Pyroelectrically caused twisting of quartz crystals

The twisting of quartz crystals is conditioned by Dauphiné twinning. Each twisted crystal shows morphological characteristics confirming this type of quartz twinning.A linear mathematical correlation based on morphological parameters measured on quartz crystals elongated and twisted along their polar [21.0] a-axis has been deduced. It was demonstrated that the twisting angle is the function of the twisting constant and some of the crystal dimensions. This constant has the same value cca 4° as the declination of the [21.0] edge of the crystal twisted around the[21.0] a-axis which also corresponds to the surface distribution of the positive charge observed with Dauphiné-twinned quartz crystal on cooling. A similar quartz feature is the twisting of a quartz crystal around the c-axis. A proposed theory describing the reasons for crystal twisting, development of different crystalforms and types of twisted quartz crystals is thus based on the pyroelectrically accelerated growth from slowly cooling and slightly supersaturated quartz-bearing solutions

Hyalophane twins from Zagradski potok in Central Bosnia

The location Zagradski potok near Busovača in Bosnia and Hercegovina has yielded numerous hyalophane specimens. Unique Karlsbad twins and different types of Manebach-Baveno combination twins were found there. The most interesting are double Manebach-Baveno combination twins. The mineral paragenesis present at this location is typical for alpine-type veins. The morphology of these minerals is described as a result of the active tectonic environment that caused theformation of so-called fadens (threads) and correlated secondary and tertiarymorphological structures that feature in the Karlsbad twins and Manebach- Baveno combination twins. The different twinnings observed in the hyalophane crystals at this location are therefore the result of the mechanical process.Different combinations of Karlsbad twins are mentioned and described: (001)- and (OlO)-Manebach-Baveno combination twins as well as (010)-(010)-Manebach- Baveno- and (O01)-(010)-Manebach-Baveno combination twins. Similar adularla morphological structures can be described with the theory proposed

Pirit z nižjo kubično simetrijo iz Lavriona, Grčija

V tej študiji smo preučili morfološke, kemijske in strukturne podrobnosti tetraedrsko oblikovanih kristalov pirita iz rudnika Jean Baptiste v Lavrionu. Pirit se pojavlja v treh generacijah. Prvo predstavljajo tetraedrski levo oziroma desno sučni kristali z najnižjo kubično 23 simetrijo. V tej generaciji so dvojčki z višjo kubično m_3 in heksagonalno 6 simetrijo. Vsi kristali druge generacije so že primarno zdvojčeni do kubične_4m3 simetrije. Nekateri pa se dvojčijo še naprej dokler ne dosežejo najvišje kubične m_3m ali heksagonalne 6mm simetrije. Kristali tretje generacije neorientirano prekrivajo kristale druge generacije. Kemijska analiza potrjuje kemijsko čist pirit, monokristalna rentgenska analiza kristalov prve in druge generacije pa za pirit značilno m_3 simetrijo. Očitno je torej, da morfološke oblike posameznih kristalov in dvojčkov kažejo na to, da imajo najnižjo kubično simetrijo, česar pa strukturna analiza ne potrjuje. Ta diskrepanca je lahko posledica spremenjenih pT pogojev in posledične transformacije prvotne strukture pirita s simetrijo 23 v sekundarno strukturo z m_3 simetrijo ali pa neoptimalnih pogojev pri določitvi strukture z rentgensko difrakcijo.V tej študiji smo preučili morfološke, kemijske in strukturne podrobnosti tetraedrsko oblikovanih kristalov pirita iz rudnika Jean Baptiste v Lavrionu. Pirit se pojavlja v treh generacijah. Prvo predstavljajo tetraedrski levo oziroma desno sučni kristali z najnižjo kubično 23 simetrijo. V tej generaciji so dvojčki z višjo kubično m_3 in heksagonalno 6 simetrijo. Vsi kristali druge generacije so že primarno zdvojčeni do kubične_4m3 simetrije. Nekateri pa se dvojčijo še naprej dokler ne dosežejo najvišje kubične m_3m ali heksagonalne 6mm simetrije. Kristali tretje generacije neorientirano prekrivajo kristale druge generacije. Kemijska analiza potrjuje kemijsko čist pirit, monokristalna rentgenska analiza kristalov prve in druge generacije pa za pirit značilno m_3 simetrijo. Očitno je torej, da morfološke oblike posameznih kristalov in dvojčkov kažejo na to, da imajo najnižjo kubično simetrijo, česar pa strukturna analiza ne potrjuje. Ta diskrepanca je lahko posledica spremenjenih pT pogojev in posledične transformacije prvotne strukture pirita s simetrijo 23 v sekundarno strukturo z m_3 simetrijo ali pa neoptimalnih pogojev pri določitvi strukture z rentgensko difrakcijo