Holtstamit

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Holtstamit
General and classification
other names

IMA 2003-047

chemical formula
  • Ca 3 Al 2 (SiO 4 ) 2 (OH) 4
  • Ca 3 (Al, Mn 3+ ) 2 [(SiO 4 ) 2 (OH) 4 ]
  • Ca 3 (Al, Mn 3+ ) 2 (SiO 4 ) 3-x (H 4 O 4 ) x (with Al> Mn 3+ )
Mineral class
(and possibly department) 		Silicates and germanates - island silicates (nesosilicates)
System no. to Strunz
and to Dana 		9.AD.25 ( 8th edition : VIII / A.08)
April 51, April 02
Crystallographic Data
Crystal system tetragonal
Crystal class ; symbol 4 / mmmTemplate: crystal class / unknown crystal class
Space group I 4 1 / acd (No. 142)Template: room group / 142
Lattice parameters a  = 12.337  Å ; c  = 11.930 Å
Formula units Z  = 8
Physical Properties
Mohs hardness 6th
Density (g / cm 3 ) calculated: 3.25
Cleavage no
Break ; Tenacity shell-like
colour brownish yellow
Line color White
transparency transparent
shine Glass gloss
Crystal optics
Refractive indices n ω  = 1.718
n ε  = 1.746
Birefringence δ = 0.028
Optical character uniaxial positive

The mineral holtstamite is a very rare island silicate from the group of garnets with the idealized composition Ca 3 Al 2 (SiO 4 ) 2 (OH) 4 . It crystallizes in the tetragonal crystal system with the structure of garnet. The round and transparent crystals are brownish-yellow in color, zoned with manganese-rich cores and reach a size of a few millimeters. The crystals are often twinned with an octahedral habit.

So far, Holtstamite has only been detected in its type locality , the Wessels Mine, a hydrothermally modified manganese deposit near Hotazel in the Kalahari manganese field in the Northern Cape Province , South Africa .

Etymology and history

Holtstamit was first described in 2005 by Ulf Hålenius, Ulrich Häussermann and Hans Harrison. They named the new mineral after the Swedish mineralogist Dan Holtstam (* 1963) in recognition of his work on manganese deposits of the Långban type.

classification

The structural classification of the International Mineralogical Association (IMA) counts the holtstamite to the garnet upper group, where together with henritermierite it forms the henritermierite group with 8 positive charges on the tetrahedrally coordinated lattice position. Formally, the Monteneveit also belongs to this group, although structurally and chemically it is closer to the minerals of the Bitikleit group.

Already in the outdated, but still in use 8th edition of the mineral systematics according to Strunz , the Holtstamite belonged to the mineral class of "silicates and germanates" and there to the department of "island silicates (nesosilicates)", where together with almandine , andradite , calderite , goldmanite , Grossular , Henritermierit , Hibschite , Hydrougrandit , katoite , Kimzeyit , Knorringit , majorite , Morimotoit , Pyrop , Schorlomit , Spessartin , Uwarowit , wadalite and Yamatoit (discredited because identical to Momoiit ) the "garnet group" with the system number. VIII / A.08 .

The 9th edition of Strunz's mineral systematics, which has been in effect since 2001, also classifies the Holtstamit under the category of "island silicates". This is further subdivided according to the possible presence of further anions and the coordination of the cations involved , so that the mineral is classified according to its composition and structure in the subsection “Island silicates without additional anions; Cations in octahedral [6] and usually greater coordination “can be found, where it is found together with almandine, andradite, calderite, goldmanite, grossular, henritermierite, katoite , kimzeyite, knorringite, majorite, momoiite, morimotoite, pyrope, schorlomite, spessartine and Uwarowit the "garnet group" with the system no. 9.AD.25 forms. The garnet compounds blythite, hibschite, hydroandradite and skiagite, which are no longer regarded as minerals, were also included in this group. Wadalite , at that time still grouped with the grenades, proved to be structurally different and is now assigned to a separate group with chloromayenite and fluoromayenite . The garnets irinarassite , hutcheonite , kerimasite , toturite , menzerite (Y) and eringaite described after 2001 would have been classified in the garnet group.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the Holtstamite to the class of "silicates and Germanates" and there in the department of "island silicate minerals ". Here it is only together with Henritermierite in the group of the " Garnet Group (Tetragonal Hydrogranate) " with the system no. April 51, 2004 to be found in the subsection “ Island silicates: SiO 4 groups only with cations in [6] and> [6] coordination”.

