Plagioclase replacement

The most common origin of secondary porosity in sandstones studied is due to dissolution of plagioclase or else dissolution of carbonate that replaced plagioclase. Replacement of silicate minerals by calcite is not uncommon, and much of the plagioclase dissolution porosity may actually be replacement-calcite dissolution porosity. We know of no way to resolve this problem. For purposes of calculations, we assumed that plagioclase was dissolved. 

In zone (1), quartz, K-feldspar, epidote, chlorite, prehnite and sphene are predominant alteration minerals. Epidote, prehnite and carbonate replace plagioclase phenocryst. Epidote often occurs as a veinlet with several millimeters wide, together with prehnite. K-feldspar, calcite and quartz tend to occur as a veinlet. Chlorite replaces pyroxene... 

Fig. 2.37. Phase diagram for Ca0-Na20 Si02-(Al203)-H20 system in equilibrium with quartz at 400°C and 400 bars. Plagioclase solid solution can be represented by the albite and anorthite fields, whereas epidote is represented by clinozoisite. Note that the clinozoisite field is adjacent to the anorthite field, suggesting that fluids with high Ca/(H+) might equilibrate with excess anorthite by replacing it with epidote. The location of the albite-anorthite-epidote equilibrium point is a function of epidote and plagioclase composition and depends on the model used for calculation of the thermodynamic properties of aqueous cations (Berndt et al., 1989).

Rb and Sr/ Sr ratios equal to or higher than those of coexisting plagioclase (White et al. 2005). Sr content in the calcite of the Chehelkureh granitoids is relatively low. The ubiquitous nature of accessory calcite documented in this study implies late-magmatic growth or subsolidus replacement of igneous phases. 

Tonalitic phase 6 dykes are quartz- and plagioclase-rich (An43.4g up to Ange-ga near skarn), and have undergone calcic metasomatism with replacement of K-feldspar by plagioclase and destruction of biotite. Dioritic phase 6 dykes are quartz... 

Both basalt samples (DC3-3600 and DH5-2831) are composed primarily of glass (40-50 percent) and plagioclase feldspar (35-40 percent). They contain in addition minor amounts of clinopyroxene (3-5 percent) orthopyroxene (1 percent), and opaques (1-2 percent). The relatively unaltered basalt (DC3-3600) possesses several intersecting fractures filled with clay. There are dark gray alteration zones adjacent to certain fractures where a smectitic clay has replaced... 

Since the primary minerals are electrically neutral a compensation must occur when differently charged elements replace each other. In plagioclase feldspars Si4+ can be replaced by Al3+ but, simultaneously Na+ is replaced by Ca2+. Large, well formed crystals are prized as gems and their colours depend on these ionic substitutions. Corundum (A1203) is colourless, yet just a few Cr3+ions turn it into a ruby (Burns, 1983). For a fuller discussion of igneous rocks the reader is referred to Dercourt and Paquet (1985). 

Like calcic pyroxene, sodium-free plagioclase occurs in two fundamentally distinct settings in CAIs it is a primary igneous phase in type B CAIs, and it occurs as a secondary phase that replaces melilite in Wark-Lovering rim sequences and in CAI interiors. The phase shows very little chemical variation (see Brearley and Jones, 1998). 

Similar effects are also observed in chondrules (e.g., Ikeda and Kimura, 1995, Kimura and Ikeda, 1998) and matrix (Krot et al., 1998a). Ikeda and Kimura (1996) have emphasized the mineralogical similarities between alteration in chondrules and CAls. In the case of chondrules, anorthite-normative mesostasis and plagioclase has been replaced by nepheline and sodalite with minor amounts of andradite, kirchsteinite, wollastonite, and hedenbergite, etc. 

Figure 9 Backscattered electron image of a region of a metasomatized type I chondnile from the CO chondrite Kainsaz (3.1). The chondnile contains clinoenstatite (cen), olivine (ol) metal, and sulfide grains (bright) and fine-grained mesostasis (mes). Primary anorthite has been extensively replaced by nephehne, which occurs as dark lamellae within the plagioclase. In addition the mesostasis has been partially replaced by salitic pyroxene which is also considered to be a metasomatic alteration product (reproduced by permission of Miner-alogical Society of America from Planetary Materials, 1998, 36, chap. 3, 3-1-3-398).

Most trace elements have values of D< C 1, simply because they differ substantially either in ionic radius or ionic charge, or both, from the atoms of the major elements they replace in the crystal lattice. Because of this, they are called incompatible. Exceptions are trace elements such as strontium in plagioclase, ytterbium, lutetium, and scandium in garnet, nickel in olivine, and scandium in clinopyroxene. These latter elements acmally fit into their host crystal structures slightly better than the major elements they replace, and they are therefore called compatible. Thus, most chemical elements of the periodic table are trace elements, and most of them are incompatible only a handful are compatible. 

Detrital feldspars in modern sediments have a compositional range that reflects their composition in the source rocks (Trevena and Nash, 1981). During diagenesis, sodium-rich K-feldspars and more calcium-rich detrital plagioclase grains are more subject to dissolution (and replacement) than potassium-rich K-feldspar or sodium-rich plagioclases (Maynard, 1984 Milliken, 1988, 1992 Milliken et al, 1989). 

The substitution of Na for Ca in plagioclase is expected to stabilize the expanded framework, since Na is slightly larger than Ca (Shannon 1976), and the difference in valence will favor the collapsed configuration in the anorthitic endmember. This effect is reinforced by the concomitant replacement of Ar with the smaller Si", which decreases the mean tetrahedral size. Consequently, the c-reflections that are diagnostic of Pi symmetry grow extremely diffuse and disappear with the substitution of Na for Ca in anorthite, suggesting a morphotropic transition to II symmetry (Adlhart et al. 1980). 

Predicting the Ca/Na ratio of the resultant brine accurately is difficult, and depends on the availability and reactivity of plagioclase and whether or not the brine remains in contact with halite -1- quartz -I- plagioclase. If a halite-saturated (or near-halite-saturated) brine flowed into a clastic section lacking halite as a phase, Na removed from the brine by albitization would cause undersaturation with respect to halite but Na could not be replaced by continued halite dissolution. 

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