74 rLORIDA GEOLOGICAL SURVEY-15TH ANNUAL REPORT

 Limonite-This is an iron mineral represented by the formula 2Fe203, 3H20. Its occurrence in clays is widespread and in a variety of forms. When present in a finely divided state it gives to the clay a yellowish or brownish color. It is always a secondary product resulting from the alteration of other minerals. It may occur in clays disseminated in fine grains, as a coating or thin film covering the clay particles, as concretions, or limonitic crusts and layers. It exerts a strong coloring and fluxing action in the burning process, if abundant and uniformly distributed. The presence of a very small per cent may produce a noticeable coloring effect and is therefore an injurious ingredient of a clay desired for whiteware. If limonite occurs in small lumps and these are not finely crushed, they appear in the burned product as unsightly black splotches. Limonite concretions are very common in many Florida clays and limonite or other hydrous iron oxides in other forms is widely distributed over the State.
 Heniatite-Fe2O3. This is another iron mineral which may be found in clays, but as it alters to limonite easily on being exposed to moisture and air, it is not so common. Its effect in clays is similar to that of limonite.
 MagnetitC-FeO4. A magnetic ore found in some clays as black magnetic grains. It is not, however, a common constituent of clays.
 Siderite-FeCO3. This is the iron carbonate which occurs in some clays and shales as concretionary masses or as disseminations. It changes to limonite if exposed to the weathering agencies. If in a finely divided state and evenly distributed through the clay, it is said to give a blue or slate-gray color to the raw clay. In burning the carbon dioxide, (C02), is driven off. The resulting effect is probably similar to that of limonite.
 Pyrite-FeS2. This mineral, the iron sulphide, is common in some clays and was noted in some of the Florida clays. It is pale or brassy yellow in color, has a metallic lustre, and occurs as small cubical grains or as nodular lumps. Pyrite is an injurious ingredient as it not only has an effect similar to that of the other iron minerals but the sulphur dioxide (S02) may, in burning, unite with steam to form sulphuric acid (H2SO4), which in turn may combine with calcium or magnesium compounds to form soluble salts. These may cause a white efflorescence or coating on the ware. If the pyrite occurs in large masses it may be screened or picked out by hand. Pyrite is sometimes an impurity in coal