QUARTZ IN THIN SECTION

QUARTZ - KWARC

The quartz is the most common of all minerals in the world. It is easy to distinguish quartz from other minerals in thin sections especially from feldspars because it is gennerally unaltered and lack of twinnings or clevage. It may contain the fluid inclusions - very small and numours - they can give quartz dusty appearance.

The colour in polarized light in crossed nicoles is gray-steel. The colour is deepend from thickness of thin section but quartz in good quality of thin section should to have gray-steel coulour, sometimes little thicker quartz will have gray-yellow or more thick will have brown to violet coulour.

Quartz - distinguishing features

Quartz is most readily distinguished under the microscope by its lack of colour, clevage and visible twinning, and by its low relief and is unattacked by acids other than HF; in hand specimen it commonly has a distinctive crystal habit and vitreous lustre. Its hardness and lack of clevage easily distinguish it from calcite. In thin section its uniaxial figure distinguishes it from cordierite and the feldspars, and in addition the alkali feldspars normally have lower refractive indices and in most cases a lower birefringence. Beryl and scapolite differ in being length-fast and optically negative; nepheline is also negative, gelatinizes with acids and is rarely completely clear. Criteria for distinguishing natural alpha-quartz from that formed originally above 573C include presence of three-fold symmetry eighter in the distribution of Dauphine in Brazil twining as reveales by etching, or in colour and inclusions.

Quartz Polymorph

Quartz is the most common natural polymorph of SiO2. The more important SiO2 polymorphs and their ranges of stability are: alpha-Quartz stable at ambient temperatures and up to 573C; betha-Quartz stable from 573 to 870C. Metastable abouve 870C; alpha-Trydymite, metastable from ambient temperatures up to 117C; betha-Trydymite metastable above 117C and is the most stable from 870 to 1470C. Metastable above 1470C. Melts at 1670C; alpha-Critobalite: metastable from ambient temperatures up to 200-275C; betha-Cristobalite: metastable above 200-275C and is the most stable polymorph from 1470 to its melting point 1713C; Coesite a high pressure phase, product at 450-800C and 3,8GPa pressure, Found in rocks subjected to the impact of large meteorites and in xenoliths in kimberlite; Stishovite a high-density polymorph pf silica, with a density of 4,3 g/cm3, senthesized at 13GPa and > 1200C. Occure in meteor crater, Arizona; Seiferitie a hard and dense polymorph of silica which is stable only at extremely high pressure; Cryptocrystalline silica (chalcedony) compact varieties containing minute crystals of quartz with submicroscopic pores; Moganite is the microcristaline length-slow fibrous polymorph of silica, it is commonly found intergrown with chalcedonic quartz; Opal consists of amorphous silica and crystaline cristobalite and/or trydimite

Picture on the left - quartz myrmekite with feldspar.

Thin section - quartz-calcite vein

In the picture on left - thin section with the quartz (gray-steel) subhedral crystals wtih calcite (dark-red-yellow) ksenomorphic crystals. The quartz-calcite vein cutting the sandstone-mudstone strongly altered with calcite (calcitization) and quartz (silification).

Thin section - quartz

In the picture on left - thin section with the quartz (gray-steel).

Thin section - quartz

In the picture on left - thin section with the quartz (gray-steel).

Thin section - quartz "Strzegom massif - Poland"

In the picture on left - thin section with the quartz (gray-steel). The myrmekite intergrows with altered feldspar.