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| Zwaan, P.C.. |
| Both rhodolite and almandine garnets occur in Umba. The rhodolites have no special properties by which they can be distinguished from other garnets in the pyrope-almandine series, except their rose-red colour. Hence a distinctive name is unnecessary. Rutile appears to be the most frequent mineral inclusion in rhodolite, whereas in almandine both apatite and rutile are common. Corundums of different colours occur. Green corundums owe their colour to iron, while blue corundums have a high Ti and a low Cr content. Common inclusions are rutile, pyrrhotite and apatite, whereas graphite especially is found in deep violet corundums. The inclusions in both the garnets and the corundums are not characteristic of this locality. The properties of emerald green and... |
| Tipo: Article / Letter to the editor |
Palavras-chave: 38.30. |
| Ano: 1974 |
URL: http://www.repository.naturalis.nl/record/317404 |
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| Zwaan, P.C.; Arps, C.E.S.. |
| Three cut sphenes, originating from the Tissamaharama area in Sri Lanka, are described. Their properties are compared with those of gem quality sphenes from Capelinha, Brazil. There is hardly any difference between the chemical data of the major elements and physical properties of the specimens of these two sources. Even the inclusions are very similar. It is the first time that fine gem quality sphene has been reported from Sri Lanka. |
| Tipo: Article / Letter to the editor |
Palavras-chave: 38.30. |
| Ano: 1980 |
URL: http://www.repository.naturalis.nl/record/317475 |
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| Zwaan, P.C.. |
| The peridots in the gem collection of our Museum were acquired a long time ago (part of them were from the collection of King William I of the Netherlands). We should be sceptical about data on the origin of this material. Firstly we do not generally know whether the stones were bought or received as a gift; we learned by experience that a person giving material (often a layman) will not be too precise when stating the locality especially as he will not see its importance. Moreover in the last century scientists could not realize yet how valuable an accurate description of the locality would be nowadays. For they did not think of the possibility that samples of one mineral from various localities might have different properties (of course they did not yet... |
| Tipo: Article / Letter to the editor |
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| Ano: 1957 |
URL: http://www.repository.naturalis.nl/record/505883 |
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| Zwaan, P.C.. |
| Our Museum can take pride in a very valuable gem collection for, besides the numerous specimens which were gathered by King William I of the Netherlands a great many of the more unusual stones are to be found in it. Especially since World War II the Museum has acquired many interesting and important stones by gifts and purchases. This enlargement is especially due to the efforts of Professor Dr. B. G. Escher, who was continually aimed at the acquisition of rare and unusual stones for the gem collection, bringing it gradually up to modern standards. As in so many gem collections, one finds stones which are not suitable for wearing, because they are soft or easily cleavable. It is surprising to see the variety of minerals that have been cut in recent years... |
| Tipo: Article / Letter to the editor |
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| Ano: 1955 |
URL: http://www.repository.naturalis.nl/record/505900 |
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| Zwaan, P.C.. |
| The X-ray powder method for determining minerals has been applied to the important rock-forming mineral group of the pyroxenes in this thesis. The purpose of the investigation was to seek the relationship between the variations of the intensities and positions of the reflections in the powder diagram and the variations in optical properties and chemical composition. For that purpose a number of pyroxenes from different localities were investigated optically, chemically and röntgenographically. The orthopyroxenes. — The optical examination of the orthopyroxenes indicates, that the variation of the optical properties is related to the chemical composition (see Table 1). A difference between plutonic and volcanic orthopyroxenes lies in the size of the optic... |
| Tipo: Article / Letter to the editor |
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| Ano: 1954 |
URL: http://www.repository.naturalis.nl/record/505841 |
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