What minerals are porphyroblasts

The veal hump near Kleinostheim am Spessart -
a classic site for staurolite!

by Joachim Lorenz, Karlstein a. Main

Staurolite crystal (crossing twin) in mica slate,
Image width 1 cm.

Important NOTE:
Evidence can only be collected with perseverance in the small quarry on the west side of the heavily overgrown mountain slope, as everything has now been searched and no fresh rock has been extracted for decades.
For a long time, mostly cracked staurolite crystals up to 2 cm in size have been found in the staurolite-garnet-plagioclase gneiss. With the appropriate perseverance, finds are still possible today. The best discoveries were made in how the gas pipeline had been laid at the foot of the hill and the road had not yet been paved.

It is a small wooded ridge that shows a small old quarry as well as rocks.

The easiest way to get there is from Bundesstraße 8 via the underpass at the train station in Kleinostheim and then parallel to the train to the west to the Schützenhaus. The last part of the road is only partially accessible (note the prohibition signs! See OKRUSCH et al. 2011, p. 149, information no. 23

In the late winter of 2012, the slope with the rocks (known in Kleinostheim as "Ellertstein" or "Kettelerfelsen", after the Mainz Bishop Wilhelm E. Freiherr v. KETTELER (* December 25, 1811 † July 13, 1877) of the calf's hump next to the Schützenheim (left A new cross was set up on the top of the rock (Main-Echo of April 3rd, 2012, p. 19). The old cross was from 1946 and was getting on in years new one, which was donated by Horst FLEISCHER. The free cutting is a great thing, you have to be grateful to those who did it, because now you can see rocks again and can climb up to the cross without injuries. April 2012 at 30 ° C and a blue sky.

At the edge of the crystalline basement of the Spessart, a staurolite gneiss stands right next to the sediments of the Main. The Spessart Edge Fault runs not far from here. During the last glacial period, the Main flowed directly past the rocks.

In the small quarry and the rocks above ("Kettelerfelsen", but strongly overgrown) consists of a barely fissured gneiss, which is very rich in the brown, up to cm-large, idiomorphic staurolite crystals. These are usually surrounded by the mica and therefore only loosen from the rock to a limited extent. Since the other rock minerals have often grown into the interior of the staurolite crystals, they break easily or have no smooth crystal surfaces.
In contrast to this, there are also small, thin, black tourmaline pillars with end faces, which, however, are not surrounded by mica and can easily be prepared.

Piece of staurolite-garnet-plagioclase-gneiss (actually a piece of mica slate, left in the quarry like from a quarry, right ground and
polished, image width 13 cm) with the feldspar grains (porphyroblasts) barely visible in the break.

The staurolite leading mica schist was formed from a clayey sea sediment (turbid deposits or turbidites) Cambrian or Ordovician age. The layers poor in mica have a greywacke as a forerunner. These sediments were probably located in a shallow sea area on the Baltic shelf. 6 kbar and temperatures of 600 - 650 ° C were derived as pressures (OKRUSCH et al. 2011: 151).

The very small quarry on the slope with the few rocks has certainly not been used for more than 30 years. Local collectors made good finds during the construction of the rifle house and the road.
The last noticeable "mining" activities probably took place on the occasion of the annual meeting of the Upper Rhine Geological Society in Aschaffenburg in 1967, when the 547 Engineer Corps of the American Army thankfully provided "fresh rock"! According to BACKHAUS, the approval for the activities was given by the municipality of "Mainaschaff" (?). The history of the rock and its use was described in detail by LANG (2013). However, there is enough rock in the area that can be searched for the staurolite crystals.
The striking rock, which is located a little to the south and above the parking lot of the Schützehaus, was exposed and protected from vegetation as a natural monument:

The rock and the slope were strongly overgrown on 04/29/2001.

The rock contains the following minerals, but in widely varying proportions:

  • quartz
  • Staurolite in up to 2 cm large, brown crystals, also as twins (usually always grown together with mica, quartz and feldspar):

Staurolite crystals with a diamond-shaped cross-section in gneiss,
Image width 3 cm

Piece of gneiss with very numerous, brown, dull,
Staurolite crystals
Image width approx. 13 cm

  • Tourmaline in black, very shiny columns (not overgrown with mica)
  • Hematite (no ilmenite!) In cm-large, coarse inclusions
  • Garnet in mm-large, red crystals
  • Cyanite (or kyanite, also called thistle) * in crystals up to 1 cm in size:

Colorless kyanite crystals in gneiss, found in 1974
Image width 2 cm
  • Sillimanite in whitish fibers in quartz
  • Muscovite and biotite
Larger pieces of rock can be found in places in the gardens of Kleinostheim. Since the rock also occurs in other parts of the Vorspessart, similar finds can of course also be made there. For example, the Wenighösbach and Feldkahl areas, but also the Aschaffenburg-Damm area, are mentioned here.

