|
PHYLLITE (Gr. 40)XAov, a leaf, probably because they yield leaf-like plates, owing to their fissility) , in See also: petrology, a See also: group of rocks which are in practically all cases metamorphosed argillaceous sediments, consisting essentially of See also: quartz, See also: chlorite and See also: muscovite, and possessing a well-marked parallel arrangement or schistosity
.
They See also: form an intermediate See also: term in the series of altered See also: clays or shaly deposits between See also: clay-slates and See also: mica-See also: schists
.
The clay-slates have a very similar See also: mineral constitution to the phyllites, but are finer grained and are distinguished also by a very much better cleavage
.
In the phyllites also See also: white mica (muscovite or sericite) is more abundant as a
See also: rule than in slate, and its crystalline plates are larger; the abundance of mica gives these rocks a glossy sheen on the smooth planes of fissility
.
Many of the best Welsh slates are See also: rich in small scales of white mica, which polarize brightly between crossed nicols
.
The Cornish slates are still more micaceous and rather coarser grained, so that they might be called mica-slates or even phyllites
.
A microscopical section of a typical phyllite shows See also: green chlorite and colourless mica both in irregular plates disposed in parallel See also: order, with a greater or smaller amount of quartz which forms small lenticular grains elongated parallel to the foliation
.
Grains of iron See also: oxide (See also: magnetite and See also: haematite) and black graphitic dust are very commonly See also: present
.
Feldspar is absent or scarce, but some phyllites are characterized by the development of small rounded grains of See also: albite, often in considerable numbers
.
The minute needles of See also: rutile, so often seen in clay-slates, are not often met with in phyllites, but this mineral forms small prisms which may be intergroivn with black magnetite; at other times it occurs as networks of sagenite
.
Other phyllites contain See also: carbonates (usually See also: calcite but sometimes See also: dolomite) in flat or spindle-shaped crystals, which often give evidence of crushing
.
Very tiny blue needles of See also: tourmaline are by no means rare in phyllites, though readily overlooked
.
Garnet occurs some-times, a See also: good example of garnetiferous phyllite being furnished by the whetstones of the See also: Ardennes, in which there are many small isotropic crystals of magnesian garnet
.
See also: Hornblende, often in branching feathery crystals, is a less frequent See also: accessory
.
In some phyllites a mineral of the chloritoid group makes its appearance; this may be ottrelite, sismondine or other varieties of chloritoid, and occurs in large sub-hexagonal plates showing complex twinning, and lying across the foliation planes of the See also: rock, so that they seem to have See also: developed after the movements and pressures which gave rise to the foliation had ceased
.
The structural variations presented by the phyllites are comparatively few
.
The most finely crystalline specimens have generally the most perfect parallel arrangement of their constituents
.
The foliation is generally flat or linear, but in some rocks is undulose or crumpled
.
From the imperfection of their cleavage phyllites are rarely suitable for roofing materials; their softness renders them valueless as road stones, but they are not uncommonly employed as inferior See also: building materials
.
They are exceedingly See also: common in all parts of the See also: world where metamorphic rocks occur; as in the Scottish See also: Highlands, See also: Cornwall, Anglesey, See also: north-west See also: Ireland, the Ardennes, the Harz Mountains, iSaxony, the See also: Alps, See also: Norway, the Appalachians, the See also: Great Lakes See also: district in See also: America, &c
.
(J
.
S
.
|
|
|
[back] PHYLE |
[next] PHYLLOCACTUS (fig. 3) |
There are no comments yet for this article.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.
Links to articles and home page are encouraged.