the 
 largest uncertainty in the influence of aerosol particles and 
 clouds on the climate system is caused by aerosol 
cloud microphysics determine cloud albedo in 
 the visible and infrared ( ir ) spectral ranges , cloud lifetime , and 
 precipitation properties .
cloud radiative 
 properties are strongly linked to the microphysical state of clouds 
 such as number concentration and size of liquid droplets and ice crystals .
aerosol particles can act as cloud condensation nuclei ( ccn ) and 
 as ice nuclei ( in ) influencing the aggregation state and the microphysical 
 properties of cloud particles .
knowledge on the glaciation of clouds 
 is essential to estimate cloud radiative forcing on the climate system . in the atmosphere ice crystals form through heterogeneous 
 and homogeneous 
 ice nucleation . at temperatures below 235
k homogeneous nucleation 
 takes place , whereas at higher temperatures ice does not form spontaneously .
in this temperature range ice nucleation occurs 
 heterogeneously ; i.e. , it is triggered by the presence of aerosol 
 particles providing foreign surfaces that reduce the energy barrier 
 for nucleation .
several mechanisms are known by which aerosol 
 particles catalyze 
 the formation of the ice phase in clouds : deposition , condensation , 
 contact , and immersion freezing .
the deposition 
 mode involves the growth of ice directly from the vapor phase , whereas 
 condensation freezing occurs if the ice phase is formed immediately 
 after condensation of water vapor on a solid particle as liquid intermediate . 
 if an in has already been immersed in a droplet and causes freezing , 
 the process is termed immersion freezing .
contact freezing happens 
 if a supercooled droplet freezes at the moment of contact with an 
 in . in mixed - phase ( liquid and ice ) and cirrus clouds the dominant 
 nucleation mechanism is suspected to be immersion or contact freezing , 
 and to a lesser extent deposition nucleation .
the ice nucleating ability of an aerosol particle in each of the 
 four modes at fixed temperature and humidity conditions depends on 
 its physicochemical properties , e.g. , surface structure , size , and/or 
 chemical composition .
although several requirements for an effective 
 in have already 
 been proposed decades ago , the exact mechanisms of nucleation are 
 still not adequately understood .
it is 
 known that the ice nucleation efficiency of a particle is not necessarily 
 determined by the entire particle but by so - called active sites on 
 the particle s surface .
however , information on the nature 
 and location of active sites is still limited and a prediction of 
 the ice nucleation efficiency of a particle based on its physicochemical 
 properties is not yet possible .
atmospheric mineral particles 
 originate from arid regions such as deserts , from volcanic eruptions 
 and from soil due to agricultural use .
they are released into the 
 air by the action of wind and are omnipresent in our atmosphere .
recent studies confirm the importance 
 of mineral dust particles 
 for ice cloud formation .
aerosol mass spectrometry suggested that 
 50% of the material in ice crystal residues in clouds over wyoming 
 was composed of mineral dusts .
another 
 in situ study analyzed the residual particles within cirrus ice crystals 
 and found an enrichment of mineral dust particles of 61% compared 
 to their near - cloud abundance .
natural dust particles rarely consist of pure mineral phases but 
 are internal mixtures of diverse mineral components covered with other 
 inorganic , organic , and/or biological substances .
the main mineral 
 dust particles found in the atmosphere are clays ( kaolinite , illite , 
 montmorillonite ) , quartz , feldspars , and calcite . within the 
 last years , extensive efforts have been made to better 
 understand and predict the role of mineral dusts in the formation 
 of atmospheric ice clouds .
several laboratory studies on the ice nucleation 
 activity ( ina ) of mineral dust particles have been performed by many 
 groups .
because different measurement techniques are applied , the 
 comparison of the results is often challenging .
an extensive comparison and overview of the studies on 
 mineral dusts was given in recent years .
most 
 studies focused on clay minerals with kaolinite being the 
 clay mineral studied most intensively . in all studies kaolinite shows 
 ina in immersion freezing mode above 243 k. in a study of all common 
 clay materials using the same method , illite was found to be the most 
 active in followed by kaolinite and montmorillonite .
the study by zimmermann et al . gives 
 a good comparison between the ina of different minerals in the deposition 
 mode .
the ina of closely related materials , like the feldspars , was 
 rather different .
in fact , microcline ( k - feldspar ) needed the lowest 
 supersaturation at 261 k to initiate ice formation ( i.e. , in activation ) . 

