Prachařite, ideally CaSb⁵⁺₂(As³⁺₂O₅)₂O₂·10H₂O, is a new mineral found in underground workings of the Plaka Mine No. 80, Plaka, Lavrion Mining District, Attica, Greece. It occurs as colourless to white, thin tabular hexagonal, in general sharp crystals up to 2.5 mm in diameter, and is associated with pharmacolite, sulphur and very rare smamite {Ca₂Sb(OH)₄[H(AsO₄)₂]·6H₂O} on a matrix composed of sphalerite, galena and carbonate gangue. Prachařite is translucent to transparent, with a glassy lustre, white streak, a good cleavage parallel to {0001} and a distinct cleavage parallel to {10\(\overline{1 }\)0}. It is non-luminescent, brittle, and has an uneven fracture, a Mohs hardness of 2–2.5 and X-ray density D_x = 2.848 g/cm³, D_calc. = 2.836–2.853 g/cm³ (for two measured compositions). Optically, it is uniaxial negative, with ω = 1.619(1) and ε = 1.553(1). Prachařite is trigonal, space group P\(\overline{3 }\)c1 (no. 165), with a = 13.951(2), c = 19.899(2) Å, V = 3354.1(10) ų and Z = 6. Strongest lines in the X-ray powder diffraction pattern are [d in Å (I) hkl]: 9.894 (100) 002; 6.045 (8) 200; 5.156 (10) 202; 4.946 (11) 004; 3.297 (19) 311, 006, 222; 2.988 (22) 400, 313, 116. Two sets of independent electron probe micro-analyses yielded (wt%): CaO 6.28/7.12, MgO 0.09/-, Zn -/0.01, Sb₂O₅ 39.22/40.19, As₂O₃ 47.59/47.39, SO₃ -/0.02, H₂O 21.65/22.04 (calculated on the basis of ideal composition derived from crystal-structure determination), total 114.83/116.77; the total is reproducibly high due to a loss of a third of all water molecules under the electron beam. The empirical formulae, based on O = 22 atoms per formula unit, for the two datasets are very similar, (Ca_0.93Mg_0.02)_Σ0.95Sb_2.02(AsO₃)_4.00·10H₂O and Ca_1.04Sb_2.03(AsO₃)_3.92·10H₂O. The ideal formula is CaSb⁵⁺₂(As³⁺₂O₅)₂O₂·10H₂O, determined with the help of a crystal-structure determination based on single-crystal X-ray diffraction datasets collected at room temperature (R1 = 2.3%). The atomic arrangement of prachařite is unusual; it is based on two different layers containing a six-membered ring of corner-sharing SbO₆ octahedra, an eight-coordinated Ca1 atom in the centre of the ring, two non-equivalent AsO₃ groups corner-linked to form a (As₂O₅)⁴⁻ diarsenite group, and, on interlayer sites, a seven-coordination Ca2 atom and three water molecules (all only weakly hydrogen-bonded), one of which is only partially occupied (split position). The mineral is named in honour of Dr Ivan Prachař, a long-term researcher of the mineralogy and underground workings of Lavrion.

Prachařite，理想情况下是 CaSb ⁵⁺ ₂ (As ³⁺ ₂ O ₅ ) ₂ O ₂ ·10H ₂ O，是在希腊阿提卡 Lavrion 矿区普拉卡 80 号普拉卡矿的地下矿井中发现的一种新矿物。它以无色至白色的薄板状六角形晶体形式出现，通常为直径达 2.5 毫米的尖锐晶体，与药石、硫和非常稀有的磷钾石 {Ca 2 Sb(OH) 4 [ _H ( _AsO 4 ₎ 2 _] · 6小时₂O}在由闪锌矿、方铅矿和碳酸盐脉石组成的基质上。Prachařite半透明至透明，具有玻璃光泽，白色条纹，平行于{0001}的解理良好，平行于{10 \(\overline{1 }\) 0}的解理明显。它不发光，脆，断口不均匀，莫氏硬度为2-2.5，X射线密度D _x  = 2.848 g/cm ³，D_计算值。 = 2.836–2.853 g/cm ³（对于两种测量的成分）。在光学上，它是单轴负的，ω = 1.619(1) 和 ε = 1.553(1)。Prachařite 是三方晶系，空间群P \(\overline{3 }\) c 1（编号 165），其中a  = 13.951(2)，c = 19.899(2) Å，V  = 3354.1(10) Å ³和Z  = 6。X 射线粉末衍射图中最强的线是 [ d in Å ( I ) hkl ]：9.894 (100) 002；6.045 (8) 200；5.156 (10) 202；4.946 (11) 004；3.297 (19) 311、006、222；2.988 (22) 400, 313, 116。两组独立的电子探针微量分析产生 (wt%)：CaO 6.28/7.12，MgO 0.09/-，Zn -/0.01，Sb 2 O 5 39.22 _/ 40.19 _， As ₂ O ₃ 47.59/47.39，SO ₃ -/0.02，H ₂O 21.65/22.04（根据晶体结构测定得出的理想成分计算），总计114.83/116.77；由于在电子束下损失了所有水分子的三分之一，因此总重现性很高。基于 O = 22 个原子/分子式单元，两个数据集的经验公式非常相似，(Ca 0.93 _Mg 0.02 ₎ Σ0.95 _Sb 2.02 ₍ AsO ₃ ) _4.00 ·10H ₂ O 和 Ca _1.04 Sb _2.03 (AsO ₃ ) _3.92 ·10H ₂ O。理想的分子式为CaSb ⁵⁺ ₂ (As ³⁺ ₂ O₅ ) ₂ O ₂ ·10H ₂ O，借助于基于在室温下收集的单晶X射线衍射数据集的晶体结构测定来确定( R 1 = 2.3%)。prachařite 的原子排列是不寻常的；它基于两个不同的层，包含一个六元环的 SbO ₆八面体，环中心有一个八配位的 Ca1 原子，两个不等价的 AsO ₃基团角连接形成一个（As ₂ O ₅ ) ⁴⁻二亚砷酸盐基团，以及在层间位点上，一个七配位的 Ca2 原子和三个水分子（都只有弱氢键），其中一个仅被部分占据（分裂位置）。该矿物以 Ivan Prachař 博士的名字命名，他是 Lavrion 矿物学和地下工作的长期研究员。