The crystal structure of nikmelnikovite, Ca12Fe2+Fe3+3Al3(SiO4)6(OH)20, a new member of the garnet supergroup from Kovdor massif, Kola Peninsula, Russia (R$\bar{3}$, a = 17.2072(6), c = 10.5689(4) Å, V = 2710.1(2) Å3 and Z = 3) has been refined to R1 = 0.046 on the basis of 1184 unique observed reflections. Nikmelnikovite is the first mineral species in the garnet supergroup that has a trigonal (rhombohedral) symmetry. The relationship between its unit cell and the pseudocubic (ideal garnet) unit cell can be described by the transformation matrix [1$\bar{1}$0 | 01$\bar{1}$ | ½½½]. The crystal-chemical relations between the ideal Ia$\bar{3}$d garnet and the nikmelnikovite structure type can be described by the following series of imaginary modifications: (1) the symmetry is lowered according to the Ia$\bar{3}$d → R$\bar{3}$ group–subgroup relationship; (2) the cation sites are split according to the following sequences: X → {X1, X2}; Y → {Y1, Y2, Y3, Y4}; Z → {Z1, Z2}; (3) the X sites remain fully occupied by Ca; (4) each Y site is occupied predominantly by a distinct chemical species: Y1 → Al (Al site), Y2 → Fe2+ (Fe1 site), Y3 → Fe3+ (Fe2 site), Y4 → vacancy (Mn site); (5) one of the Z sites (Z1) is occupied by Si, whereas the other site (Z2) is predominantly vacant. The crystal-chemical formula that takes into account the transition between the archetype and the nikmelnikovite structure type can be described as X{Ca12}Y[Fe2+Al4Fe3+2□]Z(Si6□6)O24(OH)20□4. The structural complexity of nikmelnikovite (4.529 bit/atom and 434.431 bit/cell, after H-correction) is higher than those for andradite, grossular and katoite, which is typical for low-temperature minerals formed after primary minerals with simpler structures.

俄罗斯科拉半岛科夫多尔地块石榴石超群新成员Ca 12 Fe 2+ Fe 3+ 3 Al 3 (SiO 4 ) 6 (OH) 20的晶体结构（R $\bar{3} $ , a = 17.2072(6), c = 10.5689(4) Å, V = 2710.1(2) Å 3 and Z = 3) 已细化为R 1= 0.046 基于 1184 次观察到的独特反射。Nikmelnikovite 是石榴石超群中第一个具有三角（菱形）对称性的矿物种类。其晶胞与伪立方（理想石榴石）晶胞之间的关系可以用变换矩阵[1 $\bar{1}$ 0 | 01 $\bar{1}$ | ½½½]。理想的Ia $\bar{3}$ d石榴石和 nikmelnikovite 结构类型之间的晶体化学关系可以通过以下一系列假想修改来描述： (1) 对称性根据Ia $\bar{3 }$ d → R $\bar{3}$组-子组关系；(2) 阳离子位点按照以下顺序分裂：X → { X 1, X 2}；Y → { Y 1, Y 2, Y 3, Y 4}; Z → { Z 1, Z 2}; (3) X位点仍然被 Ca 完全占据；(4) 每个Y位点主要由不同的化学物质占据：Y 1 → Al（Al 位点），Y 2 → Fe 2+（Fe1 位点），Y 3 → Fe 3+（Fe2 位点），Y4→空缺（锰位点）；(5) Z位点之一 ( Z 1) 被 Si 占据，而另一个位点 ( Z 2) 主要是空的。考虑到原型和nikmelnikovite结构类型之间转变的晶体化学式可以描述为X {Ca 12 } Y [Fe 2+ Al 4 Fe 3+ 2 □] Z (Si 6 □ 6 )O 24（哦）20 □ 4. nikmelnikovite的结构复杂性（4.529 bit/atom和434.431 bit/cell，H-校正后）高于andradite、grossular和katoite，这是结构较简单的原生矿物后形成的低温矿物的典型特征。