* The best find known to me was made by Dr. AICHERT, Hanau, around 1974.

The place is one of the most famous sites for the mineral staurolite in Germany. The island silicate staurolite belongs to a group of several minerals with very similar external properties but different chemical compositions:

  • Staurolite (Fe, Mg) Al [(OH) | O | (SiO)]
  • Magnesiostaurolite (Mg, Li) Al [(OH) | O | (SiO)]
  • Zincostaurolite (Zn, Li, Fe) Al [(OH) | O | (SiO)]

They can only be differentiated by chemical analysis. Staurolite is widespread in metamorphic rocks, but beautiful crystals are not that common. Usually mica crystals or other minerals have grown in (porphyroblast), so a pockmarked image can be observed on the crystal surfaces. Adhesions between staurolite and cyanite are known from some sites. The size of the crystals ranges from a fraction of a millimeter to 10 cm - but such large crystals are not known from the Spessart. The name is derived from the Greek "Kreuzstein", since cross-shaped adhesions (twins after (232)) are not uncommon. The brittle and very hard mineral crystallizes in the monoclinic crystal system. It is very resistant to acids and weathering, so that it can also be found in the heavy mineral fraction of sediments due to its high specific weight of 3.7 - 3.8 g / cm³.

Heavy metal zinc - a problem?
As you can see above, the heavy metal zinc is fixed in the crystal lattice of the staurolite. The very stable - chemically and mechanically - silicate staurolite (it is found in the heavy mineral fraction of sands) is completely insoluble in the soil and our climate, so that the zinc could not be released in historical periods. If someone now builds in the area of ​​distribution of the Mömbris Formation, the soil or rock can become a problem due to a locally high staurolite content, because the LAGA allocation values ​​for zinc are (far) exceeded. Then it can happen that a building owner has to dispose of the "contaminated" excavation at a dump at great expense. In Germany, for example, a large amount of hazardous waste is generated according to regulations, which is driven around for a lot of money without any sense or benefit.

Staurolites from other sites

Large staurolite crossover twin after (232) found at Coray,
Finistere in France. Here also a crystal after one has not yet been made
known twin law, which is now known as the "Coray twin"
can be designated (NESPOLO & MOËLO 2019),
Image width 6 cm

Large, simple staurolite crystal from Scaer, Finistere, France,
Image width 11cm

Free thunderstorm twins of the staurolite from Ampanihy,
Image width 6 cm

Cross-shaped fused staurolite twin from a staurolite
Mica schist (2.5 - 2.7 Ga years old!) From the Ploskay mountain (Semiostrovie)
in the Keivy Mountains in the eastern central Kola Peninsula, Murmansk Region,
Russia. The deposit is only approx. 100 x 100 m and has been around since 1979
known. The most beautiful staurolite twins in the world are likely to come from here
(LYKOVA. & PEKOV 2015). The brushed pieces from here will be on
offered to almost all mineral exchanges,
Image width 5 cm.

Crossing twin of a staurolite from Brazil (unfortunately without
Image width 5 cm

The classic: staurolite (black) and cyanite (bluish) in paragonite slate
by Pizzo Forno in Ticino, Switzerland, found in the 19th century,
Image width 14 cm

Cracked staurolite crystal in a mica slate from Mizarela, Albergaria
da Serra, Portugal. Note that the oriented ingrown mica
tracing the foliation in the staurolite,
Image width 10 cm

Large staurolite crystals (partly twinned) from Martell in South Tyrol, picture
width approx. 12 cm, collection Roman Ebensperger
seen at the stand of the South Tyrolean mineral collectors on the Munich
Mineral Days 2015

Left: Brown staurolite crystals in the gneiss of Gorob, Namib-Naukluft-Park, Namibia, image width 7 cm; right twin crystal according to (232),
Image width 4 cm.

