. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. 234 ANNALS OF THE SOUTH AFRICAN MUSEUM present as free quartz. Even the clay mineral that accounts for the balance of the Si02 and A1203 is present in very small amounts. The low fluorine content defines the mineral as dahllite. Like the francolite of Langeberg it is isotropic to very slightly anisotropic under crossed nicols. Shell debris, bryozoan remains and foraminifers are set in a collophane (dahllite)/micrite groundmass (Fig. 19). The concentration of iron oxide is considerably lower than that
. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. 234 ANNALS OF THE SOUTH AFRICAN MUSEUM present as free quartz. Even the clay mineral that accounts for the balance of the Si02 and A1203 is present in very small amounts. The low fluorine content defines the mineral as dahllite. Like the francolite of Langeberg it is isotropic to very slightly anisotropic under crossed nicols. Shell debris, bryozoan remains and foraminifers are set in a collophane (dahllite)/micrite groundmass (Fig. 19). The concentration of iron oxide is considerably lower than that of the Langeberg basal bed. Planktonic foraminifers are present. The Hoedjiespunt phosphorite is partly phosphatized micrite and partly phosphatized microcoquina. Figure 20 shows small cleavage flakes of phosphorite set in a collophane matrix, while there has also been collophane precipitation along fractures within the initial collophane matrix. An oolitic structure is shown in Figure 21. This type of structure is common in the Hoedjiespunt phosphatized wackestone. 2. Phosphate rock of the Varswater Formation Precipitation of isotropic collophane within the pore-spaces of the quartz sands has been influenced by local concentrations of organic matter. This has resulted in the development of thin lenses and concretions of phosphatic sand- stone. Figure 22 shows a tortoise bone that has acted as a nucleus for phosphate deposition which has followed the contours of the bone. Although the bone is completely phosphatized, the maximum phosphate deposition outside the bone. Fig. 19 Hoedjiespunt phosphorite showing foraminifers and shell debris. L. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original South African Museum. Cape Town : The Museum
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectnaturalhistory, booky
