Pyroxenes pigeonite

Noncumulate eucrites originally formed as quickly cooled surface lava flows (unequilibrated noncumulate eucrites), but most were subsequently metamorphosed (metamorphosed noncumulate eucrites). Unequilibrated noncumulate eucrites, also referred to as the unmetamorphosed or least-metamorphosed noncumulate eucrites or Pasamonte-type eucrites, are surface lava flows (Figure 22(c)) that cooled quickly (Walker et al., 1978). As a result of their fast cooling, their pyroxenes (pigeonite of Mg —70-20) are zoned, and exsolution lamellae are only visible by TEM. These rocks have experienced only minor metamorphism (e.g., Takeda and Graham, 1991). 

Fig. 13.45 The concentrations of FeO (total iron) of Ca-rich pyroxenes (augite) increase up-section in the Dufek Massif from 12.2% to 16.2% and in the Forrestal Range from 19.9% to 25.3%. The concentration of MgO (not shown) in the same stratigraphic interval decreases up-section from 15.4% to 12.8% (Dufek Massif) and from 11.5% to 8.4% (Forrestal Range). The Ca-poor pyroxenes (pigeonite inverted to Fe-rich hypersthene) have higher concentrations of FeO and MgO than the Ca-rich pyroxenes, but they vary similarly up-section in the Dufek Massif from 21.7% to 25.4% (FeO) and from 20.0% to 16.4% (MgO). The data for the Forrestal Range are incomplete. The stratigraphic variations of FeO and MgO in the pyroxenes of the Dufek intrusion are unambiguous evidence of the evolution of the Dufek magma by fractional crystallization (Data from Himmelberg and Ford 1976)...

Figure 5,25 Phase stability relations for natural pyroxenes in the quadrilateral. (A) Magmatic pyroxenes (not reequilibrated at low 7). (B) Three-phase region for pyroxenes of stratihed complexes and magmatic series. (C) Pyroxenes reequilibrated in subsolidus conditions. = Pbca orthopyroxene V = augite C2lc) A = pigeonite (P2jlc) 0 = olivine. From Huebner (1982). Reprinted with permission of The Mineralogical Society of America.

Heating experiments on P2ilm amphiboles (cummingtonite) show transition to the C2lm polymorph at low T (50 to 100 °C Prewitt et ah, 1970 Sueno et ah, 1972) in analogy to the behavior of pigeonitic pyroxenes (P2jlc), which show transition to the form C2/c (cf. section 5.4.3). Phase stability limits are markedly conditioned by the chemistry of the system and, particularly, by the partial pres-... 

Ross M. and Huebner J. S. (1975). A pyroxene geothermometer based on composition temperature relationship of naturally occurring orthopyroxene, pigeonite and augite. In International Conference on Geothermometry and Geobarometry, The Pennsylvania State University. 

Troctolite is thus believed to be the major rock-type forming the central peaks of Copernicus. Reflectance spectra of rays emanating from Copernicus contain more calcic-rich pyroxenes indicative of pigeonite-augite assemblages (Pieters etal., 1985). 

Metamorphosed (equilibrated) noncumulate eucrites are also collectively referred to as the ordinary eucrites. They include the Juvinas type (main group) and the Stannem and Nuevo Laredo types. They are unbrecciated or monomict-brecciated, metamorphosed basalts (Figure 22(d)) and contain homogeneous low-calcium pigeonite (Mg —42-30) with fine exsolution lamellae of high-calcium pyroxene. The high abundance... 

Figure 31 Lunar meteorite North West Africa (NWA) 773 consists of two distinct lithologies cumulate olivine norite and regolith breccia. The cumulate portion is composed of olivine, pigeonite, augite, feldspar, and opaques (troilite, chromite, Fe-metal). The breccia portion contains fragments of cumulate portion as well as silica glass, hedenbergitic pyroxene, volcanic rocks, and unusual lithic clasts with fayalite + Ba-rich K-feldspar + silica + plagioclase (photograph courtesy of M. Killgore).

Ureilites are ultramafic rocks composed dominantly of olivine and pyroxene, with <10% interstitial material rich in elemental carbon. Modal abundances of the major silicates are quite variable, with pyroxene varying from 0% to 90%. Roughly one-half of ureilites contain pigeonite as the sole pyroxene, but in the others, pyroxene assemblages include various combinations of pigeonite, orthopyroxene, and/or augite. 

Ureilites contain a variety of trace accessory phases. Metallic spherules composed of cohenite, metal, and sulfide are included in olivine and pigeonite of a few ureilites, and metal, phosphide, and sulfide are present as interstitial phases in most. The interstitial regions also commonly contain fine-grained silicates, including low-calcium pyroxene, augite and Si-Al-alkali glass. 

Northwest Africa Oil (NWA 011) has briefly been described by Afanasiev et al. (2000) and more extensively by Yamaguchi et al. (2002), the source of most information presented here. NWA Oil is composed of relatively coarse, anhedral pigeonite and augite, and fine-grained, mostly interstitial plagioclase with a recrystallized texture, interpreted to indicate that it is a recrystallized breccia. Minor phases are silica, chromite, ilmenite, calcium phosphate, ferroan olivine, troilite, and baddeleyite. Pyroxenes make up... 

In the few FA suite rocks that (as sampled) contain more than 1-2% mahc silicates, the mahcs are typically a mix of olivine and coarsely exsolved low-Ca pyroxene (igneous pigeonite) (Taylor et al., 1991 Papike et al., 1991, 1998). Another apparent geochemical discontinuity between the ferroan anorthositic and Mg-suites is manifested by Ni-Co systematics in olivine (Shearer et al., 2001b). Early models designed to... 

Among these pyroxenes, only the Ca-rich pigeonites were known to exist... 

The crystals are generally euhedral and unaltered. The dolerite also contains two and, in some cases, even three pyroxenes augite, pigeonite, and hypersthene. In addition, all specimens contain opaque minerals (magnetite and ilmenite). Quartz and alkali feldspar are prominent in the rocks of the diorite pegmatite and granophyre zones. 

The pyroxenes of the Peneplain sill consist entirely of pigeonite and augite (i.e., hypersthene is absent). Plagioclase is more abundant and more sodic than the plagioclase of the Basement sill. The micrographic intergrowth of quartz and K-feldspar is also more abundant (10-14%) in the Peneplain sill. 

Figure 4. Dpy 4ifl vs. element. Typical high-Ca and low-Ca pyroxene D patterns are shown. Diopside pattern is from [45] and the pigeonite patton is from [96]. Both experiments woe performed at one bar. These patterns are tqtpropriaie for reducing conditions where most Eu exists as Eu2+. At high fo2 where Eu2+ is destabilized, the patterns should be smooth.

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