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TRIDYMITE
* fine little crystals (size 3mm) on matrix *
* nice and rare specimen from uncommon locality *
size: 70 X 65 X 60mm
weight: 424g
origin: Vechec, Vranov nad Topľou Co., Prešov Region, Slovakia
size: 70 X 65 X 60mm
weight: 424g
origin: Vechec, Vranov nad Topľou Co., Prešov Region, Slovakia
Tridymite is a polymorph of quartz, meaning that it is composed of the same chemistry, SiO2, but has a different structure. Both quartz and tridymite are polymorphs with all the members of the Quartz Group which also include coesite, cristobalite and stishovite.
At one time tridymite was considered rare, but that was before widespread testing of many different volcanic rocks turned out to contain lots of small to microscopic crystals of tridymite. Its wide spread distribution in certain types of rocks would require an abundance rating of at least common, although its identification is not easy.
Identification of tridymite without laboratory aid is limited to crystal habit and possibly index of refraction. What is needed to identify tridymite is a well formed crystal that shows the distinctive pseudohexagonal habit or the equally distinctive twinning. An index of refraction test would clinch the identification.
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二氧化矽在自然界呈現的形態有許多種，包括：
1.石英 Quarts : 575。C以上 β 型或稱高溫水晶 ,以下 α 型也就大家最常見的水晶。分別為Hexagonal與Trigonal晶系。
2.鱗石英 Tridymite :405 。C以上 β 型,以下 α 型如上圖之標本。為Triclinic晶系。
3.方矽石 Cristobalite :405。C以上 β 型,以下 α 型。標本敝小館有採購，找到再補圖。為Tetragonal 晶系。
4.焦石英 Lechatelierite: 無晶型。閃電雷管被列在焦石英族群，茶包有賣。
5.玉髓,瑪瑙:隱晶或微晶質的石英。
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