Feldspar

“ Inclusions of magnetite, apatite and glass are common.
Phenocrysts of feldspar are scarce. In part they show the polysynthetic twinning lamination of plagioclase; in part the latter is wanting and one of the latter feldspars, which was isolated and examined for specific gravity and optical properties, was found to be sanidine. Phenocrysts of nepheline are more frequent than those of feldspar. The mineral appears partly in the form of short-prismatic crystals, partly in rounded grains. It presents distinct cleavage, parallel to (1010) and to (0001) , and the usually observed optical properties. ”

“ The cause of the crystals of feldspar in these rocks of Ascension, being all placed lengthways, is probably the same with that which elongates and flattens all the brown sphaerulitic globules (which behave like feldspar under the blowpipe) in this same direction.) These allied forces, therefore, have played an important part in the lamination of the mass, but they cannot be considered the primary force; for the several kinds of nodules, both the smallest and largest, are internally zoned with excessively fine shades of colour, parallel to the lamination of the whole ”

“ Hence crystals of feldspar, enveloped in a mass of liquified, but not highly vesicular lava, would tend to rise to the upper parts; and crystals or granules of the other minerals, thus enveloped, would tend to sink. We ought not, however, to expect any perfect degree of separation in such viscid materials. Trachyte, which consists chiefly of feldspar, with some hornblende and oxide of iron, has a specific gravity of about 2.45; whilst basalt, composed chiefly of augite and feldspar, often with much iron and olivine, has a gravity of about 3.0. ”

“ Most feldspars, unfortunately, contain far too much, but rapid screening by X-ray fluorescence or flame photometry methods can weed these out and identify specimens low enough in original strontium to be useful. Otherwise, feldspar is an excellent closed system for rubidium and strontium; it remains closed even at temperatures high enough to melt many other minerals. It is not affected at all by the same degree of heating that will drive argon out of biotite. The rubidium-strontium age of feldspar usually comes close to the time of original crystallization of the rock. ”

“ Ordinary granite, as well as syenite and eurite, usually contains two kinds of feldspar: First, the common, or orthoclase, in which potash is the prevailing alkali, and this generally occurs in large crystals of a white or flesh colour; and secondly, feldspar in smaller crystals, in which soda predominates, usually of a dead white or spotted, and striated like albite, but not the same in composition. [1]
As a general rule, quartz, in a compact or amorphous state, forms a vitreous mass, serving as the base in which feldspar and mica have crystallised ”

“ These lavas have groundmass textures that vary from almost holo-crystalline to glassy. The felted or hyalopilitic texture is the most common, and plagioclase is the principal groundmass constituent. The feldspars are lath-shaped, often with castellated terminations. In the more basic phases anhedrons of augite and of olivine appear, and magnetite grains are usually present. Flowage is often beautifully expressed by the arrangement of the slender laths of feldspar.
Among the phenocrysts feldspar is the most prominent. ”

“ The feldspar crystals have lost their luster and hardness, and even have decayed to clay. Where long- weathered granite forms the country rock, it often may be cut with spade or trowel for several feet from the surface, so rotten is the feldspar, and here the rock is seen to break down to a clayey soil containing grains of quartz and flakes of mica. ”

“ If we find feldspar and biotite in one rock, and if feldspar, tested by the rubidium-strontium method gives the same age as biotite tested by potassium-argon decay, then we can say with confidence that the rock has not been reheated since shortly after it crystallized. 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. Such information is not only valuable to pure science—it can also be useful in locating areas favorable for ore prospecting and in other practical ways. ”

“ Greenstone.—This name has usually been extended to all granular mixtures, whether of hornblende and feldspar, or of augite and feldspar. The term diorite has been applied exclusively to compounds of hornblende and triclinic feldspar. Labrador-rock is a term used for a compound of labradorite or labrador-feldspar and hypersthene; when the hypersthene predominates it is sometimes known under the name of Hypersthene-rock. Gabbro and Diabase are rocks mainly composed of triclinic feldspars and diallage. ”