Prismatic, brown staurolite crystal in the biotite gneiss of Nesodden
Akershus, Norway,
Image width 4 cm

The special show at the large mineral exchange in St.-Marie-aux-Mines
2015 showed an impressive collection of beautiful,
large and mostly twinned staurolite crystals of numerous
Locations and supplemented by images of old literature,
Taken in the swimming pool on June 26th, 2015

Unusually large staurolite, surrounded by flakes of mica
Crystal as a twin from Portugal (Sierra Da Gralheira near Porto),
Image width 10 cm

High-gloss, strip-shaped staurolite crystals in a mica slate
from Mama River Basin, Irkutskaya Oblast, Eastern Siberia in Russia,
Image width 8 cm

BACKHAUS, E. (1967): Report on the excursion in the Spessart, in the Wetterau, in the Hanau-Seligenstädter Senke and the Sprendlinger Horst from March 28th to April 1st, 1967.- Jber. And Mitt. Oberrh. geol. Ver. N. F. 49, Pp. 23 - 33, Stuttgart.
LAUF, J. R. & CLIFFORD, J. H. (2018): Connoisseur's Choice. Staurolite. Keivy Mountains, Kola Peninsula, Russia.- Rocks & Minerals Vol. 93, No. 4, Jul / Aug 2018, p. 346-356, 25 figs., [Taylor & Francis] Cincinnati OH.
LANG, E. (2013): The Ketterler memorial stone in Kleinostheim.- Spessart monthly magazine for the cultural landscape Spessart issue August 2013, 107. Volume, pp. 17 - 21, 11 Fig. [Main-Echo GmbH & Co. KG.] Aschaffenburg.
LORENZ, J. (2019): Stones around and under Karlstein. Remarkable rocks, minerals and ores - p. 8, 4 fig. - in Karlsteiner Geschichtsblätter edition 12, 64 p., Edited by the history association Karlstein [MKB-Druck GmbH] Karlstein.
LORENZ, J. with contributions by M. OKRUSCH, G. GEYER, J. JUNG, G. HIMMELSBACH & C. DIETL (2010): Spessartsteine. Spessartine, Spessartite and red sandstone - a comprehensive geology and mineralogy of the Spessart. Geographical, geological, petrographic, mineralogical and mining insights into a German low mountain range - see p. 470ff.
LYKOVA, I. S. & PEKOV, I. ​​V. (2015): 3. The Semiiostrovie Staurolithe Locality (Kola Peninsula, Russia) .- Mineralogical Almanac volume 20, issue 2, p. 94-95, 3 figs., [Mineral-Almanac Ltd.] Moscow.
MATTHES, S. (1953): Mineral sprouting and material mobilization during the metamorphosis of the paragneiss in the middle crystalline pre-spess type taking into account the staurolite genesis. Mineral. 32, Pp. 47 - 51, Berlin.
MATTHES, S. (1963): Excursion into the Crystalline of the Spessart on September 17th, 1962.- Fortschr. Miner. 41, P. 37f, Stuttgart.
MATTHES, S. & OKRUSCH, M. (1965): Spessart.- Collection of Geological Guides Volume 44, Pp. 85 - 89, Berlin.
MATTHES, S. (1978): The crystalline Spessart (excursion C on March 31, 1978) .- Jber. Mitt. Oberrhein. geol. Ver., N. F. 60, P. 48, Stuttgart.
NESPOLO, M. & MOËLO, Y. (2019): Structural Interpretation of a new twin in staurolite from Coray, Brittany, France.- European Journal of Mineralogy Volume 31, Number 4 - July, August, p. 785 - 790, 5 figs., 1 tab., [Schweizerbart Science Publishers] Stuttgart.
NEUBAUER, D. & REISS, W. (1967): Minerals from the Spessart.- The outcrop 18, Pp. 215-218, Heidelberg.
OKRUSCH, M., GEYER, G. & LORENZ, J. (2011): Spessart. Geological Development and Structure, Rocks and Minerals. - 2nd ed., Collection of Geological Guides Volume 106, VIII, 368 pages, 103 mostly colored illustrations, 2 colored geological maps (43 x 30 cm) [Borntraeger Brothers] Stuttgart.
OKRUSCH, M., STREIT, R. & WEINELT, Wi. (1967): Explanations of the geological map of Bavaria 1: 25000 sheet 5920 Alzenau i. Ufr.- p. 41 ff, p. 43 - 44, Munich.
SMOLER, M. (1987): Petrographic, geochemical and phase-petrological investigations on metasediments of the NW-Spessart / Bavaria - unpublished dissertation of the Bay. Julius Maximilians University of Würzburg, 256 pages, Würzburg.
WEINER, K. L. & HOCHLEITNER, R. (1985): Profile: Staurolite.- Lapis 10, Issue 2 February 1985, pp. 8-11, Munich.

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