the species active at the lowest supersaturation were kaolinite , illite , 
 hematite , and microcline .
kaolinite and muscovite 
 were found to be active at lower supersaturations and were therefore 
 considered rather efficient in , whereas quartz and calcite were poor 
 in .
montmorillonite was found a good in below a temperature around 
 241 k. most of the laboratory studies on mineral dust either 
 focused on 
 clay minerals that are often obtained from natural samples or on purely 
 natural dusts . concerning natural dusts , arizona test dust ( atd ) is 
 the most widely used proxy within the field .
initial freezing is reported 
 around 249 k. another natural dust sample 
 frequently studied is volcanic ash .
many of these 
 natural dusts are mixtures of different clays , quartz , and feldspars 
 with varying composition . recently atkinson et al .
performed 
 the first comparative study on ice nucleation in the immersion mode 
 looking not only at single minerals from the clay group but also at 
 k - feldspar , na / ca - feldspar , quartz , and calcite as well .
almost simultaneously 
 yakobi - hanock et al . conducted a similar 
 study on ice nucleation of 24 mineral samples in the deposition mode . 

both studies find the ice nucleating ability of k - feldspar exceptionally 
 high compared to other minerals . in the immersion mode
k - feldspar 
 already nucleates ice at temperatures of 250.5 k whereas for the other 
 minerals only lower nucleation temperatures were found . only recently was the ina of k - feldspar found 
 to be even better than that of na / ca - feldspar ( 247 k ) .
conclude that k - feldspar is the key 
 component determining the ina of atmospheric mineral dusts . as already mentioned , the majority of the studies concentrated 
 on the experimental description of the ice nucleation behavior of 
 mineral dusts . in case of immersion freezing
initial and/or median 
 freezing temperatures are reported for a vast set of atmospherically 
 relevant minerals .
where the authors relate the ina of the different 
 minerals to their particular surface charges . for example , they suggest 
 that the in properties of clays , especially of the ice active kaolinite , 
 might be due to electrostatic interactions between their charged surfaces , 
 counterions and the polar water molecule
. minerals with such ionic 
 surfaces are believed to promote ice nucleation , as they are more 
 likely to form hydrogen bonds with water molecules .
shen et al . found fluorine mica as an example of extremely 
 high ina .
overall , 
 a fundamental understanding of the heterogeneous ice nucleation 
 on mineral dusts is still missing .
for example , the nature of active 
 sites is a matter of speculation only .
the goal of our study 
 was to investigate the ina of selected mineral 
 particles as well as their chemical nature to identify possible characteristics 
 of active sites .
we performed experiments with the oil - immersion freezing 
 method with pure and pretreated ( heated , enzymatic pretreated , milled ) 
 dust particles .
in addition , we characterized the particles with field 
 emission gun scanning electron microscopy ( feg - sem ) and tunneling 
 electron microscopy ( tem ) , energy - dispersive x - ray spectroscopy ( edx ) , 
 x - ray powder diffraction ( xrd ) , and attenuated total reflection fourier 
 transform infrared spectroscopy ( atr - ftir ) . combining these data we 
 propose a possible explanation why specific minerals , in particular 
 k - feldspars , are good in , whereas others are not .
on the basis of 
 the experiments , we propose a first interpretation of the nature of 
 active sites .
oil - immersion 
 freezing experiments under a cryomicroscope were 
 carried out to test the ina of different mineral dust samples . in 
 addition , the particles were characterized regarding their surface 
 and bulk properties by the following set - ups .
nucleation properties of the samples 
 were obtained with an optical microscope and a homemade cryocell . 

the experimental setup has already been used in former studies , and 
 a detailed description can be found elsewhere . here , only a short description of the principle is given .
the core of the experiment is a thermoregulated cryocell consisting 
 of a peltier element in a teflon box .
the cryocell that has a glass 
 window on top is placed on an olympus bx51 optical microscope desk 
 below the objective .
the mineral dust samples were studied in the oil - immersion 
 mode : 
 a drop of an oil matrix ( 8085 wt % paraffin , 1520 
 wt % lanolin ) with dispersed small water droplets ( 1040 m ) 
 containing mineral dust particles was put on a glass slide and placed 
 on the peltier stage .
the peltier element was then chilled at a constant 
 cooling rate , until all visible droplets were frozen . in earlier measurements , 

