Micro hard ness anal y sis of ha lite from dif fer ent salt-bear ing for ma tions
Tomasz TOBO£A1, *, Katarzyna CYRAN2 and Marek REMBIŒ1
1 AGH Uni ver sity of Sci ence and Tech nol ogy, Fac ulty of Ge ol ogy, Geo phys ics and En vi ron men tal Pro tec tion, al. A. Mickiewicza 30, 30-059 Kraków, Po land
2 AGH Uni ver sity of Sci ence and Tech nol ogy, Fac ulty of Min ing and Geoengineering, al. A. Mickiewicza 30, 30-059 Kraków, Po land
Tobo³a, T., Cyran, K., Rembiœ, M., 2019. Micro hard ness anal y sis of ha lite from dif fer ent salt-bear ing for ma tions. Geo log i cal Quar terly, 63 (4): 771–785, doi: 10.7306/gq.1499
As so ci ate ed i tor: Wojciech Drzewicki
Micro hard ness tests were car ried out on sin gle ha lite crys tals. They were con ducted on the (001) sur face, with the in denter set in two di rec tions: par al lel to the ha lite face (010); and par al lel to the (110) face. The ha lite crys tals rep re sent salt for ma - tions of dif fer ent ages (De vo nian, Zechstein, Badenian), depths (from 1835.5 to 195 m) and in ten si ties of tec tonic dis tur - bance (hor i zon tally strat i fied, salt dome, strongly folded). The mea sure ment re sults re vealed spe cific fea tures of the ha lite crys tals ana lysed. Firstly, the data ob tained show micro hard ness ani so tropy in ha lite crys tals. More over, micro hard ness cor - re lates with the depth of the salt-bear ing for ma tions. Ha lite crys tals from deeper lev els showed higher micro hard ness, though there was no cor re la tion be tween the in ten sity of tec tonic de for ma tion and the av er age Vickers hard ness (HV). The sam ples ana lysed show a vari abil ity of HV val ues and and of the shapes of im prints. These in di cate zones where ha lite crys - tals are de formed at the atomic level and re flects the pres ence of de fects in the crys tal lat tice. Such de for ma tion is re flected in an ir reg u lar ity of the strike of cleav age planes. Con se quently, the anal y sis of im print shapes is a use ful method for the ex - am i na tion of duc tile min er als and ma te ri als.
Key words: micro hard ness, ha lite, salt for ma tion, crys tal de fects, dis lo ca tions, shape of im print.
INTRODUCTION
Rock salt has been ex ten sively stud ied over re cent years as re gards its unique me chan i cal prop er ties. Due to these, rock salt is of ten con sid ered for geotechnical and in dus trial ap pli ca - tions e.g., stor age cav erns. In con ven tional lab o ra tory tests, rock salt shows both brit tle and duc tile be hav iour. It ex hib its a brit tle be hav iour up to the elas tic stress limit; when load ing ex - ceeds the elas tic lim its. plas tic be hav iour is shown via duc tile de for ma tion. Duc tile rock salt be hav ior re sults from its crys tal - line struc ture, the in ter nal struc ture of ha lite crys tals, im pu ri ties con tent and dis tri bu tion as well as the pres ence of fluid in clu - sions (e.g., Carter and Hansen, 1983; Fossum et al., 1993;
Jeremic, 1994; Hunsche and Hampel, 1999; Schulze et al., 2001; Liang et al., 2012; Cyran et al., 2016). More over, rock salt con trib utes to the evo lu tion of salt struc tures, af fects, re gional tec ton ics and lu bri cates de for ma tion struc tures in sed i men tary bas ins (Jack son and Hudec, 2017).
Un like most rock-form ing min er als, ha lite shows no tice able rhe o log i cal prop er ties even at room tem per a ture and has the abil ity to recrystallize, par tic u larly in the pres ence of wa ter (Carter and Hansen, 1983; Drury and Urai, 1990; Carter et al., 1993; Hunsche et al., 1996; Schléder and Urai, 2005, 2007;
Desbois et al., 2012). The de for ma tion of ha lite sin gle crys tals has been stud ied by many re search ers, in clud ing Buerger (1930a, b, c), Christy (1956), Hadin and Hager (1958) and Shlichta (1968). It was ob served that ha lite crys tals de form by slip along crys tal lo graphic planes in two slip sys tems: the pri - mary slip sys tem is along the crys tal lo graphic plane (100) with sec ond ary slip sys tems (110) and (111). More over, Carter and Heard (1970) and Davidge and Pratt (1964) noted that the pri - mary slip sys tem is fa voured by lower stress and tem per a tures.
The tem per a ture de pend ence of the sec ond ary slip sys tems is much higher.
The in ner struc ture of ha lite un der goes de for ma tion un der stress in nat u ral con di tions. The re sult ing de for ma tion re sults in de fects of the crys tal lat tice. A de vi a tion from cu bic sym me try in ha lite crys tals was in di cated in blue ha lite (Zelek et al., 2008, 2014). An other re sult of de fects in a crys tal lat tice is bi refrin - gence de ter mined in blue ha lite crys tals (Buerger, 1930a;
Shlichta, 1968; Carter and Hansen, 1983; Sonnenfeld, 1995).
In den ta tion tech niques (micro hard ness tests) has been widely used as a non-de struc tive method of as sess ing the me - chan i cal prop er ties of ma te ri als. Un der stand ing why ma te ri als
* Cor re spond ing au thor, e-mail: tob@geol.agh.edu.pl
Re ceived: January 7, 2019; ac cepted: September 16, 2019; first pub lished on line: December 11, 2019
dif fer in hard ness and in deed what hard ness ac tu ally is, led to wide spread adop tion of in den ta tion meth ods in the lab o ra tory.
