Petrological description of granite-tempered ware (TF1)
TF1 fabric is characterized in the hand specimen by abundant biotite visible
on the sherd surfaces. On broken edges it can be seen that the ware also
contains abundant rock fragments, some up to 5mm across. On examination
under a binocular microscope it becomes clear that these rock fragments are
not crushed rock, since their edges are not fresh, even though angular edges
In thin-section the rock fragments are shown to be derived from a granite.
Large, composite fragments exist to demonstrate that the various minerals
present were once part of the same rock, although in most cases this rock
has decomposed into its constituents.
The most common minerals are felspars. Within this group fragments with
a perthitic texture are very common. Orthoclase felspar is also present
together with a little plagioclase felspar. Some of the perthitic and
orthoclase felspar fragments are up to 5mm across, while the plagioclase
felspar rarely occurs in fragments larger than 0.5mm across.
Biotite is the next most common mineral and is abundant as sheaves of
crystals up to 1.0mm long and 0.5mm wide.
Quartz is present in smaller quantities, being moderate in abundance. Few
fragments are larger than 0.5mm across. Some of the quartz contains fine
cracks stained by amorphous brown material and the scarcity of crystal
boundaries shows that the quartz originally occurred as large crystals.
A few thin-sections revealed inclusions with a different origin. Rounded
fragments of ferruginous sandstone between 2mm and 4mm across were present
in two sections. The sandstone contained subangular quartz fragments up to
0.3mm across in an opaque to dark brown matrix. One thin-section contained a
single rounded fragment of lava, 1mm across (B247) while another section
contained rounded white clay pellets up to 0.5mm across formed, perhaps, by
the total weathering of orthoclase felspar.
These inclusions are set in a groundmass which consists of anisotropic clay
minerals and angular fragments of biotite and quartz. Felspars may also be
present as small fragments but are difficult to identify. The
characteristics of this fabric suggest that the parent clay formed through
the weathering in situ of a coarse-grained granite, although the sparse
non-granitic rock fragments suggest that a small amount of material may
have entered the clay from other sources, such as overlying sands or even
through accidental contamination by man.
Photographs of thin sections are available
9th February 1990