollo 17 by Warner et al. (1979), as summarized by Papike et al. (1998, pp.5-34 to 5-35). The tawny oxide was more recently found on Earth, in an extensive dolerite (diabase) province in Western Australia (Rasmussen et al., 2012). It also occurs in late-stage silica-rich crystallization assemblages in martian basalts, as in NWA 856 (Leroux and Cordier, 2006) and NWA 6963 (Lindner et al., 2022) and other meteorites.

The Apollo samples yielded three minerals previously unknown on Earth (armalcolite, pyroxferroite and tranquillityite), all of which were later recognized in terrestrial settings. The formula of tranquillityite can be expressed as Fe²⁺₈(Zr,Y)₂Ti₃(SiO₄)₃O₁₂, with minor amounts of additional elements such as Ca. It is an example of a nesosilicate, with isolated SiO₄ tetrahedra - more-familiar examples are the olivine and garnet mineral groups, as well as titanite (sphene). It has hexagonal crystal structure, and occurs as foxy-red laths, translucent to near-opaque, grey in reflected light. A compilation of analytical data on lunar, martian and terrestrial tranquillityite (Lindner et al., 2022, p.2030) notes as many as 16 components. In addition to the essential Fe and Ti, Si and Zr, this mineral can contain as much as, e.g., 3.56 wt.% Y₂O₃, 1.72 wt.% CaO and 1.60 wt.% MgO.

The Moon

Probably the first thing the researchers of the recently-returned Moon rocks investigated were striking rock-forming mineral grains, crystals of pyroxene zoned from pale pigeonite cores to pink augite rims. Like Earth rocks of the basalt-diabase-gabbro clan, lunar basalts are dominated by silicates: pyroxenes and plagioclase feldspars, plus other minerals. The interstitial glass (a mesostasis) may contain such accessory minerals as ulvospinel, baddeleyite, phosphates (apatite or whitlockite) and tranquillityite (Beaty et al. , 1979). High-Ti mare basalts from Apollo 17, besides rock-forming minerals like olivine and plagioclase, contain sparse armalcolite microphenocrysts, usually mantled by ilmenite (armalcolite's name is an acronym for the three crew members of the Apollo 11 mission, that participated in the first Moon landing in 1969). Silica often occurs with residual glassy mesostasis, with late minerals such as tranquillityite, zirconolite, baddeleyite, ulvospinel and pyroxferroite (Warner al.. 1979). Not surprisingly, tranquillityite is also reported from some lunar meteorites.

Mars (via meteorites)

Leroux and Cordier (2006) studied the NWA 856 meteorite, a small (320-gram) visitor from space found in Morocco in 2001. This shergottite (martian basalt) contains silica-rich, late-stage crystallization pockets, located between maskelynite laths after feldspar, or between maskelynite and pyroxene (maskelynite is a feldspathic glass formed from pre-existing plagioclase feldspar by intense shock, likely induced by an impact event on Mars). Cristobalite and quartz occur in silica-rich glass, with late-stage accessory minerals such as ilmenite, tranquillityite, fayalite, troilite, baddeleyite, apatite and chlorapatite. A recent detailed study of another shergottite (Lindner et al., 2022) provides new data and reviews the chemistry of tranquillityite. Fragments of the NWA 6963 meteorite were uncovered in Morocco, 2011 onwards, and 8-10 kg may have been recovered. Like NWA 856, this meteorite is largely pyroxene and shocked feldspathic glass (equivalent to common terrestrial basalt, diabase and gabbro). plus accessory minerals, largely oxides, silica and phosphates. The mesostasis is largely silica plus K-feldspar and plagioclase, and minerals such as tranquillityite and the phosphate merrillite. The phosphate appears to be a crystallization product of a late magma, not due to a shock (impact) event, and the assemblage has been dated at 174±6 Ma. This indication of magmatism, with an evolved melt, at just 174 Ma is astonishing, as one might have suspected that Mars, like Earth's Moon, has been essentially dormant for much longer.

and Earth

Tranquillityite has been identified in six dolerite (diabase) dykes and sills in the northeast Pilbara craton, northern Western Australia (Rasmussen et al., 2012). As with the lunar and martian occurrences, the terrestrial tranquillityite, occurring as red translucent laths, is found in late interstitial settings in quartz and K-feldspar between pyroxene and plagioclase feldspar. It occurs with zirconolite, CaZrTi₂O₇ and baddeleyite, ZrO₂. The tranquillityite can be altered to fine-grained secondary products. The mineral, in coarse sills cutting the Eel Creek Formation, has been dated, using a SHRIMP ion microprobe, to 1064±14 Ma, suggesting that the minor intrusions are a part of the circa 1070 Ma Warakurna LIP (large igneous province). The six occurrences vary in age from Mesoproterozoic to Cambrian. The date on coarse dolerite cutting the Eel Creek Formation, on the north margin of the Pilbara craton, indicates that the large, east-west Warakurna LIP (Wingate et al., 2004), known over an area of some 1.5 million km² with abundant quartz dolerite, is actually even larger in extent. Mafic magmatism was extensive in the Mesoproterozoic, in Australia and beyond. The Warakurna magmatism is of similar age to the mineralized Nebo-Babel gabbronoritic intrusion and the Giles layered complex (Seat et al., 2011).

Kristi Tavener and Jonas Valiunas of Lakehead University prepared a polished thin section from the little cubic fragment. A quick look at the 2.4 cm² area of the polished sample reveals the ophitic texture typical of a diabase, with plagioclase crystals partly enveloped by clinopyroxene. The rough estimated mode (proportions of each mineral by area / volume) is plagioclase feldspar (63%), clinopyroxene (augite, 20%), opaque oxide (ilmenite, 10%), and olivine (7%), plus traces of evident tranquillityite and pyrite (alteration products are ascribed to the parent mineral, e.g., kaolinite after feldspar, and brown "iddingsite" after olivine). The plagioclase, clear except in heavily kaolinized patches, is not ideal for visual estimate of composition, but a result was achieved of calcic labradorite (An₆₉). The four rock-forming minerals have maximum dimensions of 1-2 mm. The mineral that I assume to be tranquillityite occurs in or adjacent to olivine, pyroxene and feldspar (unlike iddingsite, of a similar hue, which replaces olivine alone along fractures). The foxy-red (?) tranquillityite has a high relief and a rough parting, the birefringence masked by the strong body colour. It is not isotropic, though tranquillityite may contain enough uranium to become metamict (meaning the destruction of crystal structure, and loss of some inherent optical properties, by alpha radiation damage). The tranquillityite occurs in grains and clumps of grains at most 0.5x0.5 mm in section. The scattering of fine-grained pyrite, grains up to 0.1 mm in size, is largely in areas of turbid, altered feldspar.

Fig. 2: Stereomicroscope photograph in plain, transmitted light, to show the ophitic texture typical of dolerite (diabase). 25X magnification, field of view 5 mm. Here are plagioclase feldspar (clear) and pyroxene (very pale buff) plus lesser olivine (higher relief, more dappled than pyroxene), abundant ilmenite (opaque) and orange, high-relief (?) tranquillityite (just to the right and below the centre of the field of view).