no influence of the cooling rate ( 0.110 k / min ) on the median 
 freezing temperature was found within the uncertainty of the method , 
 which is about 1 k ( see supporting information ) .
whenever freezing of droplets was observed , a picture was taken 
 and the respective temperature was recorded .
frozen droplets can be 
 easily distinguished from liquid ones as they appear dark due to increased 
 light scattering and contain visible internal structures , such as 
 edges or cubes .
finally , each picture was analyzed to determine 
 the fraction of 
 frozen droplets , which was then plotted against temperature to obtain 
 a nucleation curve characteristic for a specific sample . to compare 
 different nucleation curves
, we determined the median freezing temperature , t50 , which is the temperature where 50% of all 
 droplets in the picture are frozen .
this value is more reliable than 
 the initial freezing temperature , which is the temperature where the 
 first droplet freezes , because the latter may be influenced by statistical 
 variations and is less reproducible .
sem and tem measurements were performed 
 at the ustem at the vienna university of technology . for sem ,
the 
 milled mineral dust samples were put on a graphite plate and coated 
 with 4 nm of au / pd alloy by sputtering ( program : 30s , 15 ma ) .
the powder is placed on a steel sample holder and then inserted into 
 an x - ray diffractometer ( panalyticalxpert pro ; bragg 
the 2 angle was varied between 5 and 120. 
 as x - ray source , the cu k line was used in all the experiments 
 ( 0.154  44 nm ) .
all samples were used without further treatment 
 prior to the xrd experiments except milling .
the measuring time varied 
 between 1 h for the single mineral samples and 3 h for the natural 
 samples .
the ftir study of the mineral samples 
 was taken on a bruker vector 22 .
at least 1000 scans were collected per sample to get a sufficient 
 signal - to - noise ratio at 4 cm spectral resolution .
the surface areas of the quartz 
 and montmorillonite samples were measured using a commercial liquid 
 nitrogen adsorption system ( asap2020 , micromeritics ) .
data evaluation 
 was based on the model by brunauer , emmett , and teller ( bet ) .
the surface areas of the other mineral dust 
 samples could not be measured as too little material was available . 

the geometrical surface area of those minerals was estimated on the 
 basis of the sem images .
in total the ina 
 of 13 different mineral dust samples was investigated with the cryomicroscope . 

the studied samples are calcite , gypsum , three different quartz samples 
 ( quartz i , quartz ii , quartz iii ) , microcline , albite , andesine , arizona 
 test dust ( atd ) , montmorillonite , kaolinite , limestone , and volcanic 
 ash ( table 1 ) .
the ina of the mineral dust particles was investigated 
 with pure 
 ( as purchased ) and freshly milled , heat - treated , and enzyme - treated 
 particles .
three sets of experiments were performed with pretreated 
 samples : first , the minerals were milled to initial sizes between 
 1 and 
 10 m with an agate mortar or a swing mill ( retsch mm400 ) .
suspensions 
 of mineral dust particles in milli - q water with concentrations of 
 20 and 50 mg / ml were prepared and then mixed with oil to obtain an 
 emulsion .
the water droplets in the emulsion were 1040 m 
 in size and contained around 110 particles at a concentration 
 of 20 mg / ml and about twice as much at concentrations of 50 mg / ml . 

second , the most ice 
 nucleation active samples were selected and 
 heated at 523 k for 45 h. in addition , the feldspars and quartz 
 i were also heated at 373 and 773 k to control for surface alteration 
 with temperature and to remove possible organic impurities , which 
 are known to promote ice nucleation in some cases , and to ensure that 
 the ina is only related to the mineral phases .
for all temperature - treated particles the ina was determined 
 before and after the heat treatment .
third , selected samples 
 were treated with enzymes to exclude the 
 possibility that their ina is due to adsorbed biological impurities , 
 and to specifically block nucleation sites .
for observing the impact of specific blocking , it was important 
 to select a submolecular coverage of the mineral surface .
otherwise , 
 the effect would not be related to the sites but would be due to the 
 entire coverage only .
as enzymes we used papain ( 2 mg / ml ) , pronase 
 e ( 5 mg / ml ) , cellulose onozuka ( 5 mg / ml ) , and lipase ( 2 mg / ml ) , which 
 break down proteins , polysaccharides , or lipids . three different enzymatic 
 treatment experiments were performed . after each step ice nucleation 
 measurements were performed : ( a ) first , each enzyme was added to separate 
 suspensions of pure milli - q water and mineral dust particles and left 
 for 35 h at incubation temperatures of 308 k ( lipase ) , 310 
 k ( pronase e , onozucka ) and 340 k ( papain ) , ( b ) second , the enzymes 
 were added all at once to the suspensions and the samples were left 
 in incubation in total for 5 h increasing the temperature stepwise , 
 respectively , and ( c ) in the last step , the enzymes were added and 
 nucleation measurements were conducted immediately without incubation .
in total , the ina of 13 mineral dust samples was 
 investigated with 
 the cryo - microscope ( table 1 and supporting information ) .
the most active species with t50 values > 247 k are quartz i , microcline , 
 atd , 
 and kaolinite .
all other samples had much lower t50 values ( < 242 k ) , but they were still higher than 
 those for pure water , indicating that the nucleation activation barrier 
 was still slightly reduced by the mineral dust particles .
the minerals are grouped into nonsilicates , quartz , feldspar , 
 clays , and natural samples .
the most active dusts are quartz , microcline 
 ( k - feldspar ) , atd , and kaolinite .
three different feldspar 
 samples were tested : albite ( na - feldspar ) , microcline ( k - feldspar ) , 
 and andesine ( na / ca - feldspar ) .
considerable differences in ina among 
 the feldspars were found : microcline had a t50 value of 249 k , whereas the t50 values for andesine and albite were much lower : 240 and 239 k , respectively . 
 to explain these differences the chemical bulk composition was analyzed 
 with xrd and edx .
the surface structure of microcline is rougher than that of albite , 
 but no significant difference in the morphology of the samples could 
 be identified .
these impurities appear on some albite grains , whereas 
 over 70% are highly pure albite .
edx mapping over cracks did not reveal 
 any migration of particular atoms to these sites . because the penetration 
 depth of x - rays and the exciting electrons is larger than a few hundred 
 nanometers , these analytical methods are not surface
the microcline 
 sample is crystalline and almost pure , except for the above - mentioned 
 albite content .
macroscopic defects 
 observed in the microcline sample are as common as in albite and can 
 therefore not explain the difference in the ina observed among the 
 feldspars .
the t50 for the most ice active quartz i sample was 
 249 k. the quartz ii and the quartz iii samples had t50 values of 239 and 235 k , respectively .
the sem 
 images of the quartz samples revealed that the quartz i sample ( figure 2a ) contained the largest fraction of particles with 
 diameters below 1 m followed by quartz ii and quartz iii ( figure 2b , c ) .
the diffractograms of all three tested -quartzes 
 show that no other phase was present .
the bet surface area of 
 quartz i sample was 5.5 m / g . 

