北投石是唯一以臺灣的地名所命名之礦物，是一種溫泉沉積生長具放射性之稀有 礦物，於 1907 年首次為日本人岡本要八郎在臺北的北投溪中發現。比較成分範圍不同之三 處出產天然北投石, 顯示溫泉中的 Pb/Ba 之比值愈高者， 北投石中 PbSO �痕漣t量也愈高 。再者，本研究利用硝酸鉛、硝酸鋇及硫酸銨等溶液為起始原料合成不同硫酸鋇與硫酸鋇比 例之北投石晶體。 以三種合成北投石的方法合成之北投石， 其成分涵蓋了所有 PbSO �祥P BaSO �略妝T溶範圍， 有異於前人認為固溶範圍侷限在 (Pb.Ba.)SO �� (Pb.Ba.)SO �略孜� 。 故可推斷北投石之成分與溫泉中的 Pb/Ba 比例有關。根據加熱試驗、成分分析與人工合 成之結果， 推斷北投石結晶顆粒內所存在之環帶狀現象與 Pb/Ba 比值無關，而溫泉中之雜 質離子進入北投石晶體中應是呈現暗帶之原因。北投石為 PbSO �祥P BaSO �略妝T溶體化合 物，Ba 原子取代 Pb 及 Ba 原子取代 Pb 之過程，使 a 軸長度有變大之趨勢。而根據北投 石晶體結構之解析結果， 其結構型式與重晶石、硫酸鉛礦同為 Pnma 之空間群對稱，Pb 及 Ba 原子位於相同之原子位置，即兩者呈現非序位化之相互取代現象。 北投石晶體受熱至約 960 ℃以上時會發生分解作用，PbSO �畢角懋|因分解而先發揮，並使晶體失透化。 此外， 北投石可成為耐強酸與強鹼的材料。

　　　　　Hokutolite, a radioactive mineral of hot-spring deposit and the only known mineral species named after its locality in Taiwan, was first discovered at Peitou(Hokuto) in 1907. Hokutolite is generally regarded as a lead-bearing barite. The mineral of similar occurrence has been found at three localities in the world. The fact is found that the dissimilarity in the chemical composition of the solid solution of hokutolite among them and the PbSO �� contents increase in proportion to the Pb/ Ba concentration ratio in these hot-spring waters. Besides, through the chemical reaction in precipitation process, agar gel and gelatin gel, complete solid solution of BaSO �� and PbSO �� in synthetic hokutolite crystals was grown in the present study with the solutions of Pb(NO �� ) �� Ba(NO �� ) �� and (NH �� ) �� SO �� as starting materials. Therefore, this work suggests that the chemical compositions of natural hokutolite have been controlled by the Pb/Ba ratio in the hotspring source. It is inferred that solid solution may exists in natural hokutolite and the limited PbSO �� (20to 35 mol%)contents as previously reported do not agree with the observed results. Hokutolite crystal shows a zonal structure which are concentric rhombic bands exactly in parallel with the crystal outlines. Through the heating process and chemical analysis, the present study suggests that the formation of the zonal structure may be attributable to the incorporation of cations such as Fe3+,Mn3+(impurities) into the hokutolite structure. It is therefore obvious that the Pb/Ba concentration ratio may not lead to the formation of zonal structure. The value of a0 axis increases with the substitution of Ba for Pb atom in hokutolite. The crystal structure of hokutolite is similar to those of barite (BaSO �� ) and anglesite (PbSO �� ),with a space group of Pnma. The Pb and Ba atoms were determined to be located at the same atomic positional coordinates. In other words, the Ba and Pb atoms seem to be a disordering substitution in the crystal structure of hokutolite. The work indicates that the thermal decomposition and vaporization occur above 960 ℃, and consequently the opaque crystals containing the pure BaSO �� were formed. In addition, It is shown that the hokutolite crystals hardly dissolve in strong acid and basic solutions.