Sample number: E- 605
Term: HEB
Lithic type: Melilite- Nephelinite
Location: Herchenberg / Eifel
Specimen description:
| Minerals: | black pyroxenes, ca. 2mm in size; greenish yellow, slightly weathered olivine, up to 2mm in size.; |
| Vesicles: | fine, dense matrix with very few vesicles |
| Colour: | greyish-green |
Thin section:
| Fabric (texture / structure): | the crystals are directionless |
Components:
| Groundmass: | Magnetite, isometric and hypidomorphic, in transmitted light opaque, in reflected light bright Melilite, elongated, in transmitted light colourless, light-grey in reflected light, with crossed polarizer deep dark abnormal interference colours, up to 100mm long, cross division visible Nepheline, isometric (subrounded, hexagonal, shortprismatic shape ?~ often square crystal shape), in reflected light dark, in transmitted light colourless, with crossed polarizer grey interference colours, ca. 50mm in scale, Apatite inclusions Hauyn , typical acicular inclusions Leucite, atoll structure, round, isometric, isotropic Pyroxene, in transmitted light green. |
| Phenocrysts: | Olivine, idiomorphic, fractures, no zoning, with typical triangular apex Magnetite isometric and hypidomorphic, in transmitted light opaque, in reflected light bright. Pyroxene, some crystals partly zoned with dark cores, greenish brown, 2mm in scale. Others show no zoning, no cores, hypidiomrphic to xenomorphic, ground mass up to 100mm in scale. |
| Perculiarity: | Replaced Mica, secondary mineralisation Replaced Amphibole, reacted to magnetite, cpx, plagioclase and sanidine ? ~ microaggregate Peculiarity: Calcite as reside fill, very high birefringence, diffraction of light changes while rotating, secondary mineral Pyroxeneaggregate, with many different sizes. |
Interpretation:
In this thin sections it can be seen, that there are 3 different stages of crystal growth . First the large phenocrysts (few mm in diameter) of pyroxene and olivine crystallised, what is an evidence for a relatively slow crystallisation. These crystals show the conditions of the magma chamber because they are in equilibrium with that melt. After this first crystallisation event, the microphenocysts and the mantel of the pyroxenes formed, which is a sign for a faster crystallisation, because the grain size is much smaller. His stage shows the conditions of a slow magma ascent. At last the matrix crystallised. It consists of many small crystals. This matrix is fine-grained because of rapid cooling. After that, the ? ? normal? ??? magmatic history is over. An other event is crystallisation of the water-carrying minerals, mica and amphibole, which are reorganised at the moment, when the melt lost so much H2O that they were not stable any longer. So this must have happened during the ascent. Relicts of these phases can be found in this thin section, they are called ? ? ghosts? ??? here. The very last event was a secondary mineralisation of calcite.
Sample number: E-344
Term: ADA
Lithic type: Leucitite
Location: An der Ahl (Hochsimmer), Eifel
Specimen description:
| Mineral: | pyroxene |
| Vesicles ?: | fine, dense matrix |
| Colour: | grey-green |
| Oxidation ?: | some |
Thin section:
| Fabric (texture / structure): | bimodal grain size distribution: very fine matrix, large phenocrist (up to 6 mm) |
Components:
| Groundmass: | about 70% Percentage [vol.-] Magnetite (3,5 vol. %), isometric and hypidiomorphic, in transmitted light opaque, in reflected light bright, up to 50 µm in scale, Leucite (24,5 vol. %), round/isometric, many inclusions Pyroxene (17,5 vol. %), elongated, xenomorphic Apatite (<1% vol.), long prismatic, high refraction index Nepheline (7,0 vol. % Glass (17,5 vol. %) |
| Phenocrysts: | about 30,0 vol. % Pyroxene, (20,0 vol. %) up to 6 mm in size, idiomorphic, core often idiomorphic and much more green as in HEB clearly different from much lighter rim, also as microphenocrists Nosean (5,0 vol. %), yellow rim, characteristic internal structure with erzmineral in it. (?), fractures, mostly in the middle gm-minerals included Magnetite (2,0 vol. %) (s.o.), as microphenocris Leucite (3,0 vol. %) (s.o.), as microphenocrist |
| Perculiarity: | Pyroxeneaggregates, dark objects (1.5 mm in scale, diffuse grain boundaries, hypidiomorphic, mineral mixing of dark and lighter and reddish (oxides)) with rounded pyroxeneinclusions (250 µm) unknown phase, erzminerales (yellowish ? ~ sulphide?) |
Interpretation:
The green cores of the large (and microphenocryst-) pyroxenes which were assimilated, now have new overgrowth from the same stage as the microphenocrysts/gm. The large idiomorphic minerals are the early crystallised phases. There are apart from pyroxene, nosean, magnetite and leucite. From these phases magnetite and leucite are crystalling along all the time, they occur also in the groundmass. The amount of glass shows that the melt cooled down very quickly. There was no chance to crystallise completely.
