Charakterystyka mineralogiczno-petrograficzna kaolinu z Bolesławie (Dolny Śląsk)

Leszek Stoch, Wanda Sikora




Mineral composition of kaolin found to occur at Bolesławice, near Świdnica, has been examined. This is residual kaolin formed as a result of weathering of biotite muscovite granite of Strzegom type. Kaolinite, quadz and mica are the main components of the kaolin here. There occur also small amounts of siderite, as well as grains of iron oxides and hydroxides. The kaolin under examination is coarse-grained, showing three different grain classes: > 60 mm, 60–2 mm and < 2 mm, characterized by various mineral composition. The fraction above 60 mm consists mainly of quartz, the grains of which reach even 2 mm. H reveals a wavy extinction of light. Moreover, this fraction contains kaolinite and flakes of muscovite kaolinitized to a different degree. There are observed also single biotite flakes strongly kaolinitized as well. Feldspars occur in some percent. In the fraction 60–2 mm occur: kaolinite, dioctahedral mica of muscovite type, and quartz in small amounts. The kaolinite makes about 75% of this fraction. This is Tc kaolinite. The mica content ranges from 10 to 50%. These are fine-grained secondary micas produced as a result of feldspar weathering processes. The material finer than 2 mm consists mainly of kaolinite, accompanied by dioctahedral micas. The kaolinite content reaches up to 90%. As compared with the kaolinite of the fraction 60–2 mm, its structure is disordered here. Micas that occur in this fraction can be determined, according to DTA and X-ray examinations, as illite. In addition to this, also mixed structures have been ascertained, showing randomly interstratified mica and montmorillonite layers.

The kaolin and its fractions under examination have been determined as to the content of free Fe2O3 (using Mehra'y, Jackson's method, 1960), and combined Fe2O3. The free Fe2O3 is represented by iron oxides and hydroxides found to occur in the form of single grains, and as those adsorbed on the surface of kaolinite flakes. The combined iron occurs predominantly in the structure of micas which contain about 4% Fe2O3. The mineral kaolinite contains approximately 1% of combined Fe2O3. Iron is concentrated below 60 mm, thus in fractions rich in clay minerals.
The content of combined iron remains almost the same (about 1,3%), mainly due to very small differences in the content of both micas and kaolinite in these fractions. The free Fe2O3 markedly concentrates in the fraction smaller than 2 mm.
It occurs mainly in the form of fine grains of oxides and hydroxides. A decrease in the content of iron in the kaolin examined can be realized by removing the highest possible amount of micas by dressing processes. Titanium content in kaoline is inconsiderable. It occurs in the structures of micas, and in the form of anatase and rutile grains.


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