Chemism

Holtstamite is the Al analog of henritermierite and the measured composition from the type locality is [X] Ca 3 [Y] (Al 0.96 Mn 3+ 0.68 Fe 3+ 0.37 ) [Z] Si 2 □ O 8 (OH) 3.96 .

Holtstamit forms a mixed crystal row with henritermierite according to the exchange reaction

  • [Y] Al 3+ = [Y] Mn 3+

and with a [Y] Fe 3+ hydrogen garnet that has not yet been found as a separate mineral

  • [Y] Al 3+ = [Y] Fe 3+

The silicon and OH contents vary according to the substitution

  • [Z] Si 4+ O 4 = [Z] □ OH 4

Mn 3+ is an ion with a strong Jahn-Teller effect and prefers a distorted environment with anions. Mn 3+ thus stabilizes the tetragonal symmetry compared to the otherwise cubic symmetry of the garnet structure. Therefore, it is believed that Mn-free holtstamite does not exist. Without manganese compound would Ca 3 Al 2 Si 2 □ O 8 (OH) 4 may be cubic and then a Grossular - katoite mixed crystal.

Crystal structure

Holtstamite crystallizes with tetragonal symmetry in the space group I 4 1 / acd (space group no. 142) and the lattice parameters a  = 12.337  Å and c  = 11.930 Å as well as 8 formula units per unit cell . Template: room group / 142

The structure is that of garnet , whereby in the tetragonal symmetry the X and Z positions are split into two crystallographically different positions. Calcium (Ca 2+ ) occupies the two dodecahedral X positions surrounded by 8 oxygen, aluminum (Al 3+ ) the octahedral Y position surrounded by 6 oxygen and 2/3 of the Z positions surrounded tetrahedral by 4 oxygen are silicon (Si 4+ ) occupied.

Education and Locations

So far, Holtstamite has only been detected in its type locality , the Wessels Mine near Hotazel in the Kalahari manganese field in the Northern Cape Province , South Africa . It was formed here in calcium silicate carns during the metasomatosis of limestones due to hydrothermal solutions rich in manganese and occurs together with Mn-rich vesuvianite , calcite and henritermierite .

See also

literature

  • Ulf Hålenius, Ulrich Häussermann, Hans Harryson: Holtstamite, Ca 3 (Al, Mn 3+ ) 2 (SiO 4 ) 3-x (H 4 O 4 ) x , a new tetragonal hydrogarnet from Wessels Mine, South Africa . In: European Journal of Mineralogy . tape 17 , no. 2 , 2005, ISSN  1617-4011 , p. 375-382 , doi : 10.1127 / 0935-1221 / 2005 / 0017-0375 .
  • Holtstamite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 131 kB ; accessed on May 25, 2017]).

Web links

Individual evidence

  1. ↑ a b c d e f g h i j k l m n o p q r Ulf Hålenius, Ulrich Häussermann, Hans Harryson: Holtstamite, Ca 3 (Al, Mn 3+ ) 2 (SiO 4 ) 3-x (H 4 O 4 ) x , a new tetragonal hydrogarnet from Wessels Mine, South Africa . In: European Journal of Mineralogy . tape 17 , no. 2 , 2005, ISSN  1617-4011 , p. 375-382 , doi : 10.1127 / 0935-1221 / 2005 / 0017-0375 .
  2. ^ IMA / CNMNC List of Mineral Names; March 2017 ( Memento of July 10, 2017 in the Internet Archive ) (PDF 1.66 MB)
  3. ↑ Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties . 6th completely revised and supplemented edition. Weise, Munich 2014, ISBN 978-3-921656-80-8 .
  4. ↑ a b List of localities for Holtstamite in the Mineralienatlas and in Mindat
  5. ^ A b Edward S. Grew, Andrew J. Locock, Stuart J. Mills, Irina O. Galuskina, Evgeny V. Galuskin and Ulf Hålenius: IMA Report - Nomenclature of the garnet supergroup . In: American Mineralogist . tape 98 , 2013, p. 785–811 ( rruff.info [PDF; 1.1 MB ; accessed on April 28, 2020]).
  6. ↑ Ulf Hålenius: Stabilization of trivalent Mn in natural tetragonal hydrogarnets on the join 'hydrogrossular' - henritermierite, Ca 3 Mn 3+ 2 [SiO 4 ] 2 [H 4 O 4 ] . In: Mineralogical Magazine . tape 68 , no. 2 , 2004, p. 335-341 , doi : 10.1180 / 0026461046820190 .