Petrological description of TF2
TF2 fabric is characterized in the hand specimen by sparse biotite visible
on the sherd surfaces. It is a much finer fabric than TF1 but under the
binocular microscope it can be seen to contain moderate grains of sand.
This sand consists of well-sorted, subangular and rounded grains, on average
c.0.5mm across. Larger reddish brown clay pellets are also present.
A single thin-section, grouped with TF2, contains a much finer sand, with
grains averaging c.0.2-0.3mm across, although sparse larger inclusions of
the same types as those in the remaining sherds were present.
The inclusions consist, in roughly equal quantities, of felspars,
altered felspars, quartz, and smaller quantities of lava fragments and
biotite. Rare fragments of composite rock were also present and these
tended to be larger, up to 2.0mm across.
The felspars were not normally identifiable, due to their alteration, but
fragments of plagioclase and one fragment with graphic intergrowth were
present. The rock fragments include a fragment of coarse-grained rock
composed of quartz and muscovite, and a fine-grained rock composed of
quartz, biotite and muscovite. The quartz was polycrytalline with a mosaic
texture (crystal size c.0.2mm) and this suggests that the rock might be of
high-grade metamorphic origin.
The clay pellets usually have a zoned structure, in which the zones vary in
colour and texture. The outer zone is often darker and denser than the
inner, like a crust.
The groundmass consists of anisotropic clay minerals and small
fragments of muscovite up to 0.2mm long, quartz, biotite and clay
TF2 is similar to TF1 in that it too contains moderate amounts of biotite,
black mica, and has inclusions which originate in a granite. In constrast,
however, the inclusions in TF2 are definitely a detrital sand which contains
material from several sources: acid igneous, volcanic, and metamorphic
rocks. Furthermore, the abundance of muscovite in the clay matrix, and the
absence of larger laths amongst the inclusions, suggests that the parent
clay of TF2 contained muscovite but that the sand used to temper it did
Similar suites of rock fragments and minerals were seen in sand
samples from Sully-sur-Loire (45-Loiret) and La Charité-sur-Loire
(58-Nièvre) and brick samples from Orlèans (45-Loiret).
- Samples: B103, B134, B135, B137, B143, B211, B221.
- Three of the samples (B134, B135, B211) were similar in petrology to
Rigby and Freestone's `Standard fabric' (Rigby and Freestone 1986).
- A second group (B103, B137, B143, B221) don't have the fine amphibole
and pyroxene in the matrix and fall outside the scope the `Standard
fabric' as defined in print, but are clearly related.
11th February 1990
Summary of thin-sectionsShow more details of these samples