74 
FLORIDA GEOLOGICAL SURVEY-I5TH ANNUAL REPORT 
Limonite —This is an iron mineral represented by the formula 
&Fe203, 3 H 2 O. 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 notice¬ 
able 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 dis¬ 
tributed over the State. 
Hematite —Fe203. 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. 
Magnetite —Fe304. A magnetic ore found in some clays as black 
magnetic grains. It is not, however, a common constituent of clays. 
Siderite —FeCCL. 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, 
(CO 2 ), is driven off. The resulting effect is probably similar to that of 
limonite. 
Pyrite —FeS 2 . 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 
(SO 2 ) may, in’ burning, unite with steam to form sulphuric acid 
(H 2 SO 4 ), which in turn may combine with calcium or magnesium com¬ 
pounds 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 