In or der to pre serve geo met ri cal sim i lar ity dur ing an in den ta - tion, in dent ers in the shape of a four-sided di a mond pyr a mid (the Vickers test) and three-sided pyr a mid (Berkovic test) were de vel oped (Walley, 2012). In in den ta tion tests of ma te ri als that ex hibit rhe o log i cal prop er ties, plas tic de for ma tion must be con - sid ered along with elas tic de for ma tion (Atkinson et al., 2013).
Con se quently, ha lite in micro hard ness tests should be con sid - ered as elas tic–plas tic ma te rial.
The re search de scribed in this pa per com pares the micro - hard ness of ha lite crys tals from dif fer ent salt-bear ing for ma tions.
These salt-bear ing for ma tions oc cur at dif fer ent depths, have dis tinct geo log i cal and tec tonic his to ries and are built of ha lite crys tals that un der went var i ous con di tions of stress and tem per - a ture. These con di tions had an im pact on the ha lite crys tals, re - sult ing in their de for ma tion in clud ing the cre ation of lat tice de - fects. These ha lite lat tice de fects should there fore be re flected in the re sults of micro hard ness tests. Fur ther more, micro hard ness in ha lite crys tals has so far been lit tle stud ied (Brooks et al., 1975;
Shakoor and Hume, 1981; Aptukov et al., 2010).
MATERIALS
Micro hard ness mea sure ments were car ried out on ha lite crys tals col lected from: the Lotsberg Salt For ma tion (Can ada, De vo nian); the Fore-Sudetic Monocline (Po land, Zechstein);
the K³odawa Salt Dome (Po land, Zechstein); and the Wieliczka and Bochnia Salt Mines (Po land, Badenian). The Lotsberg Salt For ma tion lo cated in Al berta is the old est salt for ma tion in the Ca na dian area (Fig.1A). It be longs to the sed i men tary rock se -
quence known as the Elk Point Group (Wardlaw and Wat son, 1966). It is un der lain by the Basal Red Beds and over lain by the Ernesta Lake For ma tion. The Lotsberg Salt For ma tion lies hor i - zon tally, and is un dis turbed by tec ton ics. It is sub di vided into a lower and up per part, sep a rated by a layer of red dolomitic shale. In both parts, the salt rocks are com posed of coarse- grained ha lite with a small amount of do lo mite and clay im pu ri - ties (Kukia³ka and Tobo³a, 2018). The ha lite crys tals range in size from ~1 cm to a few centi metres. The micro hard ness test sam ple was taken from the Pembina 22A bore hole at depth 1835.5 m.
The sec ond sam ple ana lysed was taken from the Old est Ha - lite (Na1) oc cur ring in the Lubin–G³ogów Cop per Dis trict (LGOM;
Fig. 1B). The Old est Ha lite is rep re sented by ha lite crys tals up to 15 cm in size which oc cur as veins or nests within the Na1 pri - mary salts (Tobo³a and Markiewicz, 2009). The LGOM is part of the Fore-Sudetic Monocline and it is built of Perm ian and Tri as sic strata. The Perm ian unit is sub di vided into ter res trial (Rotliegend) and ma rine (Zechstein) de pos its. The Zechstein de pos its are com posed of four evaporite cyclothems but only the low est cyclothem (PZ1) is fully de vel oped and in cludes a rock salt layer (Na1) in the LGOM area (K³apciñski, 1964a,b, 1966, 1971;
Szybist, 1976; Peryt, 1981). The salt layer (Na1) strikes WNW–ESE and dips at 3–8° (lo cally 15°) to ward the NE. The in - ter nal struc ture of the salt layer is com pli cated with var i ous types of folds, shear zones, flex ures and fault zones that in di cate large-scale salt move ment (Banaszak et al., 2007; Burliga, 2007;
Markiewicz, 2007; Tobo ³a, 2014).
The third sam ple rep re sents a “blue ha lite” (Natkaniec- Nowak and Tobo³a, 2003; Tobo³a et al., 2007; Tobo³a and Natkaniec-Nowak, 2008; Wese³ucha-Birczyñska et al., 2008;
Zelek et al., 2008, 2014; Tobo³a, 2016) while the fourth sam ple
772 Tomasz Tobo³a, Katarzyna Cyran and Marek Rembiœ
Fig. 1A – dis tri bu tion of rock salt within the De vo nian Elk Point Group in Al berta (Grobe, 2000); B – tec tonic units in Po land with lo ca tion of salt de pos its
was taken from bi tu mi nous salts (Tobo³a, 2010;
Wese³ucha-Birczyñska and Tobo³a, 2016). Both types of salt oc cur in the K³odawa Salt Dome (cen tral Po land) in the form of veins or nests within al most all the Zechstein de pos its (Fig. 1B).
The K³odawa Salt Dome is the larg est salt struc ture in Po land. It be longs to the Izbica Kujawska–£êczyca salt ridge (e.g., Werner et al., 1960; Wachowiak and Tobo³a, 2014; Tobo³a, 2016; Tobo³a and Wachowiak, 2018). The length of the dome is
~26 km in the NW–SE di rec tion and its width var ies from 0.5 to 2 km. The salt dome is built of rocks rep re sent ing the fully de vel - oped Zechstein PZ2–PZ4 cyclothems com posed of claystones, dolomites, an hyd rites, rock salts and K-Mg salts. The low est cyclothem PZ1 is known as tec toni cally trans ported blocks (Charysz, 1973; Burliga et al., 1995). All these de pos its were highly dis turbed by halotectonic and halokinetic pro cesses dur - ing up lift of the dome.
