Calcite rhomb

It is instructive to pursue the interpretation of the calcite rhomb experiment beyond the simple Huyghenian construction to learn something about the polarization of the transmitted light. The electromagnetic theory of light requires that the electric vector shall be contained in the plane of the wave front. The ordinary disturbances vibrate perpendicular to a principal section. Also the extraordinary disturbance must vibrate in the principal section plane. 

Fig. 77. Large (32-molecule) unit rhombohedron of calcite based on cleavage rhomb. The true unit cell is the small (2-molecule) steep rhombohedron shown inside.

Fig. 5. Photomicrograph of dedolomite (poikilotopic spar calcite) from the Pheil No. 3 well, Atascosa County, 2253 m, white light. Width of field is 940pm. Note that many of the dolomite rhombs are badly corroded where they have been unreplaced by the spar.

Fig. 3. Pre-quartz, early calcite. Back-scattered electron images. Scale bars 100 pm. (A) Early calcite (c) that postdates early siderite (bright rhombs) KY64A.

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. 

In samples where calcite and ankerite coexist, ankerite rhombs are systematically embedded within the calcite cement (Fig. 6B). Because of the scarcity of ankerite, textural relationships with other diagenetic cements are rarely observable. However, it is notable that in the ankerite-rich sample (30/9-1 2781.0) feldspar grains are essentially unaffected by dissolution, although they are extensively leached in ankerite-free samples of the same well. In addition, in the single instance of partial leaching of a feldspar grain found in sample 30/9-1 2781.0, ankerite was not observed to fill the intragrain porosity. These observations indicate that ankerite pre-dates feldspar dissolution and calcite cementation. 

Replacement dolomite forms euhedral to subhedral rhombs up to 85 pm in diameter in the clay matrix of sandstones and in early ferroan calcite cement. 

Calcium carbonate, upon which we shall concentrate here, is particularly convenient since it is available as well-defined highly pure cleavable rhombs of calcite, and the dislocation content is known in considerable detail 197, 201, 202). 

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