Siderite, detrital

Coal contains detrital minerals that were deposited along with the plant material, and authigenic minerals that were formed during coalification. The abundance of mineral matter in coal varies considerably with its source, and is reported to range between 9.05 and 32.26 wt% (Valkovic 1983). Minerals found in coal include (Table 2) aluminosilicates, mainly clay minerals carbonates, such as, calcite, ankerite, siderite, and dolomite sulphides, mainly pyrite (FeS2) chlorides and silicates, principally quartz. Trace elements in coal are commonly associated with one or more of these minerals (see Table 2). 

Figure 7 Replacement driven by force-of-crystal-Uzation is characterized by authigenic phases that develop euhedral faces that are not plausibly constmed as crystal growth within pore spaces (a) sphalerite replaces albitized detrital feldspar and adjacent portions of clay-rich matrix, Frio Formation, Oligocene, South Texas and (b) siderite crystal (s) attacks a detrital K-feldspar (K) in sandstone, Breathitt Formation, Pennsylvanian, eastern Kentucky.

Other common, though volumetrically minor, feldspar-replacing minerals include titanite, ana-tase, sphalerite, barite, ankerite, siderite, and fluorite. With the exception of replacement driven by force of crystallization, feldspar replacements have intracrystalline distributions that are strongly localized at sites of surface-controlled dissolution. Interestingly, replacement of detrital feldspars by authigenic clays is rarely observed in late diagenesis. 

The sample suite from each core can be divided into four factors on the basis of similarities in chemical compositions as defined by Q-mode factor modeling. Factor-1 samples are rich in trace metals due to adsorption onto clay, altered tuffaceous material, and (or) organic matter and precipitation as sulfides. The relatively high concentration of boron is probably related to its inclusion in authigenic feldspars. Factor-2 samples are rich in elements commonly associated with minerals of detrital or volcanic origin. These samples contain relatively high concentrations of analcime, dawsonite, and (or) potassium feldspar, all of which are associated with alteration of tuffaceous material. Siderite and ferroan or ankeritic... 

As a result of the anoxic, low sulphate concentrations in the Me zone, carbonates expected to form include siderite and ferroan dolomite/ankerite (Gautier Claypool, 1984). The precipitation of these carbonates occurs in sediments rich in reactive detrital iron (Coleman, 1985), as follows ... 

Siderite is most abundant (up to 14%) in finegrained sandstones rich in mica and clay pseudomatrix. It occurs as small subhedral or flattened rhombs (<3-15 pm) (Fig. lOF) that replaced the detrital clays, and expanded as well as replaced the mica flakes (Fig. 13A). In coarser-grained sand-... 

Diagenetic Ti-oxides are more abundant than iron oxides in sandstones (up to 2.6%). They occur as local aggregates of bipyramidal anatase crystals, apparently formed by the complete alteration of detrital Fe-Ti oxides, which are commonly associated with siderite and ankerite (Figs 13B and 14E) (see Morad, 1988) or are scattered in chloritized pseudomatrix and biotite. 

Rg. 2. Thin-section scale localization of authigenic si derite. Back-scattered electron images. Scale bars 10 Jim. (A) Zoned siderite (bright mineral) localized around and within a detrital chlorite (KY218B). (B) Siderite (s) localized on partially dissolved and kaolinized K-feldspar (f). 

Early calcites (both generalized and concretionary) preserve IGVs typically in the range of 30-40%. In terms of cement stratigraphy, early calcite precedes quartz cement but postdates siderite and some of the kaolinite (Fig. 3A). Early calcite is typically poikilotopic. Extensive replacement of detrital feldspars by early calcite is observed (Fig. 3B), but the calcite is pervasive in its distribution and not strongly localized on the feldspars. 

Sources for Ca, Mg, Fe and Mn are most likely ones affiliated with reacting detrital components on an intrabasinal, if not an intraformational, scale. Early in diagenesis, unstable Fe and Mn oxyhydrox-ides (e.g. Bamaby Rimstidt, 1989, and references therein) are a plausible source of materials for siderite. Mg adsorbed on to clays is a possible source for easily mobilized Mg for the early siderite and calcite. The shift from siderite to calcite precipitation may reflect the waning of these easily mobi-... 

Siderite occurs as scattered, small (10-15 pm), yellowish euhedral crystals attached to detrital grains or enclosed by later carbonate cements (Fig. 4A). The siderite has a relatively Mg-rich composition, up to 40 mol% relative to Fe (Boles, 1987), probably reflecting the Mg-rich composition of marine water (Mozley, 1989). Extensive cement zones or concretions of siderite have not been found, indicating that sideritization is not as common in this deep marine environment as in shallow marine and non-marine environments (e.g. see Mozley, 1989). 

Fig. 4. Photomicrographs of carbonate cements in sandstones of the San Joaquin basin. (A) Siderite rhombs (arrows) in pore space (dark areas). Well NCL 88-29, 2746.5 m (9010.8 ft). White bar is 0.25 mm. (B) Dolomite pore-filling sandstone from the central basin. Note high cement volume and undeformed detrital biotite (dark grains). Detrital grains are chiefly quartz and feldspar. Well NCL 88-29, 2717 m (8913 ft). White bar is 0.5 mm. (C) Calcite pore-filling cement from central basin. Note relatively high cement volume and partially crushed biotite. Well NCL 487-29,...

Siderite is present in wells 30/6-7, 30/6-6 and 30/9-B26 in small amounts (1-5%). It occurs as intergranular rhombs or spherules disseminated throughout the sandstone, and often closely associated with detrital clays or micas (Fig. 6A). Siderite is an early diagenetic phase. It is never found postdating any of the other diagenetic phases, and is systematically engulfed by calcite in pores where both carbonate cements occur (Fig. 6A). 

Siderite commonly occurs in association with (replacement of ) detrital biotites, as shown in the lower right comer of the view (arrow). Sample 30/9-B26 4286.8 crossed polars x 100. (B) Rhombs of diagenetic ankerite precipitated between the layers of an exfoliated detrital muscovite (A) and on detrital quartz (white arrow). Note that the ankerite rhombs are embedded in a late pore-filling poikilotopic calcite (C). Sample 30/6-7 2794.1 crossed polars x 100. 

Siderite exhibits the lowest Sr/ Sr ratio (0.71151) of all carbonates from the structural blocks (Table 3), still significantly higher than that of Jurassic seawater (Fig. 8B). Because siderite is often precipitated in association with detrital micas and clays, it may have incorporated some of the highly radiogenic Sr from the old detrital phyllosil-... 

Sporadic occurrences of minor amounts of early siderite formed at low temperature (20-40 °C), frequently in association with detrital biotites. The fluid from which siderite precipitated had a consistent oxygen, carbon and strontium isotopic composition over great distances, which is best explained as representing homogeneous mixing of Jurassic seawater and meteoric water at field scale. Carbon was predominantly supplied by organic sources. 

In the Upper Namur Sandstone the poikilotopic calcite cement is absent in moderately to poorly sorted quartz arenites and feldspathic quartz aren-ites that contain abundant primary porosity and some secondary porosity. Only minor patches of siderite micrite and microspar are observed in these elastics, where euhedral quartz overgrowths are well developed, albeit not volumetrically significant (< 5-10%) (Fig. 14D). The detrital grains are dominated by tangential and long contacts, with rare sutured contacts. 

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