The fifth and sixth sam ples were taken from the Wieliczka and Bochnia salt de pos its re spec tively (Fig. 1B). These sam ples rep re sent pri mary ha lite crys tals from the brecciated part of the Wieliczka salt de posit and the Mid dle Salt mem ber of the Bochnia salt de posit. Both salt de pos its are lo cated at the front of the Carpathian Thrust, within a nar row belt of folded Badenian strata, re garded as an allochthonous unit (e.g., Poborski and Skoczylas-Ciszewska, 1963; Ney et al., 1974; Garlicki, 1979).
The de vel op ment of this unit and of the Badenian salt de pos its (Wieliczka, Bochnia) is closely con nected with Carpathian orogenic move ments. There fore, the Wieliczka and Bochnia salt de pos its are strongly elon gated in an E–W di rec tion and nar row (up to 1 km) in a N–S di rec tion. The in ner struc ture of these de - pos its is com pli cated due to strong fold ing (Poborski, 1952;
Wiewórka, 1984; Cyran and Tobo³a, 2006, 2007).
METHODS
The micro hard ness tests were per formed on the ha lite sam - ples pre pared as thick sec tions (plates). The thick plates were ob tained by cut ting the sin gle ha lite crys tals par al lel to their cleav age. The di men sions of the thick plates were: thick ness
~3 mm, length and width ~2 cm. Both sur faces of the thick sec - tions were man u ally gently pol ished on felt with the ad di tion of a di a mond sus pen sion. The qual ity of pol ished sur faces was con - trolled un der a mi cro scope in re flected light.
The re sis tance of the ha lite crys tals to the stress ex erted by ex ter nal pres sure act ing on a point was mea sured as their micro - hard ness (HV). The in den ta tion tests were con ducted us ing a hard ness tester Testlab HVKD-1000IS with a py ram i dal di a mond in denter (the Vickers method). In or der to choose the op ti mal load, three force val ues were tested: 0.25 kgf = 0.245 N, 0.5 kgf = 0.49 N, 1.0 kgf = 0.98N. The op ti mal load al lowed ob ser va tions of in den ta tion marks un der the mi cro scope with a 40´ ob jec tive.
The value of 0.5 kgf = 0.49 N was cho sen. Next, the cho sen load was cal i brated based on the micro hard ness stan dard (pro vided by the man u fac turer). The micro hard ness was ex pressed as a quo tient of the in denter load (0.5 kgf = 0.49 N, in dent ing time = 10 sec.) and in den ta tion depth cal cu lat ing as a trig o no met ric re - la tion from the lengths of the di ag o nals of the in den ta tion marks (im prints). The in den ta tion tests were per formed in two ver sions.
In the first ver sion mea sure ments were made in the ar eas par al - lel to the (100) and (010) faces of the ha lite crys tal. In each area, mea sure ments were per formed along three lines that con sist of 10 points. The lines were spaced at a dis tance of 1 mm from each other and the dis tance be tween in den ta tion points in each line was also 1 mm. Thereby, 30 micro hard ness val ues (HV) were ob tained for each sam ple ana lysed and con verted into MPa. In the sec ond ver sion of the in den ta tion test, mea sure -
ments were per formed along two ad di tional lines for three se - lected sam ples. Lines were ar ranged par al lel to the (110) and (11'0) faces of the ha lite crys tals. The lines and points were sim i - larly spaced 1 mm apart, pro vid ing, an ad di tional 20 micro hard - ness (HV0.5) val ues for each sam ple. All mea sure ment re sults were used for cal cu lat ing ba sic sta tis ti cal pa ram e ters. More over, the shapes of the in den ta tion marks (im prints) were ana lysed for all sam ples. At ten tion was paid to all dis tor tions in the shape of an im print, which were clas si fied ac cord ing to the de scrip tion of Craig and Vaughan (1994). In or der to check the ir reg u lar i ties ob served in the in den ta tion marks, the lengths of the di ag o nals and po si tion of the geo met ric cen tre were ana lysed. The lengths of the two di ag o nals (Dh, Dv on Fig. 2) re corded in the in den ta tion tests were com pared in each in den ta tion mark for each sam ple ana lysed. The ra tios be tween the two di ag o nals were cal cu lated and shown on the graphs. The po si tion of the geo met ric cen tre in each in den ta tion mark and its shift in the hor i zon tal and ver ti cal di rec tion was mea sured ac cord ing to the meth od ol ogy shown in Fig ure 2. The shift in a hor i zon tal and ver ti cal di rec tion was cal - cu lated as ra tios of Dh1/Dh and Dv1/Dv, re spec tively. In this way, when there was no shift of a geo met ric cen tre, the ra tio is 0.5.
RESULTS
The re sults of the in den ta tion tests are dis played as maps (Fig. 3), which show, with help of a col our scale, the range of mea sured HV in in den ta tion points. As a re sult, there is no in for - ma tion about HV val ues in the area be tween these points. For this rea son, in or der to es ti mate HV val ues be tween in den ta tion points, a Kriging method was ap plied. The Kriging method en - ables pre dic tion of the value of a func tion at a given point by com put ing a weighted av er age based on the sur round ing mea - sured val ues.