quartz ii had specific bet area of 2.0 m / g and quartz 
 iii 0.5 m / g ( all measured before the initial freezing 
 experiment , i.e. , prior to further milling ) .
quartz i is shown in the top left image ( a ) , quartz ii at top right 
 ( b ) , and quartz iii below ( c ) .
gypsum and calcite became active 
 in at low temperatures with t50 values 
 of 239 and 237 k , respectively .
the gypsum sample contained around 
 4% of caso40.5h2o , with the rest being 
 pure caso42h2o ( gypsum ) , given by xrd 
 quantification .
montmorillonite had a t50 of 240 k. a determination of the exact mineral composition is difficult 
 as the layered structure of montmorillonite is not directly observable 
 in powder xrd . by this method
the exact quantity of those two components in montmorillonite is unknown , 
 but we expect less than 10% each .
the atd sample 
 had much smaller particle sizes than the other samples , with hardly 
 any particles larger than 5 m .
its composition ( based on powder 
 xrd phase analysis and sem - edx ) was 1520% microcline , 1520% 
 na / ca - feldspar , and 5060% quartz .
minor components were quartz , other feldspars , titanium 
 iron oxide , and aluminum oxides .
the volcanic ash was almost inactive 
 with a t50 of 238 k , but due to the k - feldspar 
 content the initial freezing temperature was much higher ( figure s1 , supporting information ) .
the natural kaolinite 
 used in this study was no single mineral component sample , as it contained 
 also quartz , muscovite , and halloysite .
natural limestone showed 
 the same ina as technical calcite with a t50 of 237 k , and it consisted only of calcite mineral on the basis 
 of the xrd and ftir analysis .
additional milling of the microcline 
 sample increased the t50 marginally ( from 
 249 to 250 k ) .
heat treatment of this sample reduces the activity 
 again , leading to the normal median freezing range 249250 
 k. for albite , milling had no effect on the t50 , but the initial freezing temperature increased significantly 
 as the albite sample contains some ( < 1% ) k - feldspar and more k - feldspar 
 surface became available during milling .
additional milling of both less 
 active quartz samples ( quartz ii and iii ) increased their t50 values : after 4 min of milling in the metal 
 swing mill the t50 values of the samples 
 both changed to 241 k and after 2 min milling time an increase by 
 up to 5 k was visible , whereas the activity of the quartz i sample 
 did not further increase . the initial freezing temperatures of the 
 samples quartz ii and iii increased from 242 to 246 k , respectively 

the surface areas of both 
 minerals were about 3 m / g after milling .
the absolute 
 number of active surface nucleation site density ns was calculated using the relation given in eq 1 , where fice is the frozen 
 fraction at the given temperature t and s is the particle surface per droplet obtained directly from bet specific 
 surface area.1 active surface site density depending on temperature plotted 
 for 
 all quartz samples .
quartz i shows the highest 
 surface site density , and the original quartz iii sample shows almost 
 no ina .
quartz ii and iii where milled for 4 min , resulting in a drastic 
 ina increase for quartz iii .
nevertheless , within the 
 measurement uncertainty the t50 change 
 only marginally by heat treatments .
temperature treatment of the quartz 
 and the kaolinite samples did not show any significant change in freezing 
 behavior .
except for the particles 
 treated with cellulose onozuka , the freezing curves of microcline 
 changed drastically after enzymatic treatment .
the t50 of the enzyme - treated microcline sample shifted to 
 lower temperatures ( 240 k for microcline and 246 k for fine milled 
 microcline ) compared to the pure microcline sample ( 249 k/250 k ) ( table 
 s1 , supporting information ) .
after 
 heating the enzyme - treated microcline samples to 773 k , the t50 values returned almost to their initial values 
 ( t50 after enzymes and heating : 248 k ) . 

