The riddle of High-Calcium Pyroxenes in S-asteroids, the earth's moon and meteorites

Norbert Brügge, Germany

Something to think about:

HCP are not present in igneous rocks on Earth. Accordingly, there is a gap at the top in the pyroxene-diagram. The mineral wollastonite is wrongly placed here, Wollastonite is not a pyroxene (A. Strekeisen). It have a different type of infinite-chain structure and is said to be secondary formed by the melting of limestone after its subduction on Earth.
In contrast, HCPs are found widespread in S-asteroids, the Earth's moon and also in meteorites. This would now also apply to the suspected HCP in ultramafic rocks on Mars (Jezero crater). It is likely that the evidence of wollastonite in meteorites is confused with terrestrial wollastonite and that they are in truth the mysterious HCPs.

Sunshine et al. (2004)
suggest that the detected HCPs in S-asteroids were created by imaginary melting processes in the rock. However, such a scenario is very hypothetical. What is the real origin of the high calcium content and why is there a lack of such evidence of HCP in igneous rocks on earth.

HCP has also been found widespread in igneous rocks on the Earth's moon (Ogawa et al. (2011).

The detection of significant proportions of the HCP in two examined achondritic meteorites is particularly interesting because they could be examined directly (Takeda et al. (2005):

"High-Ca pyroxenes have been a minor component in chondrites and HED achodrites. We report two new achondrites (NWA-2235 and 2236) which contain more high-Ca pyroxenes than previously described. We propose that the high-Ca pyroxenes are important component in partial melts of (chondritic source ?) materials. Sunshine et al. performed analyses of reflectance spectra of several S-type asteroids (Thetis and the Merxia and Agnia family members) and found that ratios of high-Ca pyroxene to total pyroxene are clearly inconsistent with any known chondritic material."


Sunshine et al. (2004) -- High-Calcium Pyroxene as an indicator of igneous differentiation in asteroids and meteorites.
                                         Meteoritics & Planetary Science 39, Nr 8, 1343-1357

Takeda et al. (2005) -- New achondrites wuth High-Calcium Pyroxene and its implication for igneous differentiation of asteroids.
                                      Lunar and Planetary Science XXXVI (2005), 1298.pdf

Ogawa et al. (2011) -- The widespread occurrence of High-Calcium Pyroxene in bright-ray craters on the Moon and implications for lunar-crust composition.
                                     Geophysical research Letters, vol. 38, L17202