The re sults of per formed in den ta tion tests in di cated a large vari abil ity in av er age micro hard ness (HV), as cal cu lated for all sam ples ana lysed. A large vari a tion in micro hard ness (HV) value and its dis tri bu tion within each thin sec tion was no - ticed for most sam ples ana lysed. More over, the re corded in - Fig. 2. Mea sure ment meth od ol ogy of a shift in the geo met ric
cen tre of an im print
774 Tomasz Tobo³a, Katarzyna Cyran and Marek Rembiœ
Fig. 3. Micro hard ness dis tri bu tion within the sam ples (maps for the first ver sion of the in den ta tion test)
den ta tion shapes (im prints) show some ir reg u lar i ties in all sam ples ana lysed.
In the first test ver sion, the high est HV val ues from all sam - ples ana lysed, both in in den ta tion points and in the av er age value of HV, were re corded in sam ple no. 1 from the Lotsberg Fm. The val ues re corded in in den ta tion points reached above 196.1 MPa with a max i mum of 252.0 MPa (Ta ble 1 and Fig. 3A), ex cept for one point in which the HV value was 176.5 MPa. HV val ues re corded in in den ta tion points that are shown in Fig ure 3A show ir reg u lar zonal dis tri bu tion within the sam ple (Fig. 3A). Ar eas char ac ter ized by high HV are sep a -
rated by zones of low HV. Each zone con sists of in ter vals rep re - sented by sev eral points char ac ter ized by the same HV value (Fig. 3A). Ar eas of high HV are elon gated in the left part of the map but in the right part there are two small ar eas di vided by a low HV zone. More over, high vari abil ity in HV value is vis i ble on the right side of the map (Fig. 3A).
The shapes of im prints in the ha lite from Pembina are the most ir reg u lar among all the sam ples ana lysed in the first test ver sion. The im prints are elon gated lat er ally (Figs. 4A–C and 5A). The geo met ric cen ter of the im print is shifted lat er ally and slightly ver ti cally (Fig. 4A–C). The ir reg u lar shape of im prints is
First test ver sion Sec ond test ver sion
Sam ple no. 1 2 3 4 5 6 2 4 6
Lo ca tion Pembina LGOM K³odawa
blue ha lite
K³odawa bi - tu mi nous
salts Wieliczka Bochnia LGOM
K³odawa bi - tu mi nous
salts Bochnia
Depth [m] 1835.5 950.0 630.0 720.0 135.0 195.0 950.0 720.0 195.0
In den ta tion
point HV value [MPa]
1 234.4 210.9 198.1 212.8 181.4 181.4 188.3 192.2 163.8
2 233.4 207.9 221.6 198.1 177.5 200.1 182.4 193.2 165.7
3 218.7 207.9 170.6 209.9 193.2 192.2 182.4 195.2 163.8
4 211.8 206.9 176.5 204.0 177.5 179.5 181.4 183.4 162.8
5 216.7 204.0 176.5 197.1 162.8 182.4 183.4 200.1 161.8
6 200.1 205.9 191.2 204.0 171.6 148.1 185.4 200.1 164.8
7 214.8 205.9 186.3 202.0 179.5 170.6 182.4 196.1 163.8
8 239.3 208.9 180.4 223.6 164.8 176.5 189.3 191.2 167.7
9 176.5 199.1 205.0 203.0 163.8 180.4 186.3 187.3 160.8
10 217.7 203.0 222.6 197.1 163.8 178.5 189.3 188.3 162.8
11 214.8 205.0 194.2 202.0 247.1* 173.6 188.3 226.5 159.9
12 220.7 200.1 197.1 200.1 160.8 181.4 188.3 211.8 161.8
13 230.5 204.0 172.6 194.2 162.8 179.5 177.5 199.1 160.8
14 234.4 200.1 192.2 192.2 160.8 174.6 183.4 189.3 161.8
15 228.5 190.3 191.2 210.9 172.6 172.6 181.4 176.5 160.8
16 202.0 205.9 186.3 215.8 158.9 169.7 179.5 182.4 165.7
17 214.8 205.9 202.0 206.9 165.7 170.6 178.5 186.3 163.8
18 205.9 201.0 185.4 214.8 158.9 167.7 183.4 187.3 173.6
19 227.5 202.0 198.1 208.9 166.7 171.6 180.4 184.4 162.8
20 252.0 206.9 189.3 196.1 190.3 169.7 185.4 188.3 160.8
21 218.7 204.0 198.1 206.9 173.6 165.7
22 221.6 206.9 206.9 194.2 182.4 169.7
23 201.0 205.9 212.8 224.6 160.8 166.7
24 205.0 199.1 191.2 204.0 160.8 164.8
25 221.6 201.0 187.3 219.7 162.8 169.7
26 223.6 201.0 189.3 214.8 290.3* 170.6
27 213.8 207.9 177.5 209.9 254.0* 170.6
28 231.4 197.1 184.4 227.5 194.2 176.5
29 207.9 203.0 204.0 183.4 165.7 167.7
30 205.0 189.3 190.3 196.1 167.7 166.7
Av er age 218.1 203.2 192.6 205.8 170.4 173.6 183.8 193.0 163.5
x ±5.59 ±1.88 ±4.94 ±3.97 ±4.20 ±3.51 ±1.69 ±5.21 ±1.45
Me 218.2 204.0 191.2 204.0 165.7 171.1 183.4 190.3 162.8
s 14.4 4.9 12.8 10.2 10.4 9.1 3.5 10.8 3.0
CV 6.6 2.4 6.6 5.0 6.1 5.2 1.9 5.6 1.8
* – re jected due to ab nor mally high val ues caused by the ingrowth of anhydrite crys tals; x – mea sure ment er ror de ter mined by the Stu - dent-Fisher method; Me – me dian; s – stan dard de vi a tion; CV– co ef fi cient of vari a tion
T a b l e 1 HV val ues reg is tered in the in den ta tion test
776 Tomasz Tobo³a, Katarzyna Cyran and Marek Rembiœ
Fig. 4. The shapes of im prints reg is tered in the first test ver sion [par al lel to the (100) and (010) faces]
A–C – Pembina; D–F – LGOM, G–I – K³odawa blue ha lite; J–L – K³odawa bi tu mi nous salts; M–O – Wieliczka; P–S – Bochnia
fur ther in di cated by the ra tio be tween di ag o nals which var ies from 0.86 to 1.10 (Fig. 6A). A con vex shape of the im print edges (Fig. 4A) pre vails in the vast ma jor ity of in den ta tion points, though, a few im prints are slightly sigmoidal (Fig. 4B,C). How - ever, the point at which the HV value was the low est showed a more reg u lar shape (Fig. 4A).