the particles mixed with enzymes at low temperatures and direct freezing 
 measurements resulted in the same t50 values 
 as the enzyme - treated samples with incubation .
for albite the 
 same procedure was applied and no distinct loss 
 in ina was observed . in addition , heat and enzyme treatment resulted 
 in no freezing behavior change .
a clear loss of ina could 
 be observed for quartz i after the sample was treated with enzymes . 

although for treatment with papain the activity was almost unchanged , 
 a treatment with lipase and pronase e clearly shifted the freezing 
 curves to lower temperature values ( papain , 249 k ; pronase e , 247 
 k ; lipase , 247 k ) .
when all enzymes were applied together , t50 was 244 k. the activity was always restored 
 to the original level after 4 h heating at 773 k.
there is still no molecular description of the exact nature of 
 nucleation sites on mineral dust particles in the literature .
adsorption 
 of water through surface oh groups of the silica is reported , which is in agreement with later studies on 
 kaolinite .
surface amphotericity and 
 size matching between the ice structure and the solid surface has 
 a strong influence on ina . obviously , it is not the perfect quartz 
 surface itself that nucleates ice at a higher temperature ( see the 
 low in activity of the coarse quartz iii ) , but rather local defects , 
 which would support the concept of active sites
. those defects may 
 be atomic lattice distortions caused by impurities , leading to a better 
 structure matching between the ice and the particle surface , or crystallographic 
 dislocations .
on the basis of the experiments , we suggest that the 
 defect density is different in the three quartz samples , and that 
 it is increased by mechanical milling , which is known to generate 
 nucleation sites .
this would explain 
 the reported variations of ina , which range from quartz with t50 = 243 k being the second best in after the 
 feldspars to studies finding quartz one 
 of the worst in together with most oxides .
the minerals investigated in this study exhibit median freezing 
 temperature ( t50 ) values varying over 
 a range of 13 k between the most ( atd , 250 k ) and least ( calcite , 
 237 k ) potent in ( figure 1 ) .
structural analysis 
 showed that there is no direct correlation of the ina with the crystal 
 structure of the minerals .
this is especially true for the feldspar 
 group that showed considerable differences in their ina .
milling of 
 the samples increased the ina , which indicates that the freshly produced 
 surfaces provide nucleation active sites that are not accessible otherwise . 

suppressing ina 
 with specific enzymes with particular chemical functionalities , which 
 can interact with possible surface functional groups on the minerals , 
 was successful in several cases .
subsequent heat treatment to remove 
 the enzymes from the surface resulted in a reactivation of the nucleation 
 activity of the respective mineral .
all these experiments point 
 to the crucial importance of the mineral 
 surface itself and the involvement of the surface chemistry of these 
 particles , as there is no spectroscopic evidence for any particularity 
 to distinguish active surface sites of different quality .
the ice nucleation 
 property of specific quartz samples is not a result of their perfect 
 quartz structure , but rather of local defects acting as nucleation 
 sites .
it is still not possible to study these sites directly with conventional 
 methods , but some conclusions can be taken from the immersion freezing 
 experiments carried out in this study . as the silanol groups thought 
 to play an important role in the ice nucleation process are present in almost all silicates that show 
 different t50 values in aqueous immersion , 
 they alone do not act as good in .
ice nucleation is rather a complex 
 interplay between the forming ice structure and the local surface 
 structure of the mineral particle and therefore the arrangement of 
 the functional groups ( on the surface ) . the local electronic configuration , 
 as well as distance and arrangement of functional groups influence 
 the capability of a particle to act as good in .
possible functional 
 groups are metal - hydroxyl , fluorine , or ionic oxygen species .
the functional groups need to be able to act 
 as a hydrogen bond donor and/or acceptor .
a certain particle surface 
 is able to act as in , if the functional groups are arranged properly . 
 here , we define a molecular site analogous to ( molecular ) catalysis 
 as an arrangement of functional groups able to stabilize water molecules 
 in an ice - like structure .
a single molecular site may stabilize ice 
 embryos , but to form a good nucleation site , a larger area with domains 
 of molecular sites is needed .
these domains nucleate ice at a given 
 temperature , if the stabilized ice cluster is almost as large as the 
 critical ice cluster at this temperature .
the molecular sites are 
 of different composition , size , and concentration on different minerals 
 and samples .
the assumption of specific molecular sites is based on 
 the fact that with only partial surface coverage with enzymes the 
 ina is lowered , but not totally lost .
in addition , the ina of the 
 mineral particles can be increased by increasing the available surface 
 area .
the higher ina of quartz i is attributed to an increased concentration 
 of functional groups by the manufacturing ( milling ) process .
for example , 
 quartz iii froze almost heterogeneously before milling , whereas introducing 
 further defects ( by milling ) increased the activity more than would 
 be expected by just increasing the specific surface area .
molecular 
 sites are of different form and surface density on mineral dust particles . 

