Hematite Characterisation

It is clear from the results of calculations presented in Table 6.7 that both aggregate size and structure have a substantial effect on ease of fluid flow through the (assumed) ftactal aggregates making up the cake. Aggregates characterised by Case 4A (r = 0.7 Jim and D=2.4 for slowly formed aggregates and r =3.5 J.m and D=2.2 for rapidly, formed aggregates) are considered most similar to the hematite... 

Table A3.3 lists the characteristics of the different hematite sizes and compares selected characteristics. Specific surface area decreases with particle size and the ferric chloride floes have a much higher surface area, most likely due to a smaller precipitate size or amorphous nature. Zeta potential decreases with particle size (see Chapter 4 for method description and charge characterisation as a function of solution chemistry).

Alabaster is a form of gypsum (hydrated calcium sulfate q.v), occurring as a fine-grained, massive mineral which is used as an ornamental stone. It can become coloured, a property dependent on associated minerals - for example, hematite q.v.) will impart a red colour. Oriental alabaster is a term for a stalagmitic variety of calcite q.v) characterised by well-marked banding it should therefore not be confused with alabaster proper. 

In a recent paper, Oliveira et al. (2002) have Raman spectroscopy to characterise various modem purple iron oxide pigments called caput mortuum, supplied by Kremer. They note that variation in colour in these samples is related to particle size the darker hues have the largest particle size. However, thq consider real caput mortuum to be that used in Roman contexts, in this case (Rushton Roman villa, UK) the pigment is identified as hematite mixed with calcium carbonate and kaohnite qq.v.), and again is unrelated to wet chemically precipitated compounds. 

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