High HV val ues were reg is tered in sam ple no. 2 from the LGOM (Ta ble 1 and Fig. 3B). The av er age HV re corded in this sam ple was 203.0 MPa. This sam ple is char ac ter ized by the low est vari abil ity of HV val ues among all sam ples ana lysed. HV val ues reg is tered in in den ta tion points range from 189.3 MPa to 210.9 MPa (Ta ble 1). In the map, there are only four HV in ter - vals ar ranged in sev eral ir reg u lar zones (Fig. 3B). The in ter vals of the low est HV val ues are lo cated in the cen tral area and in the right lower cor ner of the map. The zones dis tin guished by high HV com prise the top left and right parts of the map (Fig. 3B).
All in den ta tion marks re corded in sam ple no. 2 are dis tin - guished from other im prints ana lysed in the first test ver sion by a reg u lar shape (Fig. 4D–F). The ra tio be tween di ag o nals is
close to 1.0 in most cases (Fig. 5B). How ever, some ir reg u lar i - ties in im print shapes are vis i ble. There is a slight lat eral shift of the geo met ric cen tre (Fig. 6B) and the edges are slightly con - vex and sigmoidal (Fig. 4 E, F).
In the K³odawa blue ha lite sam ple, zonation and large vari - abil ity of HV val ues is clearly vis i ble. The HV val ues mea sured at all in den ta tion points are the range of 170.6 and 222.6 MPa while the av er age HV reached a value of 192.2 MPa (Ta ble 1).
There are two cir cu lar ar eas of high HV lo cated in the right up - per and left up per cor ners of the map (Fig. 3C). An elon gated zone of the low est HV is sit u ated at the left side of the map near an area of higher HV val ues. Gen er ally, the cen tral area of the map is dom i nated by in ter vals of low HV. How ever, in den ta tion points marked by high HV are grouped in the left and right parts of the map (Fig. 3C).
In blue ha lite from K³odawa (sam ple no. 3) an ir reg u lar shape of the im prints dom i nates (Fig. 4G–I). The dis tor tion of im print shape is re flected by elon ga tion in the lat eral and ver ti - cal di rec tions. This ir reg u lar ity of im print shape is re flected by Fig. 5. The ra tio be tween the length of di ag o nals re corded in the first test ver sion
the ra tio be tween di ag o nals which var ies from 0.86 to 1.08 (Fig. 5C) and by the shift of the geo met ric cen tre (Fig. 6C). The edges of im prints are very ir reg u lar, mostly con vex (Fig. 4G, H) and slightly sigmoidal (Fig. 4I).
The micro hard ness char ac ter is tics of bi tu mi nous ha lite from K³odawa (sam ple no. 4) are dif fer ent from that de scribed for the blue ha lite. The av er age HV of bi tu mi nous ha lite is like that cal cu lated for the LGOM and reaches 205.80 MPa. How - ever, HV val ues re corded in in den ta tion points range from 183.4 to 227.5 MPa. This va ri ety is re flected in the map (Fig. 3D) where sev eral ir reg u lar zones marked by dif fer ent HV in ter vals are vis i ble. The zones of high HV are con cen trated in the lower part of a map. The points at which the low est HV were
cal cu lated are lo cated in the bot tom right and a left cen tral ar eas of the map (Fig. 3D).
The shape of im prints in sam ple no. 4 are also very ir reg u - lar. The edges of im prints re corded for bi tu mi nous ha lite are more ir reg u lar than the edges in blue ha lite (Fig. 4J–L). The edges of im prints are char ac ter ized by sigmoidal and slightly con vex shapes (Fig. 4J–L). The ir reg u lar shape of an im print is re flected by the ra tio be tween di ag o nals that var ies from 0.88 to 1.10 (Fig. 5D) and a shift of the geo met ric cen tre in both lat eral and ver ti cal di rec tions (Fig. 6D).
The av er age HV cal cu lated for sam ple no. 5 from Wieliczka is lower than val ues of other sam ples and amounts to 179.5 MPa. HV val ues mea sured in in den ta tion points vary and
778 Tomasz Tobo³a, Katarzyna Cyran and Marek Rembiœ
Fig. 6. Shifts in the geo met ric cen tre of im prints. Di a gram for the first test ver sion (par al lel to the (100) and (010) faces)
range from 158.9 to 194.2 MPa (Fig. 3E). The dis tri bu tion of HV in ter vals within the sam ple shown in the map is ir reg u lar but dom i nated by zones of low HV val ues. The in ter vals of the high - est HV com prise three cir cu lar ar eas at the top, bot tom and right side of the map. Con se quently, in ter vals of low HV val ues fill the re main ing space in the map (Fig. 3E).