we assume that the domains of arranged molecular sites conform to 
 a material specific distribution with larger domains , resulting in 
 larger stabilized ice clusters and higher nucleation temperatures 
 being less frequent . with this idea it is possible to explain the 
 increase in nucleation temperatures with increasing surface area .
the feldspar family has 
 higher t50 values than the quartz sample 
 of comparable surface area in agreement with atkinson et al . and yakobi - hancock et al .
the structure of feldspars is 
 closely related to quartz , but si is partly substituted by al .
the 
 sio4/alo4 tetrahedra in feldspar are slightly tilted due to the charge compensating 
 cations ; surface distortions from the 
 basic quartz structure may even be larger and defects ( in particular 
 ionic defects ) are much more common , so more frequent molecular sites 
 are expected for feldspars .
nevertheless , the feldspars have 
 quite large differences in t50 values 
 with microcline being active at a much higher temperature in this 
 study and in the study by atkinson et al .
the latest studies showed also good ice nucleation activities in the deposition 
 mode for the orthoclase phase of k - feldspar , so the increased activity 
 of k - feldspar need not be phase specific .
the sem images of the feldspar 
 samples are quite similar and a morphological difference can not explain 
 the different ice nucleation behavior .
on the basis of the tem - edx 
 measurements , if ice nucleation occurs at macroscopic defect sites 
 such as cracks , it is not due to a local element accumulation , but 
 rather to the steric configuration there .
extrinsic ina by organic 
 adsorbates on microcline or its inhibition 
 on the plagioclases can be excluded on the basis of the enzymatic 
 and temperature treatment experiments .
the inhibition of ice nucleation 
 sites by enzymes may be understood as steric hindrance or blocking 
 of the site .
further experiments revealed that the loss of ina is 
 a result of a reversible active site hindrance / blocking and not a 
 destruction , as the initial activity was regained after enzyme removal . 

our results indicate that the higher ina of the k - feldspar sample 
 is an intrinsic property and not a result of adsorbed organic / biological 
 material .
the investigated feldspars have different counter 
 cations with different ionic radii ( rk > rna  rca ) .
in addition the plagioclases ( na / ca - feldspars ) have a higher al / si 
 ordering than the k - feldspars .
the measurements leave only the 
 conclusion that the difference 
 in the ina of the feldspars is a result of the difference in ionic 
 radius of the cations and therefore the local chemical configuration 
 at the surface .
the surface cations released into the surface bilayer 
 may interact with water to enhance / inhibit ice formation .
the resulting 
 depletion of cations in the outermost layer may be different for each cation due to the differences in ionic 
 radii .
the ion charge density of the cations of the mineral was already 
 suggested to influence ice nucleation on mineral surfaces .
the cations around the surface have different 
 affinity to water molecules and potential bonds are of different strength . 