Sample number:E-189
Term:NDM
Lithic type:Tephrite
Location:lava stream of Niedermendig, Eifel
Specimen description:
| Minerals: | pyroxene, hauyn |
| Vesicles ?: | present |
| Colour: | pale-brown |
| Oxidation ?: | no |
| Perculiarity: | quarz-zenolite with green reaction-reams |
Thin section:
| Fabric (texture / structure): | big pyroxenes without orientation |
Components:
| Groundmass: | Hauyn: Idiomorphic as well as hypidomorphic in shape. This minerals show a typical grid-like internal structure. Leucite:granular shape with concentric patterns. Plagioclase Magnetite: Mostly hypidomorphic in external shape. Apatite: small crystals with prismatic habit Nephelinite: The external form isxenomorphic. This mineral-phase fills the interstices. |
| Phenocrysts: | Altered amphibole: (3 vol.-%) Big black aggregates, composed mainly of magnetite (ca. 40%), pyroxene and plagioclase. These aggregates have hexagonal shapes. Pyroxene:(<5 vol.-%) small (<100µm) light-gray minerals. These minerals are mainly hypidiomorphic in shape. Idiomorphic ones are also present. These minerals show resorbed rims. Magnetite: (<1 vol.-%) Black minerals. Hypidiomorphic in shape. |
Interpretation:
The age relationships of some of the minerals were determined by checking which mineral is overgrown by another. One can find some pyroxenes thatt overgrow amphiboles, suggesting that amphiboles crystallised prior to the pyroxenes. The same can be seen between pyroxene and magnetite showing that magnetite is earlier than pyroxene.
The minerals hauyne, leucite, plagioclase, apatite and magnetite are smaller than the former ones, suggesting not having had enough time and place to grow further. Thus, these minerals represent a second stage of growth.
Sample number: E-41
Term: RO 1
Lithic type:Basanit
Location:Rothenberg / Dike, Eifel
Specimen description:
| Minerals: | arge pyroxenes (up to 1cm in size), many olivines (2mm), much more phenocrysts than in the other samples |
| Vesicles ?: | no, dense matrix |
| Colour: | grey |
| Oxidation ?: | fresh |
| Perculiarity: | mantle xenoliths |
Thin section:
| Fabric (texture / structure): | there is no orientation |
Components:
| Groundmass: | Pyroxene (up to 200mm) Phlogopite (up to 0.5 mm), brownish ? ? red || {100}; 3. order, {100} Ore-mineral (up to 50mm), isometric and hypidiomorphic, in transmitted light opaque, in reflected light bright Plagioclase shows twinning lamellae, low interference colour Leucite (up to 150mm), atoll structure with many inclusions Nepheline in interstices, xenomorphic |
| Phenocrysts: | Olivine (about 5 Vol. %) fractures, hypidiomorphic, no zoning Pyroxene (about 10 Vol, %), fractures, idiomorphic, # 90", two different kinds of cores, but both are not as green as those in the NDM-tephrite and the ADA-leucitite. One type is a very core consisting of an aggregate of many different pyroxenes and few olivines. The other type are cores which are partly broken and later normaly overgrown. Phlogopite ( 2 Vol. %), tabular, pleochroitic |
| Perculiarity: | Aggregates of minerales (predominantly pyroxene, but also olivine and phlogopite) |
Interpretation:
The basanite of the Rothenberg dike contains large early crystallised phenocrysts of green pyroxene. These pyroxenes were in equilibrium with a highly differentiated magma (see below). After this crystallisation they were intruded by a new host, the basanitic magma of the Rothenberg. There they were not stable and the magma began to resorb the green pyroxenes. Afterwards a new pyroxene phase, very undifferentiated, grew over the green core. This lighter phase was in equilibrium with the host magma because the small pyroxenes (micro phenocrysts and ground mass) have the same colour. Besides, olivine and phlogopite started to crystallise in large depth. The magma rose fast, because crystals with medium size are ? ?Smissing? ???. In this phase of evolution mantle xenoliths were brought from the magma to the uppermost crust. At last the ground mass crystallized in the dike. At this state, in addition to pyroxene, plagioclase, and magnetite, leucite was formed because of silica-undersaturation of the magma. The intersticies were filled by nepheline.