The im prints in sam ple no. 5 are dis tin guished from other sam ples ana lysed in the first test ver sion. The shape of im prints ap pears to be elon gated in a ver ti cal di rec tion at first sight (Fig. 4M–O). How ever, dis tor tion in the shape of im prints is caused pri mar ily by a shift of the geo met ric cen tre (Fig. 6E). Ad - di tion ally, the ra tio be tween di ag o nals ranges from 0.92 to 1.08 (Fig. 5F). The edges of im prints are mostly sigmoidal, con cave and more reg u lar than in the sam ples from K³odawa (Fig. 4M–O).
The low est av er age HV was cal cu lated for sam ple no. 6 from Bochnia (Ta ble 1 and Fig. 4) and amounts to 173.6 MPa.
Al though the HV re corded in the in den ta tion points ranges from 148.1 to 200.1 MPa, the great ma jor ity of HV val ues are be low 176.5 MPa. This is re flected in the dis tri bu tion of HV in ter vals within the sam ple shown in the map (Ta ble 1 and Fig. 3F).
There is only one area of high HV value vis i ble, in the left top of the map. Low HV in ter vals are lo cated in the cen tral and right parts of the map (Fig. 3F).
The in den ta tion shapes in sam ple no. 6 are more reg u lar (Fig. 4P–S) than im prints from K³odawa, Pembina and Wieliczka. The reg u lar shape is re flected by the ra tio be tween di ag o nals that is close to 1.0 (Fig. 5E). How ever, there are some ir reg u lar i ties in the edges of the im prints (Fig. 4P–S) as well as a slight shift of the geo met ric cen tre mostly in a lat eral di - rec tion (Fig. 6F). A con vex shape of edges dom i nates in most im prints.
In the sec ond test ver sion, fea tures such as ir reg u lar dis tri - bu tion of HV in ter vals, high vari abil ity of HV val ues in in den ta - tion points and dis tor tion of im prints were ob served.
The av er age HV val ues cal cu lated in the sec ond test ver - sion are slightly lower than in the first test ver sion for all sam ples ana lysed. More over, the range of HV val ues cal cu lated in in - den ta tion points is nar rower com pared to the re sults from the first test ver sion.
In the sam ple from the LGOM, the av er age HV re corded in the in den ta tion points is 183.4 MPa. As in the first test ver sion, the sam ple from the LGOM is char ac ter ized by the low est vari - abil ity of HV val ues among all sam ples ana lysed and ranges from 177.5 to 189.3 MPa (Ta ble 1 and Fig. 7A). In the map, the low est HV in ter vals are lo cated in two zones at the top of the map. How ever, ir reg u lar zones of high HV com prise the left side and right bot tom of the map (Fig. 7A).
The shape of im prints in sam ple no. 7 is rather reg u lar as in the first test ver sion (Fig. 8A–C). The reg u lar ity of an im print is marked by a ra tio be tween di ag o nals that is close to 1.0 (Fig. 9A). How ever, there are some slight dis tor tions in the shape of edges that are slightly con cave, with a slight shift of the geo met ric cen tre in a lat eral and ver ti cal di rec tion (Fig. 10A).
The high est av er age HV (193.0 MPa) in the sec ond test ver sion was reg is tered for bi tu mi nous ha lite from K³odawa. HV val ues re corded in the in den ta tion points com prise a wide va ri - ety of val ues from 176.5 to 226.5 MPa (Fig. 7B). A map of HV reg is tered in the in den ta tion points shows sev eral ir reg u lar zones marked by dif fer ent HV in ter vals. The zones of high HV are lo cated in the left part and at the bot tom of the map (Fig. 7B). The points at which the low est HV were re corded are grouped in the top cen tral area of the map.
Fig. 7. Micro hard ness dis tri bu tion within the sam ples (maps for the sec ond ver sion of the in den ta tion test)
780 Tomasz Tobo³a, Katarzyna Cyran and Marek Rembiœ
Fig. 8. The shape of im prints reg is tered in the sec ond test ver sion (par al lel to the (110) and (11'0) faces): A–C–
LGOM, D–F – bi tu mi nous salts, K³odawa; G–I – Bochnia
Fig. 9. Ra tio be tween the length of di ag o nals re corded in the sec ond test ver sion
The shape of im prints in bi tu mi nous ha lite from K³odawa is very ir reg u lar. The edges of im prints are sigmoidal and slightly con cave (Fig. 8D, E). More over, the ra tio be tween di ag o nals var ies from 0.82 to 1.40 (Fig. 9B). The geo met ric cen tre of the im prints is slightly shifted in a lat eral di rec tion (Fig. 10B).
The low est av er age HV in the sec ond test ver sion was de - ter mined for sam ple no. 9 from Bochnia (Ta ble 1 and Fig. 7C) and amounts to 163.8 MPa. The HV re corded in the in den ta tion points var ies in a nar row range from 159.9 to 173.6 MPa. There is only one area of high HV value vis i ble in the right top of the map, as in the first test ver sion. In ter vals of low HV com prise the cen tral and left parts of the map (Fig. 7C).
The in den ta tion shapes in the sam ple from Bochnia ap - pears less reg u lar than in the im prints from the LGOM be cause of the con cave edges (Fig. 8F–H). How ever, there is only a slight shift of the geo met ric cen tre in a lat eral di rec tion (Fig. 10C) and the ra tio be tween di ag o nals is close to 1.0 (Fig. 9C).
DISCUSSION
The sam ples ana lysed from dif fer ent salt-bear ing for ma - tions are char ac ter ized by high vari abil ity in av er age HV and a broad range of HV val ues re corded in the in den ta tion points.
