Primary Clays

Primary clay deposits are formed when a rock formation is chemically attacked by water. The size and shape of the deposit depends on the size and shape of the parent rock [6]. The mineral constituents and impurities of a primary clay deposit are also determined by the composition of the parent rock, the degree of completion of the reaction, the impurities that are removed by solution during or after reaction, and the impurities brought in during or after formation [3]. The residual clay deposits formed by conversion of feldspar almost always contains silica (quartz) and mica as major mineral impurities. The soluble cations such as potassium, sodium, and calcium are dissolved and removed during or after conversion [2]. Most primary deposits contain a high proportion of impurity phases, with typical clay contents ranging from 10 to 40% by volume [21]. However, primary deposits tend to be low in iron-bearing impurities (reported subsequently as Fe2O3), TiO2, and organics. The major mineral impurities can be removed by beneficiation techniques such as air or water flotation to yield usable clay, while removal of other impurities may require more involved treatment processes [1]. Though not mineralogically correct, clays that are white in color and have minimal iron-based impurities are often referred to as “kaolin,” regardless of the crystalline phases present. To avoid confusion, the term “china clay” will be used for iron-free, white burning clays in this article. Most of the commercially important primary clay deposits are considered as china clays. Industrially significant primary clay deposits in the United States are found in North Carolina with minor deposits in Pennsylvania, California, and Missouri [22]. Perhaps the most famous primary china clay deposits

Table 3 Typical compositions (weight percent) of some primary china clays [3,22]

Location

SiO2

Al2O3

Fe2O3

TiO2

CaO

MgO

K2O

Na2O

H2Oa

North Carolina

46.2

38.4

0.6

Trace

0.4

0.4

0.6

0.1

13.3

California

45.3

38.6

0.3

Trace

0.1

0.2

1.0

1.4

13.3

England

48.3

37.6

0.5

0.2

0.2

Trace

1.3

0.3

12.0

‘Called “loss on ignition” in most older texts, now considered chemically combined water

are those found in Cornwall, England, the source of English china clay [22]. Typical compositions of some primary china clays (after removal of accessory minerals) are given in Table 3 [3,22]. In the raw state, high purity clays can be nearly white in color, although commercial deposits vary in color from white to ivory. Likewise, the color upon firing varies from white to ivory depending upon the impurity content. The high­est quality clays are termed “white burning” because of the lack of coloring from impurities after heating.