The discovery of a (Mg,Al)-Spinel bearing rock type exposed on the lunar surface  has sparked extensive analyses to constrain the occurrence and petrologic characteristics of Mg-spinel on the Moon [e.g., 2-12]. Mg-spinel has been found in lunar samples, but always in association with relatively abundant olivine [e.g. 3]. It is important to note that exposures of the new Mg-spinel rock type identified to date  contain no detectible mafic minerals (olivine, pyroxene). This is interpreted to mean the new rock type is feldspathic, a ‘pink spinel anorthosite’ , with the spinel/plagioclase ratio currently under investigation.
The spectroscopic detection of spinel-rich regions across the Moon is based on two strong tetrahedral absorptions due to ferrous iron that occur near 2 and 3 µm due to crystal field splitting of iron d-orbitals in a tetrahedral environment. Many optical and instrumental effects can mimic a 2 µm absorption, but the presence of these dual bands and the absence of ferrous absorption near 1 µm is important for Mg-spinel identification. Using these criteria, the number of confirmed Mg-spinel locations continues to grow. All exposures are small (≤1 km) and are characterized by Mg-spinel in a feldspathic environment.
Three of the key detections (Moscoviense, Theophilus, Thomson/Ingenii) are associated with very thin crust and located on the rings of major basins [e.g., 13]. The multiple exposures at Thomson [9, 12] are particularly noteworthy. Thompson is a mare-filled 117 km diameter crater that occurs within the Ingenii basin located on the NW rim of SPA. The distinct spinel exposures occur on both the northern and southern walls of Thomson. A fresh crater on the NW rim of Thomson has excavated pure anorthosite as well as minor low-Ca pyroxene. Small craters in the Thomson ejecta expose local areas with more abundant low-Ca pyroxene. The geologic context - large feldspathic crater (Thomson), within a mascon basin (Ineinii), within SPA - suggests a deep-seated origin for the Thomson spinel. The size of Thomson and Ingenii also suggest they would have excavated through any impact melt produced by SPA giving them access to the lower crust and/or mantle exposed by SPA at this location. The feldspathic context and nearby association of the Thomson spinel exposure with pure anorthosite implies a crustal origin. Since the mare fill of Ingenii and Thomson clearly occurred well after these major impact events, the origin of spinel pre-dates the mare basalt, and may pre-date SPA itself.
1] Pieters C et al., JGR, 2011. 2] Dhingra D et al., GRL, 2011. 3] Gross J & Treiman A 2011, JGR 4] Jackson C et al., 2012, LPSC43, 2335. 5] Prissel T et al, 2012, LPSC43, 2743; --2013 LPSC44 #3066. 6] Kaur et al., 2012, LPSC43, 1434. 7] Bhattacharya et al., 2012, Current Science. 8] Taylor & Pieters 2013 LPSC44 #2785. 9] Pieters C. et al., 2013, LPSC44 #2545, 10] Sun et al., 2013 LPSC44 #1393 11] Srivastava & Gupta 2013, LPSC44 #1509. 12] Kaur et al., 2013, LPSC44 #1547, 13] Wieczorek M et al., 2013 Science.