This vari abil ity of HV val ues and HV dis tri bu tion within each sam ple stud ied was greater in the first ver sion of the in den ta tion
tests per formed par al lel to the (100) and (010) faces. More over, the av er age HV cal cu lated for the mea sure ments in this di rec - tion were higher than the av er age HV de ter mined in the di rec - tion par al lel to the (110) and (11'0) faces. This sup ports the hy - poth e sis that the micro hard ness of these crys tals is anisotropic and re lated to the di rec tion of the spe cific crys tal lo graphic plane (Brookes et al., 1975; Craig and Vaughan, 1994; Stevenson et al., 2002).
The av er age HV cal cu lated in all ana lysed sam ples cor re - lates with the depth of salt-bear ing for ma tions re lated to these sam ples (Fig. 11). This cor re la tion prob a bly re sults from the lithostatic stress re lated to the weight of over bur den rocks, the geo ther mal gra di ent and the rhe o log i cal prop er ties of ha lite (Peng and Zhang, 2007; Urai et al., 2008). Gen er ally, all these pro cesses are con nected with the broadly rec og nized diagenesis and com pac tion of the de pos its and they are well rec og nized in such rocks. How ever, changes in phys i cal and me chan i cal prop er ties in a sin gle ha lite crys tal in re la tion to the depth have not been con sid ered so far. The cor re la tion de - scribed above may re sult from ha lite’s abil ity to recrystallize and un dergo visco-plas tic be hav iour (Shlichta, 1968; Guillope and Poirier, 1979; Carter and Hansen, 1983; Roedder, 1984; Ter Heege et al., 2005) as well as to de for ma tion of the crys tal’s in - ter nal struc ture un der var i ous stress and tem per a ture con di - tions (Hansen and Carter, 1980). None the less, the changes in stress and tem per a ture with depth and their in flu ence on the de pos its are com plex is sues that can not be con sid ered lin early Fig. 10. Shifts in the geo met ric cen tre of im prints
Di a gram for the sec ond test ver sion [par al lel to the (100) and (11'0) faces]
(Piniñska, 2011). A de vi a tion from cu bic sym me try in ha lite crys tals was in di cated in blue ha lite sam ples by the use of sin - gle crys tal X-ray dif frac tion meth ods (Zelek et al., 2008, 2014).
More over, po lar ized light mi cros copy ob ser va tions re vealed op ti cal ani so tropy (bi refrin gence) of blue ha lite crys tals re sult ing from the crys tals’ de for ma tion at the atomic level. The same op - ti cal fea tures were found in ha lite from De vo nian rock salt de - pos its (Pembina) and in bi tu mi nous salts from K³odawa. Op ti cal ani so tropy (bi refrin gence) rec og nized in crys tals from Pembina and K³odawa in di cate the de for ma tion of the ha lite in ter nal struc ture. Con se quently, it is pos si ble that de for ma tion of the ha lite crys tals at the atomic level re sulted in their higher micro - hard ness. It should be em pha sized that a cor re la tion be tween av er age HV and the depth of salt-bear ing de pos its is not as so - ci ated with tec tonic dis tur bances. The geo log i cal con di tions of the Pembina de posit which is part of the Lotsberg Salt For ma - tion are not af fected by tec ton ics. The Lotsberg For ma tion is lo - cated at the larg est depth among all for ma tions ana lysed but it is char ac ter ized by hor i zon tal, un dis turbed strata (Wardlaw and Wat son, 1966; Kukia³ka and Tobo³a, 2018). In con trast, the Badenian Salt For ma tion is strongly folded but the tec tonic move ments that had an im pact on this for ma tion took place at shal low depth (e.g., Poborski, 1952; Poborski and Skoczylas- Ciszewska, 1963; Ney et al., 1974; Garlicki, 1979; Wiewórka, 1984; Cyran and Tobo³a, 2006, 2007). In case of the Zechstein salt for ma tion, halokinetic and halotectonic move ments took place at a great depth that in cluded burial of salt de pos its.
HV val ues re corded in in den ta tion points vary in a wide range for all sam ples ana lysed and sim i lar HV val ues are dis - trib uted un equally within each sam ple stud ied. Ar eas dis tin - guished by high HV are prob a bly as so ci ated with a larger num - ber of dis lo ca tions (struc tural im per fec tions in the crys tal lat tice)
pres ent in the nat u ral crys tals. Oth er wise, zones of low HV are char ac ter ized by a lower num ber of dis lo ca tions which may re - sult from par tial recrystallisation. The pres ence of de for ma tion and its in flu ence on in den ta tion prop er ties has been de scribed for many ma te ri als (Craig and Vaughan, 1994; Rabier et al., 2010).
More over, a vari abil ity of HV val ues is con sis tent with dis tor - tion in im print shape. In ha lite from Pembina and K³odawa, a large vari abil ity in HV val ues and their dis tri bu tion within a sam - ple is as so ci ated with ir reg u lar shapes of im prints. Oth er wise, the sam ples from the LGOM are char ac ter ized by reg u lar in - den ta tion shapes and low vari abil ity of HV val ues. Oc cur rence of dis tor tion struc tures in the im prints ana lysed is prob a bly af - fected by sev eral fac tors: ir reg u lar shape of cleav age planes, sus cep ti bil ity to plas tic de for ma tion and oc cur rence of dis lo ca - tions. These dis lo ca tions are both nat u ral and in duced as a re - sult of the in den ta tion test (Craig and Vaughan, 1994;
Stevenson et al., 2002). A con vex shape of an im print was mostly ob served for sam ples from Pembina, the LGOM and K³odawa that are char ac ter ised by high HV. The con vex shape re flects pil ing up of ma te rial against the faces of the in denter due to the incompressibility as so ci ated with plas tic de for ma tion.
