Associated with the limestones and calc-gneisses are quartzites and tourmaline-biotite schists which probably represent the lowest portions of the shales immediately overlying the limestones.
The commonest hornfelses (the "biotite hornfelses") are dark-brown to black with a somewhat velvety lustre owing to the abundance of small crystals of shining black mica.
In biotite hornfelses a faint striping may indicate the original bedding of the unaltered rock and corresponds to small changes in the nature of the sediment deposited.
Slates, shales and clays yield biotite hornfelses in which the most conspicuous mineral is black mica, in small scales which under the microscope are transparent and have a dark reddish-brown colour and strong dichroism.
Sphene, biotite and iron oxides are the other common constituents, but these rocks show much variety of composition and structure.
They are excessively variable in their mineralogical composition, and very often alternate in thin seams with biotite hornfels and indurated quartzites.
In these biotite hornfelses the minerals, which consist of aluminium silicates, are commonly found; they are usually andalusite and sillimanite, but kyanite appears also in hornfelses, especially in those which have a schistose character.
The aplites, pegmatites, graphic granites and muscovite granites are usually richest in silica, while with increase of biotite and hornblende, augite and enstatite the analyses show the presence of more magnesia, iron and lime.
According to the terminology of the French school it signifies a granite in which both kinds of mica (muscovite and biotite) occur, and corresponds to the German Granit, or to the English "muscovite biotite granite.
Often they enclose dark crystals of biotite and hornblende, arranged zonally.
These are probably due to the adhesion of the biotite crystals to one another as they separated from the rock magma at an early stage in its crystallization.
Exceptions to this sequence are unusual; sometimes the first of the felspars have preceded the hornblende orbiotite which may envelop them in ophitic manner.
The biotiteis brown; the hornblende brown or greenish brown; the augite usually green.
In the hand specimen many of the hornblende and biotite dacites are grey or pale brown and yellow rocks with white felspars, and black crystals of biotite and hornblende.
Though consisting mainly of biotite granite, these later intrusions pass by intermediate stages into diorite, as in the area between Balmoral and the head-waters of the Gairn.
The biotite and hornblende are yellow or brown and richly pleochroic.
Eastwards down the Dee and the Don and northwards across the plain of Buchan towards Rattray Head and Fraserburgh there is a development of biotite gneiss, partly of sedimentary and perhaps partly of igneous origin.
Biotite and primary hornblende suffer comparatively little change; olivine disappears, and garnet, talc and tremolite or anthophyllite take its place.
The biotite and hornblende have much the same characters in both of these groups of rocks, and are often surrounded by black borders produced by corrosion and partial resorption by the magma.
Their commonest minerals are olivine, anorthite, hornblende, augite, biotiteand leucite.
The commonest accessory minerals are tourmaline, topaz, apatite, fluorspar and iron oxides; a little felspar more or less altered may also be present and a brown mica which is biotite or lithionite.
Biotite is, however, readily distinguished by its darker colour, strong pleochroism, and small optic axial angle.
In the dyke-rocks known as mica-traps or mica-lamprophyres biotite is especially abundant.
The magnesia-micas are now referred to the species biotite and phlogopite, which differ in that the former contains a considerable but widely varying amount of iron.
In volcanic rocks, and in nearly all other kinds of igneous rocks with the exception of granite, biotite occurs to the exclusion of the muscovite.
Although biotite (black mica) is much more common and widely distributed than white mica, yet it is of far less economic importance.
In the ejected blocks of crystalline limestone of Monte Somma, Vesuvius, the most perfectly developed crystals of biotite (figs.
There are many tracts of land along the river suitable for cultivation, though for long stretches the banks show only rounded surfaces of biotite gneiss, smooth and glaciated.
Biotite occurs in some of these rocks, and melanite also is present.
Of these the former consists of orthoclase, nepheline, sodalite, diopside and aegirine, biotite and sphene.
Hornblende andbiotite occur also, but are less common.
While biotite is the commonest dark colored constituent of granite, hornblende is more apt to take its place in syenite.
The porphyritic crystals are of lath-shaped, striated feldspar, with which are associated crystals of either biotite or hornblende or both.
Biotite is a common accessory in amphibolite, and garnet and magnetite frequently occur.
The distinguishing features of muscovite and biotite are as follows:— 13.
The crystallization of biotite is hexagonal, and of muscovite monoclinic; but both occur commonly in flat six-sided forms.
Conversely, if the biotite comes out much younger than the feldspar, we can be sure that something has happened to this rock long after it first crystallized.
A dark red, finely laminated rock consisting chiefly of decomposed biotite and feldspar, occasional quartz grains and sericite and much iron oxide.
The principal minerals are, plagioclase, biotite and microcline, with smaller quantities of quartz, iron oxide, pyroxene and garnet.
The labradorite contains inclusions of rutile and biotite and has a well-developed wedge structure and cross fracture due to the pressure and shearing which it has undergone.
A white rock having a peculiar mottled appearance due to the inclusions of decomposing biotite which project from the surrounding mass of calcite.
Under the microscope some of the biotite has a green coloration from decomposition and is surrounded by strong pleochroic halos.
The rock, a biotitegranite gneiss [4] having a strike N.
A spherical shell of darkened biotite is first produced and the inner colouration is only effected as the more feeble ionisation along the track of the ray in course of ages gives rise to sufficient alteration of the mineral.
The heavy iron atom of the biotite is, doubtless, chiefly responsible for this in biotite haloes.
The fine striated appearance of the biotite is due to the cleavage of that mineral, which is cut across in the section.
This experiment has, however, been anticipated by some scores of millions of years, for here is the same effect in a biotite crystal (Pl.
We can evidently attack 240 the biotite with a known number of alpha rays and determine how many are required to produce a certain intensity of darkening, corresponding to that of a halo with a nucleus of measurable dimensions.
We must infer that solutions or gases able to establish the radioactive nuclei moved along these conduits, and we are entitled to ask if all the haloes in this biotite are not, in this sense, of secondary origin.
It is well known that there is in biotite a maximum absorption of a plane-polarised light ray, when the plane of vibration coincides with the plane of cleavage.
It is apparent that the experiment on the biotite has a direct bearing on the validity of the radioactive method of estimating the age of the rocks.
These sedimentary gneisses (or paragneisses, as they are often called) are often rich in biotite and garnet and may contain kyanite and sillimanite, or less frequently calcite.
Muscovite and biotite may both occur in the same rock; in other cases only one of them is present.
A special group of monchiquites rich in deep brown biotite has been called fourchites (after the Fourche Mountains, Arkansas).
The hornblende and biotite are brown or greenish brown, and as a rule their crystals even when small are very perfect and give the micro-sections an easily recognizable character.
Both in the hand specimens and in microscopic slides of lamprophyric rocks biotite and hornblende are usually conspicuous.
The presence or absence of the four dominant minerals, orthoclase, plagioclase, biotite and hornblende, determines the species.
The biotite is always brown or yellow; the hornblende usually green, but sometimes brown or yellowish brown in those diorites which have affinities to lamprophyres.
More rarely biotite and augite exhibit the same relations to the plagioclase.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "biotite" in a variety of sentences. We hope that you will now be able to make sentences using this word.