surface calcium ions on calcite are known to bind the water quite 
 tightly and thus inhibit ice nucleation by fixing the water molecules 
 in ice structure mismatching locations . the difference in ina of microcline and the plagioclases may also 
 be a result of the more random cation / aluminum distribution in the 
 k - feldspars compared to the na / ca feldspars .
the tendency of the surface to interact with water 
 molecules is increased by this process as dangling bonds remain at 
 the surface .
the hydration shells of ca , na , and 
 k ions have different sizes and shapes .
ca and na belong to the chaotrope family ( structure breaking 
 ions ) whereas k is a kosmotrope ( structure making ion ) .
small ions with high charge density are considered to be chaotrope 
 and have a strong interaction with water
water 
 hydrogen bonds are broken to form the hydration shells with larger 
 residence times of water molecules compared to the shells of kosmotropic 
 ions .
these strong chaotrope - water interactions inhibit an ice - like 
 structuring of water molecules in the vicinity of the ions .
kosmotropes 
 like k on the other hand have a weaker interaction with 
 water than the intermolecular water  water interaction .
the 
 k ions form hydration shells , but the water molecules 
 are bonded more weakly and have high exchange rates .
therefore , any 
 thermodynamic phase change of water by k ions is kinetically 
 less hindered than by chaotrope ions .
in feldspar the cations 
 released into the water stay close to the 
 surface due to surface charging and charge compensation and are then 
 able to interact with water molecules . in the case of ca and na , ice nucleation is inhibited by their chaotropic 
 behavior , whereas k has a positive or at least a neutral 
 effect .
in addition , it was shown that koh is easily incorporated 
 into the ice structure .
the size of k is around the size of a h3o ion , whereas 
 na and ca are far too small to fit well into 
 the ice structure .
this would further lower the negative effect of 
 the k ions to the formation of ice - like structures close 
 to the vicinity of the surface bilayer .
this is in agreement with 
 a former study where lower ice nucleation 
 temperatures were found for micas containing al ions 
 instead of k ions , where again the aluminum ion has a 
 much higher charge density .
our 
 kaolinite sample contains 
 small amounts of k - feldspar , which gives the sample most of its ina 
 together with a slightly larger surface area .
any exact explanation 
 of the ice nucleation behavior of our montmorillonite sample is difficult , 
 as exact mineral composition analysis and structure determination 
 are lacking .
a distinct phase identification by xrd was not possible 
 due to the layered stacking structure of montmorillonite . on the basis 
 of the sem - edx , the larger amounts of mg compared to 
 k , na , and ca found suggest chaotropic 
 influence of the magnesium ions if the surfaces of the mineral support 
 ice - like arranged water molecules .
nevertheless , the montmorillonite 
 acts as a heterogeneous in , but at rather low temperatures compared 
 to k - feldspar .
as already reported , the calcite 
 surface has a strong affinity to water molecules .
the water molecules are tightly bound to the surface and 
 can not arrange in ice - like structures .
the water molecules are possibly 
 bound to hydrolysis species , chemisorbed on the surface of calcite . in gypsum , the strong calcium 
water interaction 
 may lead to a similar behavior . in agreement with the feldspars , 

the investigated volcanic ash 
 acts as a rather poor in with a t50 of 
 238 k. the ash sample from the eyafjallajkull eruption 2010 
 is mainly composed of the weak in albite ( na - feldspar ) .
the higher 
 initial freezing temperature can be easily explained by the inhomogeneity 
 of the sample .
the titanium  iron oxide which was found in the 
 ash seems to have no influence on the ina , but no experiments on single 
 minerals could be performed .
this partly explains why in a former 
 study volcanic ash showed both high in activity , as well as almost 
 none .
k - feldspar rich volcanic ash has 
 a larger tendency to act as a good in than ash containing mainly plagioclases 
 ( na / ca - feldspar ) .
it can not be stated directly that only the mineral 
 composition determines the ina as volcanic ash eruptions are often 
 accompanied by sulfuric acid and other gases that may alter the surface 
 and ice nucleation properties .
the atd sample had a slightly larger surface area than the other 
 dust samples , probably leading to a slightly overestimated t50 value compared to the other samples .
still , 
 the atd sample was a rather good in , which was not surprising due 
 to its k - feldspar content .
in addition , the quartz content may also 
 act as in , depending on the preprocessing of the sample .
the feldspar family has 
 higher t50 values than the quartz sample 
 of comparable surface area in agreement with atkinson et al . and yakobi - hancock et al . for the deposition mode
the structure of feldspars is 
 closely related to quartz , but si is partly substituted by al .
the 
 sio4/alo4 tetrahedra in feldspar are slightly tilted due to the charge compensating 
 cations ; surface distortions from the 
 basic quartz structure may even be larger and defects ( in particular 
 ionic defects ) are much more common , so more frequent molecular sites 
 are expected for feldspars . nevertheless , the feldspars have 
 quite large differences in t50 values 
 with microcline being active at a much higher temperature in this 
 study and in the study by atkinson et al .
the latest studies showed also good ice nucleation activities in the deposition 
 mode for the orthoclase phase of k - feldspar , so the increased activity 
 of k - feldspar need not be phase specific .
the sem images of the feldspar 
 samples are quite similar and a morphological difference can not explain 
 the different ice nucleation behavior . on the basis of the tem - edx 
 measurements , if ice nucleation occurs at macroscopic defect sites 
 such as cracks , it is not due to a local element accumulation , but 
 rather to the steric configuration there .
extrinsic ina by organic 
 adsorbates on microcline or its inhibition 
 on the plagioclases can be excluded on the basis of the enzymatic 
 and temperature treatment experiments .
the inhibition of ice nucleation 
 sites by enzymes may be understood as steric hindrance or blocking 
 of the site .
further experiments revealed that the loss of ina is 
 a result of a reversible active site hindrance / blocking and not a 
 destruction , as the initial activity was regained after enzyme removal . 