“ The streak is white, but the color of the mineral is highly variable, although potassium and sodium-bearing feldspar are commonly white or pink and most plagioclase feldspar is gray.
Feldspars are essential parts of the crystalline igneous rocks. Their decomposition products are present in most soils. In Illinois relatively small feldspar crystals can be found associated with quartz and other minerals in granite and gneiss boulders and pebbles in glacial drift. ”

“ Feldspars are minerals, not rocks, but mention of them is made here because, with quartz, they make up such an overwhelming percentage of earth materials. It is estimated that the feldspars make up 50 per cent of all the igneous rocks and 16 per cent of the sedimentary rocks. As the igneous rocks are so much more abundant than the sedimentary rocks, the percentage of feldspars in the earth approaches the former rather than the latter figure. In most rocks feldspar is in too small grains and is too intimately associated with other minerals to be of commercial importance ”

“ The minerals observed in the microliths of the general lava of 1866 are feldspar and titaniferous oxide of iron. The predominant feldspar in large crystals is the labrador, but anorthite, and some oligoclase and sanidine are also found in the same condition. The microlithic feldspar is albite with a considerable proportion of oligoclase, and the whole is cemented with a vitreous matter which represents the residue of the crystallization, the part of the rock to which the latter owed its fluidity, at the moment when the minerals which had made a portion of it were already crystallized. ”

“ With the kaolinisation of the feldspars in the upper strata of the archæan rocks, and the alteration of the feldspar amphibolites (diorite schists) into chloritic schists, easily soluble mineral constituents of the country were replaced by more resistive ones, and the rocks, after alteration or decomposition, became far more permeable for water than they were in their primary state. ”

“ Fel ́spar, or Feldspar, a very important group of mineral silicates of aluminium, with potassium, sodium, or calcium, ranging from orthoclase, the potassium species, with 64.7 per cent of silica, to anorthite, the calcium species, with only 43.3. Albite, the sodium felspar, has 68.8 per cent of silica, and the species between this and anorthite are regarded as mixtures of albite and anorthite molecules. ”

“ Davidson above the Comstock 183 Lode in Nevada, etc., are typical localities for finding diorite.
Olivine-Gabbro
The combination of plagioclase feldspar with augite (or any of the pyroxenes) and olivine makes olivine-gabbro. The feldspar is usually one of those with considerable calcium in it, like labradorite; and as the dark minerals predominate, the rock is dark-colored. It is an intrusive rock, usually in dikes or stocks, where it solidified far below the surface, and was only exposed after the overlying rocks were weathered off. ”

“ Like labradorite the sheen is visible only when the stone is held at a certain angle. Some specimens of the mineral carnallite, which is a chloride of potassium and magnesium, exhibit a similar sheen, and being soluble in water the crystals of hematite can be separated out. They are then seen to be perfect little hexagons of a blood-red color. The sheen of sunstone is best visible when the stone is held in the sunlight or strong artificial light. The variety of Feldspar to which the sunstone most in use at the present time belongs is oligoclase, a soda-lime triclinic Feldspar. ”

“ Mineral synthesis determines the individuals belonging to a family and distinguishes the true isomorphism of the series in question. The artificial reproduction of the feldspar series proved that the two members, albite and anorthite, were isomorphous and could be united in all proportions, some new forms being found which were unknown in Nature. Other mineral types which are suggested by, but are absent in Nature have been formed artificially, thus completing a mineral series, making the limits of isomorphism more clear. ”

“ Having unequal oblique axes; as, anorthic crystals.
AÏnor¶thite (?) , n. [Gr. ? priv. + ? straight (? sc. ? right angle) ; not in a right angle.] A mineral of the feldspar family, commonly occurring in small glassy crystals, also a constituent of some igneous rocks. It is a lime feldspar. See Feldspar.
AÏnor¶thoÏscope (?) , n. [Gr. ? priv. + ? straight + Ïscope.] (Physics) An optical toy for producing amusing figures or pictures by means of two revolving disks, on one of which distorted figures are painted. ”