It oc curs when move ment of dis lo ca tions is re strained by other in ter nal de fects and plas tic strain con cen trates near the in den - ta tion area (Giannakopoulos and Suresh, 1999; Zhang et al., 2011; N’jock et al., 2015). How ever, the con cave shape re flects a sink ing-in of ma te ri als around the in denter when the plas tic - ally de formed re gion is pushed out from the in denter with the im print sink ing be low the ini tial sur face level. Sink ing-in oc curs when dis lo ca tion move ment is easy (Giannakopoulos and Suresh, 1999; Zhang et al., 2011; N’jock et al., 2015). A con - cave shape was ob served mostly in the sam ple from Wieliczka and prob a bly rep re sents recrystallised ha lite from a boul - der-rich part of the Wieliczka de posit.
An other ana lysed pa ram e ter was the shift of the im prints’
geo met ric cen tre within each sam ple. A shift may in di cate crys - tal de for ma tion. How ever, a shift of im print geo met ric cen tre may re sult from prep a ra tion of a thick plate. In this case, all val - ues should dif fer equally from 0.5 (0.5 rep re sents a reg u lar shape in which a geo met ric cen tre is lo cated in the mid dle of all sides) but there is no dis per sion of these val ues. In the im prints ana lysed, a small dis per sion of shift val ues was in di cated in sam ples from the LGOM and Bochnia (Fig. 6B, F). In the di a - gram pre pared for the sam ple from the LGOM (Fig. 6B) all points are grouped near a value of 0.5. How ever, all points in the di a gram pre pared for the sam ple from Bochnia are con cen - trated in the area of low H1/H ra tio (left). This shift may in di cate some rough ness in the sur face of the thick plate. In the sec ond test ver sion, the dis tri bu tion of points in the di a gram for Bochnia is sim i lar to that in the first test ver sion. The di a gram for the LGOM shows a lin ear points dis tri bu tion (Fig. 10A) that may in - di cate a zone with de fects of crys tal struc ture. The sam ples from Pembina, K³odawa and Wieliczka are char ac ter ised by higher dis per sion of shift val ues in the first test ver sion (Fig. 6A, C–E). The high est dis per sion was noted in the sam ple from Pembina and cor re lates with the high est HV val ues and a con - vex shape of im prints. Be cause thick plates were pre pared by cut ting the sin gle ha lite crys tals par al lel to their cleav age, the shift of an im print’s geo met ric cen tre is caused by ir reg u lar strike (de for ma tion) of cleav age planes. The ir reg u lar strike of the cleav age planes is prob a bly as so ci ated with de for ma tion of ha lite crys tals caused by stress.
HV val ues re corded in in den ta tion points dur ing the in den ta - tion tests are higher that the re sults re ported by Shakoor and Hume (1981) but lower than data pro vided for sam ples pre vi - ously de formed by uni ax ial com pres sion (Aptukov et al. 2010).
782 Tomasz Tobo³a, Katarzyna Cyran and Marek Rembiœ
Fig. 11. Di a gram of HV vs. depth of sam ples
Con se quently, HV val ues reg is tered for the sam ples ana lysed are in the range be tween data re ported in those pa pers. This may in di cate de for ma tion of the crys tal struc ture re sult ing from nat u ral con di tions such as stress and tem per a ture.
CONCLUSIONS
The re sults of in den ta tion tests in di cated sev eral as pects of micro hard ness char ac ter is tics in ha lite crys tals. Firstly, the HV val ues re corded at the in den ta tion points dif fer de pend ing on the mea sure ment di rec tion. Higher HV val ues were ob tained for mea sure ments par al lel to the (100) and (010) faces of the ha lite crys tal while HV val ues reg is tered par al lel to the (110) and (11'0) faces were lower. These re sults dem on strate an ani so - tropy of ha lite micro hard ness. More over, in the first ver sion of the in den ta tion test [par al lel to the (100) and (010) faces] a larger vari abil ity of HV val ues was no ticed.
Sec ondly, anal y sis of the re sults re vealed a cor re la tion be - tween the HV of ha lite and the depth of ha lite oc cur rence. The
av er age HV of ha lite crys tals that oc cur at great depth is higher than the av er age HV cal cu lated for crys tals lo cated at shal low depth. How ever, there is no cor re la tion be tween the in ten sity of of tec tonic de for ma tion and av er age HV.
Other as pects in clude a vari abil ity of HV val ues within the sam ples and the dis tor tion of an im print’s shape. These as - pects in di cated zones where ha lite crys tals are de formed at the atomic level and de fects in the crys tal lat tice. De for ma tion of ha lite crys tals is re flected by ir reg u lar ity in the strike of the cleav age planes. Con se quently, anal y sis of im print shapes (dis tor tion of shape, shift in the geo met ric cen tre, pro por tion of axes and di ag o nals) pro vides a use ful method for ex am i na tion of me chan i cal prop er ties in duc tile min er als and ma te ri als.
Ac knowl edge ments. The au thors would like to thank Dr. M. Schramm and an anon y mous re viewer for valu able com - ments and sug ges tions which were very use ful in prep a ra tion the fi nal ver sion of this pa per. This work was fi nan cially sup - ported by the AGH Uni ver sity of Sci ence and Tech nol ogy (Krakow, Po land), re search pro jects No 16.16.140.315.
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