Structure of type A CAI-like melts: A view from multi-nuclear NMR study of melilite (Ca2Al2SiO7-Ca2MgSi2O7) glasses

Sun Young Park, Changkun Park, Hyun Na Kim, Seoyoung Lee, Sung Keun Lee

Chemical Geology
In Press, Journal Pre-proof, Available online 16 September 2020

LINK

“Highlights

• The coordination number of Al in melilite glasses and melts were observed by NMR (predominant 4 coordinated Al and small fraction of five coordinate Al).
• The network connectivity and structural disorder in type A CAI-like melts were revealed using NMR.
• Despite the strong preference of Si over Al for NBOs, Ca-O-Al is observed in type A CAI-like melts.
• The structural changes should be considered to interpret the melt properties including viscosity, diffusivity, and oxygen isotopic composition.”

“Exploring the polymerization and structural disorder of Ca-Al-rich inclusion (CAI)-like melts is a key question in cosmochemistry due to strong implications for macroscopic properties of melts (i.e., viscosity and diffusivity). Here, we report experimental results showing the effect of composition on the structure of melilite glasses and melts [åkermanite (Åk, Ca2MgSi2O7) – gehlenite (Gh, Ca2Al2SiO7)] in type A CAIs with varying composition using high-resolution solid-state nuclear magnetic resonance (NMR). The 27Al magic angle spinning (MAS) and 3Q (triple quantum) MAS NMR spectra of melilite glasses show predominant [4]Al. A non-negligible fraction of [5]Al is observed in Åk50Gh50 and Åk72Gh28 glasses and it slightly increases with increasing åkermanite content. The 17O 3QMAS NMR spectra of melilite glasses show that bridging oxygens (BOs, Si–O–Si, Al–O–Al, and Si–O–Al) and non-bridging oxygens (NBOs, Ca–O-Si, Ca-O-Al, and mixed {Ca, Mg}–NBO) are partially resolved. Despite the strong preference of Si over Al for NBOs, for the first time, Ca-O-Al is observed in natural melts (i.e., gehlenite and Åk25Gh75 glasses and melts). The results show that Al-NBO (~150 ppm in MAS dimension) can be distinguished from Si-NBO (~110 ppm) in melilite glasses and melts. The fraction of NBO increases with increasing åkermanite content. The experimental results suggest that composition-induced structural changes should be considered to interpret the melt viscosity, diffusivity, and oxygen isotopic composition of CAI-like melts in the early Solar System.”