“ A similar change takes place with the micas, augite, and hornblende; but these species, being usually rich in iron, make clays which are much darker colored than those derived from feldspars. The fact that the feldspars contain little or no iron undoubtedly explains their low specific gravity and light colors, as compared with the other minerals just named. The only common minerals for which the feldspars are liable to be mistaken are quartz and the carbonates. From the latter they are easily distinguished by their superior hardness and non-effervescence with acids ”

“ Of their minerals felspar is usually the most abundant, and is principally labradorite and bytownite, though anorthite occurs in some, while oligoclase and orthoclase have been found in others. The felspar is sometimes very clear and fresh, its crystals being for the most part short and broad, with rather irregular or rounded outlines. Albite twinning is very frequent, but in these rocks it is often accompanied by pericline twinning by which the broad or narrow albite plates are cut transversely by many thin, bright and dark bars as seen in polarized light. ”

“ The rock looks like a diorite in the hand specimen; it weathers to rusty looking blocks of remarkable hardness. Under the microscope the mineral which looks greyish-black in the hand specimen is found to be olivine, colourless where unaltered in thin section; but the usual alteration to serpentine has gone on along irregular cracks, and the separated granules or iron oxide give the black colour to the mass. The felspar, which forms considerably more than half the rock, is a very fresh, though occasionally much cracked, labradorite. ”

“ The felspar (plagioclase) is in the form usually of broad regular crystals, 3 to 4 mm. in size, and giving extinctions of andesine-labradorite (28°) ; they are “clouded” through the presence of fine alteration products associated with numerous fissures. The relation between the hornblende and the plagioclase is not constant. This appears to be partially due to the occurrence of traces of a groundmass.
The diorite forming blocks in the agglomerate of the coast cliffs, west of the Salt Lake Passage, is a remarkable rock showing large blackish hornblende crystals, in the shape of blades 25 mm. ”

“ If the felspar is in crystals, it is easily recognized by its cleavage: but when in grains the blow-pipe must be used, for the edges of the grains can be rounded in the flame, whereas those of quartz are infusible. If the geologist is desirous of distinguishing the three varieties of felspar above enumerated, or hornblende from augite, it will often be necessary to use the reflecting goniometer as a test of the angle of cleavage, and shape of the crystal. ”

“ The narrow veins are composed of a much closer-grained basalt in which a few scattered felspars are visible. Mr. Harker remarks, with regard to a thin slice of one of these rocks [7113] , that "the microscope shows this, too, to be an olivine-basalt. The porphyritic felspars are twinned on the Carlsbad and albite laws. Olivine and pseudomorphs after it are well represented. Magnetite is only sparingly present. The general mass of the rock consists of very small striated prisms of labradorite, granules of augite, and interstitial matter which must be partly glassy." ”

“ The structure is not essentially different from that of granite. The ferromagnesian minerals crystallize comparatively early and have some idiomorphism; the felspar usually follows and only in part shows good crystalline outlines. Orthoclase and quartz, if present, are last to separate out, and fill the spaces between the other minerals; often they interpenetrate to form micropegmatite. In many diorites the plagioclase felspar has crystallized before the hornblende, which consequently has less perfect outlines and forms irregular plates which enclose sharply formed individuals of felspar. ”

“ FELSPAR (Orthose, Fr.; Feldspath, Germ.) is a mineral crystallizing in oblique rhomboidal prisms, susceptible of two cleavages; lustre more pearly than vitreous; spec. grav. 2·39 to 2·58; scratches glass; yields no water when calcined; fusible at the blowpipe into a white enamel; not affected by acids. The liquid left from its analytical treatment with nitrate of baryta, nitric acid, and carbonate of ammonia, affords on evaporation an alkaline residuum which precipitates platina from its chloride, and appears from this, as well as other tests, to be potash. Felspar consists of—silica, 66·75 ”