Djerfisherite: nebular source of refractory potassium

Abstract

Djerfisherite is an important carrier of potassium in highly reduced enstatite chondrites, where it occurs in sub-round metal-sulfide nodules. These nodules were once free-floating objects in the protoplanetary nebula. Here, we analyze existing and new data to derive an equation of state (EOS) for djerfisherites of ${\text{K}}_{ 6} ({\text{Cu}},{\text{Fe}},{\text{Ni}})^{B} ({\text{Fe}},{\text{Ni}},{\text{Cu}})_{24}^{C} {\text{S}}_{ 2 6} {\text{Cl}}$ structural formula. We use this EOS to calculate the thermal stability of djerfisherite coexisting in equilibrium with a cooling vapor of solar composition enriched in a dust analogous to anhydrous, chondritic interplanetary dust (C-IDP). We find that condensed mineral assemblages closely match those found in enstatite chondrites, with djerfisherite condensing above 1,000 K in C-IDP dust-enriched systems. Results may have implications for the volatile budgets of terrestrial planets and the incorporation of K into early formed, highly reduced, planetary cores. Previous work links enstatite chondrites to the planet Mercury, where the surface has a terrestrial K/Th ratio, high S/Si ratio, and very low FeO content. Mercury’s accretion history may yield insights into Earth’s.

Appendices

Appendix 1: ordering calculation

$$ \left( { 1- {\text{X}}_{ 2} - {\text{X}}_{ 3} - \frac{24}{25}({\text{s}}_{1} + {\text{ s}}_{2} )} \right) ( {\text{X}}_{ 2} - \frac{1}{25}{\text{s}}_{ 1} ) = \exp \left( \frac{25}{48} \right)\Updelta \bar{\text{G}}_{Ni - Fe}^{{{\text{o - }}ORD}} /RT )\left( { 1- {\text{X}}_{ 2} - {\text{X}}_{ 3} + \frac{1}{25}({\text{s}}_{1} + {\text{ s}}_{2} )} \right) \left( {\text{X}}_{ 2} + \frac{24}{25}{\text{s}}_{ 1} \right) $$

defining ${\text{K}}_{ 1} \equiv { \exp }\left( {\frac{25}{48}\Updelta \bar{G}_{Ni - Fe}^{{{\text{o - }}ORD}} /RT} \right)$, and ${\text{D}} \equiv {\text{X}}_{ 2} -\left( {{\text{X}}_{ 2} } \right)^{ 2} -{\text{X}}_{ 2} {\text{X}}_{ 3} + \frac{1}{25}{\text{X}}_{ 2} {\text{s}}_{ 2}$ and ${\text{E}} \equiv \frac{24}{25}( 1-{\text{X}}_{ 3} + \frac{1}{25} {\text{s}}_{ 2} )-\frac{23}{25} {\text{X}}_{ 2} ,$ and ${\text{F}} \equiv \frac{24}{625}$, and ${\text{X}} \equiv {\text{X}}_{ 2} -\left( {{\text{X}}_{ 2} } \right)^{ 2} -{\text{X}}_{ 2} {\text{X}}_{ 3} -\frac{24}{25}{\text{X}}_{ 2}{\text{s}}_{ 2} ,$ and ${\text{Y}} \equiv \frac{1}{25}(- 1- 2 3 {\text{ X}}_{ 2} + {\text{X}}_{ 3} + \frac{24}{25} {\text{s}}_{ 2} ),$ and ${\text{Z}} \equiv \frac{24}{625}$ we have $0 = \left( {{\text{K}}_{ 1} {\text{D}} - {\text{X}}} \right) + \left( {{\text{K}}_{ 1} {\text{E}} - {\text{Y}}} \right){\text{s}}_{ 1} + \left( {{\text{K}}_{ 1} {\text{F}} - {\text{Z}}} \right)\left( {{\text{s}}_{ 1} } \right)^{ 2}$.

Substituting X₂ for X_3, and X₃ for X₂, and s₁ for s₂, and s₂ for s₁, and $\Updelta \bar{G}_{Cu - Fe}^{{{\text{o - }}ORD}}$ for $\Updelta \bar{G}_{Ni - Fe}^{{{\text{o - }}ORD}}$ in the first expression above, leads to an analogous quadratic equation in s₂ in similarly substituted variables.

Appendix 2: EPMA method

Electron probe microanalysis was performed with one micron spot size, at 15 keV accelerating voltage and 20 nA beam current. Natural and synthetic standards were used to calibrate on the Kα lines for 16 elements using wavelength dispersive spectrometers (WDS) and one element using energy dispersive analysis (EDS). Unknowns were analyzed in five passes as listed in Table 4, and an EDS spectrum was collected on each spot with a 240 s dwell time.

Table 4 EPMA conditions