our results indicate that the higher ina of the k - feldspar sample 
 is an intrinsic property and not a result of adsorbed organic / biological 
 material .
the investigated feldspars have different counter 
 cations with different ionic radii ( rk > rna  rca ) .
in addition the plagioclases ( na / ca - feldspars ) have a higher al / si 
 ordering than the k - feldspars .
the measurements leave only the 
 conclusion that the difference 
 in the ina of the feldspars is a result of the difference in ionic 
 radius of the cations and therefore the local chemical configuration 
 at the surface .
the surface cations released into the surface bilayer 
 may interact with water to enhance / inhibit ice formation .
the resulting 
 depletion of cations in the outermost layer may be different for each cation due to the differences in ionic 
 radii .
the ion charge density of the cations of the mineral was already 
 suggested to influence ice nucleation on mineral surfaces .
the cations around the surface have different 
 affinity to water molecules and potential bonds are of different strength . 

surface calcium ions on calcite are known to bind the water quite 
 tightly and thus inhibit ice nucleation by fixing the water molecules 
 in ice structure mismatching locations .
the difference in ina of microcline and the plagioclases may also 
 be a result of the more random cation / aluminum distribution in the 
 k - feldspars compared to the na / ca feldspars . as already mentioned , 
 feldspar surfaces are cation deficient in 
 aqueous solution .
the tendency of the surface to interact with water 
 molecules is increased by this process as dangling bonds remain at 
 the surface .
the hydration shells of ca , na , and 
 k ions have different sizes and shapes .
ca and na belong to the chaotrope family ( structure breaking 
 ions ) whereas k is a kosmotrope ( structure making ion ) .
small ions with high charge density are considered to be chaotrope 
 and have a strong interaction with water . the weaker water 
water 
 hydrogen bonds are broken to form the hydration shells with larger 
 residence times of water molecules compared to the shells of kosmotropic 
 ions .
these strong chaotrope - water interactions inhibit an ice - like 
 structuring of water molecules in the vicinity of the ions .
kosmotropes 
 like k on the other hand have a weaker interaction with 
 water than the intermolecular water  water interaction .
the 
 k ions form hydration shells , but the water molecules 
 are bonded more weakly and have high exchange rates . therefore ,
any 
 thermodynamic phase change of water by k ions is kinetically 
 less hindered than by chaotrope ions .
in feldspar the cations 
 released into the water stay close to the 
 surface due to surface charging and charge compensation and are then 
 able to interact with water molecules . in the case of ca and na , ice nucleation is inhibited by their chaotropic 
 behavior , whereas k has a positive or at least a neutral 
 effect .
in addition , it was shown that koh is easily incorporated 
 into the ice structure .
the size of k is around the size of a h3o ion , whereas 
 na and ca are far too small to fit well into 
 the ice structure .
this would further lower the negative effect of 
 the k ions to the formation of ice - like structures close 
 to the vicinity of the surface bilayer .
this is in agreement with 
 a former study where lower ice nucleation 
 temperatures were found for micas containing al ions 
 instead of k ions , where again the aluminum ion has a 
 much higher charge density .
the ice 
 nucleation behavior of various mineral samples was investigated with 
 a special focus on feldspars , which are known to be among the most 
 ice nucleation active species .
the feldspars are more ice nucleation active than most quartz samples , 
 and k - feldspar is by far the most active in of the feldspar family
. 
 the size of the cation , and its binding energy toward water are the 
 key factors determining ina .
different quartz samples showed 
 a large discrepancy of their ina . 
 with the presented idea of domains of molecular sites able to bind 
 and arrange water molecules in an ice - like structure acting as ice 
 nucleation sites
, we suggest that the history of the quartz particles 
 and dust particles in general has an important influence .
ina is enhanced 
 by introducing more defects to a quartz surface by mechanical milling . 

the nucleation sites suggested here are not necessarily the same on 
 each particle , but rather caused by a stochastic arrangement of functional 
 groups which are able to bind water molecules .
therefore , nucleation 
 site sizes , as well as the corresponding nucleation temperature , conform 
 to a statistical distribution .
consequently , the freezing curves determined 
 with our experimental setup can be shifted to higher temperatures 
 by increasing the particle surface for active in like k - feldspar . 
 still , an open task for future work will be to understand the processes 
 that lead to site formation on the molecular level .
 mineral 
 dusts originating from earth s crust are known to 
 be important atmospheric ice nuclei . in agreement with earlier studies , 
 
 feldspar was found as the most active of the tested natural mineral 
 dusts . 
 here we investigated in closer detail the reasons for its activity 
 and the difference in the activity of the different feldspars . 
 conclusions 
 are drawn from scanning electron microscopy , x - ray powder diffraction , 
 infrared spectroscopy , and oil - immersion freezing experiments . 
 k - feldspar 
 showed by far the highest ice nucleation activity . 
 finally , we give 
 a potential explanation of this effect , finding alkali - metal ions 
 having different hydration shells and thus an influence on the ice 
 nucleation activity of feldspar surfaces . 