Dissakisite- (La)

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Dissakisite- (La)
General and classification
other names

IMA 2003-007

chemical formula CaLaAl 2 MgSi 3 O 12 (OH), more precisely
Ca (La, Ce, Th) (Mg, Fe 2+ ) (Al, Fe 3+ , Cr 3+ ) 2 [O | OH | SiO 4 | Si 2 O 7 ]
Mineral class
(and possibly department)
Silicates and germanates - group silicates (sorosilicates)
System no. to Strunz
and to Dana
9.BG.05 ( 8th edition : VIII / C.23)
Crystallographic Data
Crystal system monoclinic
Crystal class ; symbol monoclinic prismatic 2 / m
Room group (no.) P 2 1 / m (No. 11)
Lattice parameters a  = 8.93  Å ; b  = 5.69 Å; c  = 10.18 Å
β  = 115.15 °
Formula units Z  = 2
Physical Properties
Mohs hardness 6.5 to 7
Density (g / cm 3 ) measured: 3.79; calculated: 3.84
Cleavage imperfect after (001)
Break ; Tenacity shell-like, brittle
colour black to dark brown
Line color greenish gray
transparency translucent to opaque
shine Glass gloss
radioactivity weak
Crystal optics
Refractive indices n α  = 1.7395 (25)
n β  = 1.7434 (25)
n γ  = 1.7495 (25) (589 nm)
Birefringence δ = 0.010
Optical character biaxial positive
Axis angle 2V = 77.0 to 77.5 °
Pleochroism weak:
X = light brown
Y = Z = light greenish brown

Dissakisite- (La) (pronounced: Lanthanum dissakisite ) is a very rarely occurring mineral from the mineral class of " silicates and germanates ", more precisely a group silicate from the epidote group . It crystallizes in the monoclinic crystal system with the chemical composition Ca (La, Ce, Th) (Mg, Fe 2+ ) (Al, Fe 3+ , Cr 3+ ) 2 [O | OH | SiO 4 | Si 2 O 7 ] or somewhat simplified CaLaAl 2 MgSi 3 O 12 (OH).

Dissakisite- (La) could only be found in the form of granular or kidney - shaped mineral aggregates , which consist of translucent crystals with a needle-like or thick-leaved habit of up to 2.5 cm in size and whose crystal surfaces have a glass-like sheen . The color of the mineral varies between dark brown and black, while the color of the streak is greenish-gray.

Dissakisit- (La) is closely related to Dissakisit- (Ce) .

Etymology and history

Dissakisit- (La) was first discovered on the Hochwart above the Ultental / Vedetta Alta in Italy and described by Simone Tumiati, Gaston Godard, Silvana Martin, Paolo Nimis, Volkmar Mair and Bernard Boyer.

It was recognized by the IMA as an independent mineral in 2003 under the register no. IMA2003-007. The test results and the recognized name were published two years later in the American Mineralogist .


Since the Dissakisit- (La) was only discovered in 2003, it is not listed in the Strunz system of minerals (8th edition), which has been outdated since 2001 . Only the "Lapis mineral directory" published in 2008, which is still based on the classic system of Karl Hugo Strunz out of consideration for private collectors and institutional collections , lists the mineral under system no. VIII / C.23-60 .

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), assigns the dissakisite (La) also to the class of "silicates and germanates" and there in the department of " group silicates (sorosilicates) " a. This department is, however, further subdivided according to the type of group formation of the silicate complexes and the coordination of the cations , so that the mineral is classified according to its composition and structure in the subdivision of “group silicates with mixed SiO 4 and Si 2 O 7 groups; Cations in octahedral [6] and greater coordination ”can be found, where the“ epidote group ”with the system no. 9.BG.05 and the other members allanite (Ce) , allanite (La) , allanite (Y) , clinozoisite , dissakisite (Ce) , dollaseite (Ce) , epidote (Pb) , epidote ( Sr) , epidote , Ferriallanit- (Ce) , Khristovit- (Ce) , Manganiandrosit- (Ce ), Manganiandrosit- (La) , Tweddillit , Mukhinit , Piemontit- (Sr) , Piemontit , Uedait- (Ce) and Vanadoandrosit- ( Ce) forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the Dissakisit- (La) to the class of "Silicates and Germanates", but there in the more finely divided department of "Group silicates: insular, mixed, individual and larger tetrahedral groups" . Here it is also in the "epidote group (allanite subgroup)" with the system no. 58.02.01c and the other members allanite (Ce), allanite (La), allanite (Y), dissakisite (Ce), manganiandrosite (La), ferriallanite (Ce), västmanlandite (Ce), manganiandrosite - (Ce) and vanadoandrosite- (Ce) within the subdivision of “ Group silicates: insular, mixed, single and larger tetrahedral groups with cations in [6] and higher coordination; Single and double groups (n = 1,2) ”.

Crystal structure

Dissakisit- (La) crystallizes monoclinically in the space group (space group no. 11) with the lattice parameters a  = 8.93  Å ; b  = 5.69 Å; c  = 10.18 Å and β = 115.15 ° and 2 formula units per unit cell .


Due to its content of thorium and other slightly radiating isotopes, often added to existing rare earths , Dissakisite- (La) is classified as weakly radioactive and has a specific activity of around 1929  Bq / g (for comparison: natural potassium 31.2 Bq / g ).

Education and Locations

Dissakisite- (La) is formed by hydration and accumulation in LILE (light incompatible elements) and L REE (light rare earth elements) -containing peridotites . Accompanying minerals include various amphiboles and apatites , calcite , dolomite , clino- and orthopyroxenes , olivine , pentlandite , phlogopite , spinel , uraninite , thorite , thorianite , zircon , small amounts of clinochlor and various copper sulphides .

In addition to its type locality Hochwart in Ultental in Italy, the mineral has so far (as of 2011) only been detected at Kyschtym in the Russian Oblast Chelyabinsk in the southern Urals and at Rybníček in Okres Vyškov in the Czech Republic.

See also

Individual evidence

  1. a b IMA / CNMNC List of Mineral Names (English, PDF 1.8 MB)
  2. a b Stefan Weiß: The large Lapis mineral directory . 5th edition. Christian Weise Verlag, Munich 2008, ISBN 3-921656-17-6 .
  3. a b Webmineral - Dissakisite- (La) (English)
  4. a b American Mineralogist Crystal Structure Database - Dissakisite- (La) (English, 2006)
  5. American Mineralogist Crystal Structure Database - Dissakisite- (La) (English, 2006)
  6. Mindat - Dissakisite- (La) (English)
  7. Mineral Atlas: Dissakisite- (La)


Web links