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Copyright of Mineralogy & Petrology is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Mineralogy & Petrology is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Mineralogy & Petrology is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Mineralogy & Petrology is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Mineralogy & Petrology is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Eifelite of variable composition is uniaxial positive with n near 1.543 and n near 1.544, a between 10.14 and 10.15 Å, and c about 14.22 Å, space group P 6/ m 2/ c 2/ c. There is a complete series of solid solution between the eifelite end member KNaMgSiO and roedderite, KNaMgSiO, following the 2 Na⇌Mg substitution. Both eifelite and roedderite have milarite-type structures, but Na is always in six-coordinated sites: In roedderite Na occupies solely a newly defined B′-position which is slightly displaced along c from the ideal B-position lying on the (001/2)-mirror plane in KMgSiO. In eifelite Na is located both in B′ and in the A-positions, where it partially replaces Mg. Eifelite has the highest cation occupancy of all osumilite group minerals known thus far. Both eifelite and roedderite occur in vesicles of contact metamorphosed basement xenoliths ejected with the leucite tephrite lava of the Quaternary Bellerberg volcano in the Eifel, West Germany. They are considered to be precipitates from highly alkaline, MgSi-rich, but Al-deficient gas phases that originated through interaction of gaseous igneous differentiates with the xenoliths. [ABSTRACT FROM AUTHOR]

Copyright of Tschermaks Mineralogische und Petrographische Mitteilungen (1889-1927) is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Tschermaks Mineralogische und Petrographische Mitteilungen (1889-1927) is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The conditions under which covering ('decorating', 'masking') takes place have been studied in the sea urchins Paracentrotus lividus, Psammechinus miliaris and 2 other species. Covering occurs equally in darkness or light. It requires suitable objects and locomotion or searching activity of the tube feet. The covering reaction which may follow chemical, mechanical or optical stimuli may be purely the result of an increase in locomotory or general activity. Initial selection of different covering objects depends on the tube feet reflexes; size, form and weight of the object are important. Whether an object is accepted or not, depends on its surface and structure, the amount of water movement and the general activity level of the sea urchin. Transparency and colour of an object do not appear to be important. It is suggested that the covering process can be explained in terms of local tube feet and spine reflexes. The loading-up of objects may be understood as 'relative walking': the same reflexes which move the urchin on stable ground, draw loose particles towards the animal and then upwards. [ABSTRACT FROM AUTHOR]